Mutations in blaKPC-3 That Confer Ceftazidime-Avibactam Resistance Encode Novel KPC-3 Variants That Function as Extended-Spectrum β-Lactamases

Evolution of ceftazidime/avibactam resistance and plasmid dynamics in OXA-48-producing Klebsiella spp. during long-term patient colonization

PURPOSE

To prospectively monitor the evolution of the resistome of OXA-48-producing Klebsiella species in a patient with long-term colonization, with a particular focus into the plasmid dynamics and the evolution of ceftazidime/avibactam resistance.

METHODS

All OXA-48-producing Klebsiella spp. isolates from a single patient admitted to a hospital during seven months were prospectively collected. MICs were determined through reference broth microdilution. Multilocus sequence types, SNPs analysis, resistance mechanisms, genetic context of β-lactamases and plasmid dynamics were determined by WGS and bioinformatic analysis. The impact of β-lactamase variant obtained after ceftazidime/avibactam exposure was determined via cloning experiments.

RESULTS

Four isolates, two before (one OXA-48-producing K. pneumoniae and one CTX-M-15-like-producing K. pneumoniae) and two after treatment with ceftazidime/avibactam (one OXA-48- and CTX-M-15-like-producing K. pneumoniae and one OXA-48- and CTX-M-15-like-producing K. aerogenes) were collected. The plasmid dynamics analysis demonstrated that the IncL and IncFIIK plasmids, in which blaOXA-48 and blaCTX-M-15-like genes were located, respectively, exhibited a high degree of conservation indicating a potential for both intra- and interspecies transmission. The K. pneumoniae isolate obtained after treatment, which differed from the previous isolate by just six SNPs, exhibited resistance to ceftazidime/avibactam through P167S substitution in CTX-M-15, which is now designated CTX-M-273. Cloning experiments demonstrated enhanced resistance to ceftazidime/avibactam.

CONCLUSION

The transfer of plasmid-borne β-lactamase resistance genes between intra- and interspecies bacterial populations enables the rapid diversification of the bacterial genome. The emergence of ceftazidime/avibactam resistance through the modification of CTX-M-enzymes represents a mechanism by which OXA-48-producing Enterobacterales may evolve toward ceftazidime/avibactam resistance in vivo.

Spectrum of cefepime-taniborbactam coverage against 190 β-lactamases defined in engineered isogenic Escherichia coli strains

Cefepime-taniborbactam is a β-lactam/β-lactamase inhibitor combination in clinical development for the treatment of Enterobacterales and Pseudomonas infections, including carbapenem-resistant Enterobacterales and multidrug-resistant Pseudomonas aeruginosa. Taniborbactam is a novel cyclic boronate with direct inhibitory activity against clinically relevant Ambler class A, B, C, and D β-lactamases. To further characterize the spectrum of β-lactamase coverage by cefepime-taniborbactam, we constructed 190 isogenic strains of Escherichia coli that constitutively expressed a different β-lactamase. Synthetic codon-optimized genes encoding the mature periplasmic protein linked to the TEM-1 signal sequence were used for optimized expression and periplasmic localization of the β-lactamase. The repertoire of β-lactamases consisted of 50 Ambler class A, 34 class B (metallo), 48 class C, and 58 class D enzymes known to mediate β-lactam resistance in the clinical isolates of Enterobacterales and P. aeruginosa. Overall, in the 190 isogenic strains, the MIC50/MIC90 values were 8/128 µg/mL for cefepime and >128/>128 µg/mL for ceftazidime. Cefepime-taniborbactam (MIC50/MIC90 of 0.25/8 µg/mL) showed greater activity than ceftazidime-avibactam (MIC50/MIC90 of 4/>128 µg/mL) and similar activity to aztreonam-avibactam (MIC50/MIC90 of 0.5/4 µg/mL). Cefepime-taniborbactam inhibited strains overproducing metallo-β-lactamases, including clinically important NDM and VIM enzymes, whereas ceftazidime-avibactam showed no coverage. Among the 129 β-lactamase-overproducing strains with increased cefepime MIC ≥16-fold relative to the control strain, taniborbactam potentiated cefepime MIC by ≥8-fold for 113 strains overexpressing β-lactamases (42 Ambler class A, 24 B, 23 C, and 24 D). Cefepime-taniborbactam demonstrated broader activity relative to ceftazidime-avibactam and comparable activity with aztreonam-avibactam in the overall coverage of both serine- and metallo-β-lactamases from all four Ambler classes.

Geographic variations in distributions of carbapenemase-encoding genes, susceptibilities, and minimum inhibitory concentrations of inpatient meropenem-resistant Enterobacterales to ceftazidime-avibactam, meropenem-vaborbactam, and aztreonam-avibactam across four global regions: 2020-2022 data from the Antimicrobial Testing Leadership and Surveillance

PURPOSE

To evaluate the susceptibility profiles of regional inpatient meropenem-resistant (MEM-R) carbapenemase-producing Enterobacterales (CPE) isolates and their MIC values to ceftazidime-avibactam (CZA), meropenem-vaborbactam (MVB), and aztreonam-avibactam (ATM-AVI) METHODS: The 2020-2022 Antimicrobial Testing Leadership and Surveillance database were analyzed. Carbapenemase-encoding genes in CPE isolates were identified using multiplex PCR and Sanger sequencing. Susceptibility breakpoints for CZA and MVB recommended by CLSI 2024 and EUCAST 2025 against Enterobacterales were applied.

RESULTS

A total of 2,318 CPE isolates (78.2% were Klebsiella pneumoniae) were tested globally. Notable diversity in carbapenemase-encoding gene distributions was observed among CPE isolates from Africa/the Middle East (10 countries; n=361), Asia (7 countries, excluding India and Pakistan; n=182), Europe (17 countries; n=1,002), and Latin America (10 countries; n=773). Metallo-β-lactamase-encoding genes, predominantly blaNDM-1, were more frequently detected in CPE isolates from Africa/the Middle East (75.3%, except blaNDM-5 in Kuwait) and Asia (67%, except blaIMP-8 in Taiwan) compared to other regions. Among KPC variants, the KPC-2 enzyme was the predominant one in CPE isolates in Europe (43.4%, except for KPC-3 prevalent specifically in Italy) and in Latin America (62.1%). The susceptibility rates of all analyzed CPE isolates harboring only a single blaKPC gene to CZA and MVB were 99.4% and 93.5%, respectively, based on the CLSI 2024 susceptibility breakpoints. The MIC50/90 values of CPE isolates to ATM-AVI were 0.12/0.25 mg/L and 0.5/1 mg/L, respectively, regardless of collection region, dual carbapenemase production, or infection source.

CONCLUSIONS

The trends in resistance to novel antibiotics among contemporary CPE isolates need close monitoring.

Past, present, and future perspectives on aztreonam and avibactam

INTRODUCTION

Aztreonam is a monobactam antibiotic approved in 1986 to treat infections caused by aerobic Gram-negative bacteria, but, together with cephalosporins, lost clinical utility due to the emergence of extended-spectrum β-lactamases (ESBLs) and novel (serine) carbapenemases. Avibactam was the first in a novel non-β-lactam β-lactamase inhibitor class to effectively inhibit these enzymes. It has been approved in combination with ceftazidime to treat Gram-negative infections caused by bacteria that produce AmpC, ESBLs and serine carbapenemases, and with aztreonam to treat patients infected with metallo-β-lactamase-producing enteric bacteria. Combinations of avibactam with ceftazidime and/or aztreonam have been used successfully to treat enteric pathogens producing multiple classes of β-lactamases.

AREAS COVERED

Development of aztreonam, avibactam, and avibactam combinations are placed into a historical perspective, based on both preclinical and clinical data. A search of MEDLINE (Ovid) was used to identify relevant literature.

EXPERT OPINION

Avibactam combined with ceftazidime and aztreonam in either dual or triple combinations provides the opportunity to treat previously untreatable Gram-negative infections that produce multiple β-lactamases. Aztreonam combinations should be particularly attractive, due to stability to metallo-β-lactamase hydrolysis and its safety advantage in treating penicillin-allergic patients. Other inhibitor combinations in development may challenge these combinations.

A systematic review and individual bacterial species level meta-analysis of in vitro studies on the efficacy of ceftazidime/avibactam combined with other antimicrobials against carbapenem-resistant Gram-negative bacteria

BACKGROUND

Carbapenem-resistant Gram-negative bacteria (CR-GNB) develop resistance to many antimicrobials. To effectively manage infections caused by these organisms, novel agents and/or combinations of antimicrobials are required.

OBJECTIVES

Evaluated the in vitro efficacy of ceftazidime/avibactam in combination with other antimicrobials against CR-GNB.

METHODS

PubMed, Web of Science, Embase and Scopus were searched. Study outcomes were quantified by counting the number of isolates exhibiting synergy, defined as a fractional inhibitory concentration index ≤ 0.5 for checkerboard and Etest, and a >2 log cfu/mL reduction for time-kill studies. The proportion of synergy was calculated as the ratio of isolates exhibiting synergy to the total number of isolates tested. These proportions were analysed using a random-effects model, following the Freeman-Tukey double-arcsine transformation.

RESULTS

Forty-five in vitro studies were included. A total of 734 isolates were tested, and 69.3% of them were resistant to ceftazidime/avibactam. The combination of ceftazidime/avibactam with aztreonam showed a high synergy rate against carbapenem-resistant Klebsiella pneumoniae (effect size, ES = 0.91-0.98) and Escherichia coli (ES = 0.75-1.00). Ceftazidime/avibactam also demonstrated a high synergy rate (ES = 1) in time-kill studies when combined with azithromycin, fosfomycin and gentamicin against K. pneumoniae. Compared to ceftazidime/avibactam alone, a higher bactericidal rate was reported when ceftazidime/avibactam was combined with other antimicrobials against carbapenem-resistant K. pneumoniae (57% versus 31%) and E. coli (93% versus 0%).

CONCLUSIONS

Ceftazidime/avibactam frequently demonstrates synergistic bactericidal activity when combined with various antimicrobials against CR-GNB in in vitro tests. Further pre-clinical and clinical studies are warranted to validate the utility of ceftazidime/avibactam-based combination regimens for CR-GNB infections.

Clinical impact of ceftazidime/avibactam on the treatment of suspected or proven infections in a large cohort of patients with haematological malignancies: a multicentre observational real-world study

OBJECTIVES

To evaluate clinical impact of ceftazidime/avibactam on treating infections due to MDR Gram-negative bacteria in patients with haematological malignancies (HMs).

METHODS

We conducted a retrospective, observational study at 17 Italian haematological wards that included patients with HMs receiving ceftazidime/avibactam for the treatment of suspected or proven infections. The primary endpoint was all-cause mortality 30 days after infection onset. Secondary endpoints included the development of in vitro ceftazidime/avibactam resistance, adverse reactions and infection relapse.

RESULTS

Of 198 patients enrolled, 66 had fever of unknown origin and 132 had microbiologically proven infections (MPIs). Enterobacterales were responsible for 98 MPIs, with KPC producers accounting for 75% of these, and carbapenem-resistant Pseudomonas aeruginosa caused 25% of MPIs. The overall 30-day mortality rate was 17.7%. Infection relapse occurred in four patients with MPI. Patients who died within 30 days of infection onset tended to have pre-existing cerebrovascular diseases, a Charlson Comorbidity Index > 4 and septic shock at infection onset and had received inadequate initial antibiotic therapy. Thirty-day mortality was independently associated with septic shock at infection onset and inappropriate initial antibiotic therapy.

CONCLUSIONS

Our study provides further evidence about the effectiveness of ceftazidime/avibactam in treating infections in patients with HMs.

Unseen Enemy: Mechanisms of Multidrug Antimicrobial Resistance in Gram-Negative ESKAPE Pathogens

Multidrug antimicrobial resistance (AMR) represents a formidable challenge in the therapy of infectious diseases, triggered by the particularly concerning gram-negative Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. (ESKAPE) pathogens. Designated as a "priority" in 2017, these bacteria continue to pose a significant threat in 2024, particularly during the worldwide SARS-CoV-2 pandemic, where coinfections with ESKAPE members contributed to worsened patient outcomes. The declining effectiveness of current treatments against these pathogens has led to an increased disease burden and an increase in mortality rates globally. This review explores the sophisticated mechanisms driving AMR in gram-negative ESKAPE bacteria, focusing on Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Enterobacter spp. Key bacterial mechanisms contributing to resistance include limitations in drug uptake, production of antibiotic-degrading enzymes, alterations in drug target sites, and enhanced drug efflux systems. Comprehending these pathways is vital for formulating innovative therapeutic strategies and tackling the ongoing threat posed by these resistant pathogens.

Identification of blaKPC-90 in Klebsiella pneumoniae associated with ceftazidime-avibactam resistance and the translocation & truncation of resistant genes mediated by IS26

OBJECTIVES

In this study, we discovered blaKPC-90 in ceftazidime-avibactam resistant clinical isolates of K. pneumoniae from a patient with multiple comorbidities and investigated the resistance & transfer mechanism of blaKPC-90.

METHODS

K. pneumoniae strains carrying blaKPC-2 and blaKPC-90 were isolated from the patient. Antimicrobial susceptibility tests and whole genome sequencing were performed to investigate the phenotype & genotype of strains. Conjugation assays, cloning experiment, kinetic parameters measuring, outer membrane protein SDS-PAGE and qRT-PCR were performed to explore the spread and antimicrobial resistance mechanisms.

RESULTS

KPC-90 isolates had an insertion of two amino acids (Thr180_Ser181 ins Tyr Thr) compared to the wildtype KPC-2. Antimicrobial susceptibility testing of isolates with KPC-90 vs. KPC-2 showed ceftazidime-avibactam MICs of >128 vs. 1-2 mg/L, meropenem-vaborbactam MICs of 4 vs. 1 mg/L, meropenem MICs of 4-8 vs. >128 mg/L and imipenem MICs of 0.5-1 vs. 64 mg/L. Analysis of kinetic parameters of KPC-90 compared to KPC-2 showed decreased hydrolysis of carbapenems and increased IC50 of avibactam. Genetic characterization of the plasmid revealed that IS26 could mediate the intramolecular inversion, translocation and truncation of the resistance determinant region.

CONCLUSION

We have described the case of a patient infected with blaKPC-90-carrying K. pneumoniae strains and investigated the mechanism of resistance to carbapenems and ceftazidime-avibactam associated with blaKPC-2 and its variants. We have also focused on the functional diversity of IS26 in relation to antimicrobial resistance. In the future, it is crucial to pay more attention to the evolution and horizontal transmission of blaKPC.

Mechanisms leading to in vivo ceftazidime/avibactam resistance development during treatment of GES-5-producing Pseudomonas aeruginosa infections

The mechanisms underlying ceftazidime/avibactam resistance development in four ceftazidime/avibactam susceptible/resistant pairs of GES-5-producing ST235 Pseudomonas aeruginosa clinical isolates were investigated. In three of the cases, ceftazidime/avibactam resistance was driven by a single mutation leading to GES-27 (P162Q), GES-29 (P162A), or the novel GES-60 (N136S), as confirmed through cloning experiments. Moreover, these mutations were associated with increased cefiderocol MICs but reduced carbapenem, particularly imipenem/relebactam, resistance. Understanding the complexity of resistance mechanisms to the growing repertoire of antipseudomonal β-lactams is crucial to guide optimized treatments and antimicrobial stewardship measures.

Emergence of Escherichia coli ST131 carrying carbapenemase genes, European Union/European Economic Area, August 2012 to May 2024

Analysis of 594 isolates of Escherichia coli sequence type (ST)131 and its single locus variants carrying carbapenemase genes from 17 European Union/European Economic Area countries revealed acquisition of 18 carbapenemase variants, mainly in ST131 clades A and C. Most frequent were bla OXA-244 (n = 230) and bla OXA-48 (n = 224), detected in 14 and 12 countries, respectively. Isolates carrying bla OXA-244 have increased rapidly since 2021. The increasing detection of carbapenemase genes in the E. coli high-risk lineage ST131 is a public health concern.

Ceftazidime-avibactam resistance in KPC-producing Klebsiella pneumoniae accompanied hypermucoviscosity acquisition

BACKGROUND

Antimicrobial resistance and bacterial hypermucoviscosity, associated with escalating production of capsules, constitute major challenges for the clinical management of Klebsiella pneumoniae (K. pneumoniae) infections. This study investigates the association and underlying mechanism between ceftazidime-avibactam (CAZ-AVI) resistance and bacterial hypermucoviscosity in Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae (KPC-Kp).

RESULTS

The proportion of CAZ-AVI-sensitive clinical isolates exhibiting the hypermucoviscous phenotype was significantly lower than that of the resistant strains (5.6% vs. 46.7%, P < 0.001). To further verify the correlation and molecular mechanism between CAZ-AVI resistance and hypermucoviscosity, 10 CAZ-AVI-resistant isolates were generated through in vitro resistance selection from CAZ-AVI-sensitive KPC-Kp. The results showed the same association as it showed in the clinical isolates, with four out of ten induced CAZ-AVI-resistant isolates transitioning from negative to positive in the string tests. Comparative genomic analysis identified diverse mutations in the wzc gene, crucial for capsule polysaccharide (CPS) synthesis, in all four CAZ-AVI-resistant hypermucoviscous KPC-Kp strains compared to the parent strains. However, these mutations were absent in the other six KPC-Kp strains that did not exhibit induced hypermucoviscosity. Cloning of the wzc gene variants and their expression in wild-type strains confirmed that mutations in the wzc gene can induce bacterial hypermucoviscosity and heightened virulence, however, they do not confer resistance to CAZ-AVI.

CONCLUSIONS

These results indicated that resistance to CAZ-AVI in KPC-Kp isolates may be accompanied by the acquisition of hypermucoviscosity, with mutations in the wzc gene often involving in this process.

ESKAPE pathogens: antimicrobial resistance, epidemiology, clinical impact and therapeutics

The rise of antibiotic resistance and a dwindling antimicrobial pipeline have been recognized as emerging threats to public health. The ESKAPE pathogens - Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp. - were initially identified as critical multidrug-resistant bacteria for which effective therapies were rapidly needed. Now, entering the third decade of the twenty-first century, and despite the introduction of several new antibiotics and antibiotic adjuvants, such as novel β-lactamase inhibitors, these organisms continue to represent major therapeutic challenges. These bacteria share several key biological features, including adaptations for survival in the modern health-care setting, diverse methods for acquiring resistance determinants and the dissemination of successful high-risk clones around the world. With the advent of next-generation sequencing, novel tools to track and combat the spread of these organisms have rapidly evolved, as well as renewed interest in non-traditional antibiotic approaches. In this Review, we explore the current epidemiology and clinical impact of this important group of bacterial pathogens and discuss relevant mechanisms of resistance to recently introduced antibiotics that affect their use in clinical settings. Furthermore, we discuss emerging therapeutic strategies needed for effective patient care in the era of widespread antimicrobial resistance.

International and regional spread of carbapenem-resistant Klebsiella pneumoniae in Europe

Carbapenem-resistant Klebsiella pneumoniae (CRKP) are of particular concern due to the spread of antibiotic resistance genes associated with mobile genetic elements. In this study, we collected 687 carbapenem-resistant strains recovered among clinical samples from 41 hospitals in nine Southern European countries (2016-2018). We identified 11 major clonal lineages, with most isolates belonging to the high-risk clones ST258/512, ST101, ST11, and ST307. blaKPC-like was the most prevalent carbapenemase-encoding gene (46%), with blaOXA-48 present in 39% of isolates. Through the combination and comparison of this EURECA collection with the previous EuSCAPE collection (2013-2014), we investigated the spread of high-risk clones circulating in Europe exhibiting regional differences. We particularly found blaKPC-like ST258/512 in Greece, Italy, and Spain, blaOXA-48 ST101 in Serbia and Romania, blaNDM ST11 in Greece, and blaOXA-48-like ST14 in Türkiye. Genomic surveillance across Europe thus provides crucial insights for local risk mapping and informs necessary adaptions for implementation of control strategies.

First report of KPC variants conferring ceftazidime-avibactam resistance in Colombia: introducing KPC-197

Resistance to ceftazidime-avibactam (CZA) due to Klebsiella pneumoniae carbapenemase (KPC) variants is increasing worldwide. We characterized two CZA-resistant clinical Klebsiella pneumoniae strains by antimicrobial susceptibility test, conjugation assays, and WGS. Isolates belonged to ST258 and ST45, and produced a KPC-31 and a novel variant KPC-197, respectively. The novel KPC variant presents a deletion of two amino acids on the Ω-loop (del_168-169_EL) and an insertion of two amino acids in position 274 (Ins_274_DS). Continued surveillance of KPC variants conferring CZA resistance in Colombia is warranted.

IMPORTANCE

Latin America and the Caribbean is an endemic region for carbapenemases. Increasingly high rates of Klebsiella pneumoniae carbapenemase (KPC) have established ceftazidime-avibactam (CZA) as an essential antimicrobial for the treatment of infections due to MDR Gram-negative pathogens. Although other countries in the region have reported the emergence of CZA-resistant KPC variants, this is the first description of such enzymes in Colombia. This finding warrants active surveillance, as dissemination of these variants could have devastating public health consequences.

Detection of KPC-216, a Novel KPC-3 Variant, in a Clinical Isolate of Klebsiella pneumoniae ST101 Co-Resistant to Ceftazidime-Avibactam and Cefiderocol

BACKGROUND

Carbapenemase-producing Klebsiella pneumoniae (CP-KP) represents a global threat to public health, with limited antimicrobial therapeutic options. In this study, we analyzed a ceftazidime/avibactam (CAZ-AVI)-resistant K. pneumoniae isolate obtained from a patient previously exposed to CAZ-AVI expressing a novel K. pneumoniae carbapenemase (KPC)-3 variant.

METHODS

Antimicrobial susceptibility testing was performed using reference broth microdilution. Whole-genome sequencing (WGS) was performed using Illumina and Nanopore Technologies. Short- and long-reads were combined with Unicycler. Assemblies were investigated for multilocus sequence typing (MLST), antimicrobial resistance genes, porins, and plasmids.

RESULTS

The K. pneumoniae isolate (KP_RM_1) was resistant to CAZ-AVI, expanded-spectrum cephalosporins, amikacin, ertapenem, and cefiderocol (FDC) but was susceptible to tigecycline, colistin, trimethoprim/sulfamethoxazole, meropenem-vaborbactam, and imipenem-relebactam. WGS revealed that the KP_RM_1 genome is composed of a single chromosome of 5 Mbp and five circular plasmids. Further analysis showed the presence of novel blaKPC-216 located on a 72 kb plasmid. KPC-216 differs from KPC-3 by a Lysin (K) insertion at position 168 (+K168).

CONCLUSIONS

We report the identification of a new KPC-3 variant associated with CAZ-AVI resistance. The KPC variants associated with CAZ-AVI resistance should be determined to promptly inform clinicians and start the appropriate antimicrobial therapy.

Comparative In Vitro Activity of Ceftazidime-Avibactam, Imipenem-Relebactam, and Meropenem-Vaborbactam against Carbapenem-Resistant Clinical Isolates of Klebsiella pneumoniae and Pseudomonas aeruginosa

Co-infection with carbapenem-resistant Klebsiella pneumoniae (CRKP) and Pseudomonas aeruginosa (CRPA) is associated with poor outcomes and historically relied on combination therapy with toxic agents for management. However, several novel β-lactam/β-lactamase inhibitor combination agents have been developed, offering potential monotherapy options. Here, we compare the in vitro activity of ceftazidime-avibactam (CZA), imipenem-relebactam (IRL), and meropenem-vaborbactam (MVB) against both CRKP and CRPA clinical isolates. Minimum inhibitory concentrations (MICs) for each agent were determined using broth microdilution. Carbapenemase gene detection was performed for representative isolates of varying carbapenem resistance phenotypes. IRL demonstrated excellent activity against CRKP and CRPA with susceptibility rates at 95.8% and 91.7%, respectively. While CZA and MVB showed comparable susceptibility to IRL against CRKP (93.8%), susceptibility of CRPA to CZA was modest at 79.2%, whereas most CRPA strains were resistant to MVB. Of the 35 CRKP isolates tested, 91.4% (32/35) carried a blaKPC gene. Only 1 of 37 (2.7%) CRPA isolates tested carried a blaVIM gene, which conferred phenotypic resistance to all three agents. None of the CRKP strains were cross-resistant to all three agents. Source of infection and co-infection did not significantly influence antimicrobial activity for IRL and CZA; none of the CRPA isolates from co-infected patients were susceptible to MVB. Our results suggest that novel β-lactam agents with antipseudomonal activity and stability against carbapenemases, such as IRL and CZA, offer potential monotherapy options for the treatment of co-infection involving both CRKP and CRPA, but not MVB.

Activity of cefiderocol and innovative β-lactam/β-lactamase inhibitor combinations against isogenic strains of Escherichia coli expressing single and double β-lactamases under high and low permeability conditions

OBJECTIVES

To analyse the impact of the most clinically relevant β-lactamases and their interplay with low outer membrane permeability on the activity of cefiderocol, ceftazidime/avibactam, aztreonam/avibactam, cefepime/enmetazobactam, cefepime/taniborbactam, cefepime/zidebactam, imipenem/relebactam, meropenem/vaborbactam, meropenem/xeruborbactam and meropenem/nacubactam against recombinant Escherichia coli strains.

METHODS

We constructed 82 E. coli laboratory transformants expressing the main β-lactamases circulating in Enterobacterales (70 expressing single β-lactamase and 12 producing double carbapenemase) under high (E. coli TG1) and low (E. coli HB4) permeability conditions. Antimicrobial susceptibility testing was determined by reference broth microdilution.

RESULTS

Aztreonam/avibactam, cefepime/zidebactam, cefiderocol, meropenem/xeruborbactam and meropenem/nacubactam were active against all E. coli TG1 transformants. Imipenem/relebactam, meropenem/vaborbactam, cefepime/taniborbactam and cefepime/enmetazobactam were also highly active, but unstable against most of MBL-producing transformants. Combination of β-lactamases with porin deficiency (E. coli HB4) did not significantly affect the activity of aztreonam/avibactam, cefepime/zidebactam, cefiderocol or meropenem/nacubactam, but limited the effectiveness of the rest of carbapenem- and cefepime-based combinations. Double-carbapenemase production resulted in the loss of activity of most of the compounds tested, an effect particularly evident for those E. coli HB4 transformants in which MBLs were present.

CONCLUSIONS

Our findings highlight the promising activity that cefiderocol and new β-lactam/β-lactamase inhibitors have against recombinant E. coli strains expressing widespread β-lactamases, including when these are combined with low permeability or other enzymes. Aztreonam/avibactam, cefiderocol, cefepime/zidebactam and meropenem/nacubactam will help to mitigate to some extent the urgency of new compounds able to resist MBL action, although NDM enzymes represent a growing challenge against which drug development efforts are still needed.

Antimicrobial and Diagnostic Stewardship of the Novel β-Lactam/β-Lactamase Inhibitors for Infections Due to Carbapenem-Resistant Enterobacterales Species and Pseudomonas aeruginosa

Carbapenem-resistant Gram-negative bacterial infections are a major public health threat due to the limited therapeutic options available. The introduction of the new β-lactam/β-lactamase inhibitors (BL/BLIs) has, however, altered the treatment options for such pathogens. Thus, four new BL/BLI combinations-namely, ceftazidime/avibactam, meropenem/vaborbactam, imipenem/relebactam, and ceftolozane/tazobactam-have been approved for infections attributed to carbapenem-resistant Enterobacterales species and Pseudomonas aeruginosa. Nevertheless, although these antimicrobials are increasingly being used in place of other drugs such as polymyxins, their optimal clinical use is still challenging. Furthermore, there is evidence that resistance to these agents might be increasing, so urgent measures should be taken to ensure their continued effectiveness. Therefore, clinical laboratories play an important role in the judicious use of these new antimicrobial combinations by detecting and characterizing carbapenem resistance, resolving the presence and type of carbapenemase production, and accurately determining the minimum inhibitor concentrations (MICs) for BL/BLIs. These three targets must be met to ensure optimal BL/BLIs use and prevent unnecessary exposure that could lead to the development of resistance. At the same time, laboratories must ensure that results are interpreted in a timely manner to avoid delays in appropriate treatment that might be detrimental to patient safety. Thus, we herein present an overview of the indications and current applications of the new antimicrobial combinations and explore the diagnostic limitations regarding both carbapenem resistance detection and the interpretation of MIC results. Moreover, we suggest the use of alternative narrower-spectrum antibiotics based on susceptibility testing and present data regarding the effect of synergies between BL/BLIs and other antimicrobials. Finally, in order to address the absence of a standardized approach to using the novel BL/BLIs, we propose a diagnostic and therapeutic algorithm, which can be modified based on local epidemiological criteria. This framework could also be expanded to incorporate other new antimicrobials, such as cefiderocol, or currently unavailable BL/BLIs such as aztreonam/avibactam and cefepime/taniborbactam.

Activity of imipenem/relebactam and comparators against KPC-producing Klebsiella pneumoniae and imipenem-resistant Pseudomonas aeruginosa

PURPOSE

Relebactam is a novel β-lactamase inhibitor, which, when combined with imipenem/cilastatin, is active against both class A and class C β-lactamases. To evaluate in vitro antimicrobial activity of imipenem/relebactam against a collection of recent clinical isolates of carbapenem-non-susceptible P. aeruginosa and K. pneumoniae ST258 and ST512 KPC producers belonging to different lineages from hospitals in Southern Spain.

METHODS

Six hundred and seventy-eight isolates were tested: 265 K. pneumoniae (230 ST512/KPC-3 and 35 ST258/KPC-3) and 413 carbapenem-non-susceptible P. aeruginosa. Imipenem, piperacillin/tazobactam, ceftazidime, cefepime, aztreonam, ceftolozane/tazobactam, meropenem, amikacin, ciprofloxacin, colistin, and ceftazidime/avibactam were used as comparators against P. aeruginosa. Against K. pneumoniae ceftazidime, cefepime, aztreonam, and ceftolozane/tazobactam were not tested, and tigecycline was studied instead. MICs were determined in duplicate by broth microdilution according to EUCAST guidelines.

RESULTS

Imipenem/relebactam displayed potent in vitro activity against both sequence types of KPC-3-producing K. pneumoniae. MIC50 and MIC90 values were 0.25 mg/L and 1 mg/L, respectively, with percent of susceptible isolates >97%. Only three K. pneumoniae ST512/KPC-3 isolates and one ST258/KPC-3 were resistant to imipenem/relebactam. Relebactam sensitized 98.5% of K. pneumoniae isolates resistant to imipenem. The activity of imipenem/relebactam against P. aeruginosa was moderate (susceptibility rate: 62.7%). Analysis of the acquired and mutational resistome of isolates with high levels of resistance to imipenem/relebactam has not shown a clear association between them.

CONCLUSION

Imipenem/relebactam showed excellent activity against K. pneumoniae KPC-3. The activity of imipenem/relebactam against imipenem-resistant P. aeruginosa was moderate.

Rapid Detection of Ceftazidime/Avibactam Susceptibility/Resistance in Enterobacterales by Rapid CAZ/AVI NP Test

We developed a novel culture-based test, the Rapid CAZ/AVI NP test, for rapid identification of ceftazidime/avibactam susceptibility/resistance in Enterobacterales. This test is based on glucose metabolization upon bacterial growth in the presence of a defined concentration of ceftazidime/avibactam (128/53 μg/mL). Bacterial growth is visually detectable by a red to yellow color change of red phenol, a pH indicator. A total of 101 well characterized enterobacterial isolates were used to evaluate the test performance. This test showed positive percent agreement of 100% and negative percent agreement of 98.5% with overall percent agreement of 99%, by comparison with the MIC gradient strip test (Etest) taken as the reference standard method. The Rapid CAZ/AVI NP test had only 1.5% major errors and 0% extremely major errors. This test is rapid (result within 2 hours 45 minutes), reliable, affordable, easily interpretable, and easy to implement in clinical microbiology laboratories without requiring any specific equipment.

Reviewing novel treatment options for carbapenem-resistant Enterobacterales

INTRODUCTION

Carbapenem resistant Enterobacterales (CRE) are a major threat to global health and hospital-onset CRE infections have risen during the COVID-19 pandemic. Novel antimicrobials are now available for the treatment of CRE infections. There remains an urgent need for new antimicrobials for CRE, especially for those producing metallo-β-lactamases.

AREAS COVERED

This article discusses previously published research supporting currently available novel antimicrobials for the treatment of CRE infections. Newer compounds currently being evaluated in clinical trials are covered. A literature search was conducted in PubMed over all available dates for relevant published papers and conference abstracts with the search terms, 'CRE,' 'carbapenem-resistant Enterobacterales,' 'β-lactam-β-lactamase inhibitor,' 'KPC,' 'NDM,' 'metallo-β-lactamase,' 'ceftazidime-avibactam,' 'meropenem-vaborbactam,' 'imipenem-cilastatin-relebactam,' 'cefiderocol,' 'eravacycline,' 'plazomicin,' 'taniborbactam,' 'zidebactam,' and 'nacubactam.'

EXPERT OPINION

Novel antimicrobials for CRE infections have been developed, most notably the β-lactam-β-lactamase inhibitor combinations, though treatment options for infections with metallo-β-lactamase producing Enterobacterales remain few and have limitations. Development of antibiotics with activity against metallo-β-lactamase producing Enterobacterales is eagerly awaited, and there are promising new compounds in clinical trials. Finally, more clinical research is needed to optimize and individualize treatment approaches, which will help guide antimicrobial stewardship initiatives aimed at reducing the spread of CRE and development of further resistance.

In vitro development of imipenem/relebactam resistance in KPC-producing Klebsiella pneumoniae involves multiple mutations including OmpK36 disruption and KPC modification

OBJECTIVES

In order to inform and anticipate potential strategies aimed at combating KPC-producing Klebsiella pneumoniae infections, we analysed imipenem/relebactam and ceftazidime/avibactam single-step mutant frequencies, resistance development trajectories, differentially selected resistance mechanisms and their associated fitness cost using four representative high-risk K. pneumoniae clones.

METHODS

Mutant frequencies and mutant preventive concentrations were determined using agar plates containing incremental concentrations of β-lactam/β-lactamase inhibitor. Resistance dynamics were determined through incubation for 7 days in 10 mL MH tubes containing incremental concentrations of each antibiotic combination up to their 64 × baseline MIC. Two colonies per strain from each experiment were characterized by antimicrobial susceptibility testing, whole genome sequencing and competitive growth assays (to determine in vitro fitness). KPC variants associated with imipenem/relebactam resistance were characterized by cloning and biochemical experiments, atomic models and molecular dynamics simulation studies.

RESULTS

Imipenem/relebactam prevented the emergence of single-step resistance mutants at lower concentrations than ceftazidime/avibactam. In three of the four strains evaluated, imipenem/relebactam resistance development emerged more rapidly, and in the ST512/KPC-3 clone reached higher levels compared to baseline MICs than for ceftazidime/avibactam. Lineages evolved in the presence of ceftazidime/avibactam showed KPC substitutions associated with high-level ceftazidime/avibactam resistance, increased imipenem/relebactam susceptibility and low fitness costs. Lineages that evolved in the presence of imipenem/relebactam showed OmpK36 disruption, KPC modifications (S106L, N132S, L167R) and strain-specific substitutions associated with imipenem/relebactam resistance and high fitness costs. Imipenem/relebactam-selected KPC derivatives demonstrated enhanced relebactam resistance through important changes affecting relebactam recognition and positioning.

CONCLUSIONS

Our findings anticipate potential resistance mechanisms affecting imipenem/relebactam during treatment of KPC-producing K. pneumoniae infections.

In vitro synergy screens of FDA-approved drugs reveal novel zidovudine- and azithromycin-based combinations with last-line antibiotics against Klebsiella pneumoniae

Treatment of infections caused by multi-drug resistant (MDR) enterobacteria remains challenging due to the limited therapeutic options available. Drug repurposing could accelerate the development of new urgently needed successful interventions. This work aimed to identify and characterise novel drug combinations against Klebsiella pneumoniae based on the concepts of synergy and drug repurposing. We first performed a semi-qualitative high-throughput synergy screen (sHTSS) with tigecycline, colistin and fosfomycin (last-line antibiotics against MDR Enterobacteriaceae) against a FDA-library containing 1430 clinically approved drugs; a total of 109 compounds potentiated any of the last-line antibiotics. Selected hits were further validated by secondary checkerboard (CBA) and time-kill (TKA) assays, obtaining 15.09% and 65.85% confirmation rates, respectively. Accordingly, TKA were used for synergy classification based on determination of bactericidal activities at 8, 24 and 48 h, selecting 27 combinations against K. pneumoniae. Among them, zidovudine or azithromycin combinations with last-line antibiotics were further evaluated by TKA against a panel of 12 MDR/XDR K. pneumoniae strains, and their activities confronted with those clinical combinations currently used for MDR enterobacteria treatment; these combinations showed better bactericidal activities than usual treatments without added cytotoxicity. Our studies show that sHTSS paired to TKA are powerful tools for the identification and characterisation of novel synergistic drug combinations against K. pneumoniae. Further pre-clinical studies might support the translational potential of zidovudine- and azithromycin-based combinations for the treatment of these infections.

Clinical efficacy and drug resistance of ceftazidime-avibactam in the treatment of Carbapenem-resistant gram-negative bacilli infection

OBJECTIVE

To examine the clinical efficacy, safety, and resistance of Ceftazidime-Avibactam (CAZ-AVI) in patients with Carbapenem-resistant Gram-negative bacilli (CR-GNB) infections.

METHODS

We retrospectively analyzed relevant data of CR-GNB infected patients receiving CAZ-AVI treatment, analyzed relevant factors affecting drug efficacy, and compared the efficacy and safety with patients receiving Polymyxin B treatment.

RESULTS

A total of 139 patients were included. Agranulocytosis, septic shock, SOFA score, and CAZ-AVI treatment course were independent risk factors affecting the prognosis of patients with CR-GNB infection treated with CAZ-AVI while prolonging the treatment course of CAZ-AVI was the only protective factor for bacterial clearance. The fundamental indicators showed no statistically significant differences between CAZ-AVI and Polymyxin B treatment groups. At the same time, the proportion of patients treated with monotherapy was significantly higher in the CAZ-AVI group than in the Polymyxin B group (37.2% vs. 8.9%, p < 0.05), the 30-day mortality rate of the CAZ-AVI treatment group (27.7% vs. 46.7%, p = 0.027) was lower than that of the Polymyxin B treatment group. The 30-day clinical cure rate (59.6% vs. 40% p = 0.030) and 14-day microbiological clearance rate (42.6% vs. 24.4%, p = 0.038) were significantly higher in the CAZ-AVI than in the Polymyxin B treatment group. Eighty nine patients were monitored for CAZ-AVI resistance, and the total resistance rate was 14.6% (13/89). The resistance rates of Carbapenem-resistant Klebsiella pneumoniae (CRKP) and Carbapenem-resistant Pseudomonas aeruginosa (CRPA) to CAZ-AVI were 13.5 and 15.4%, respectively.

CONCLUSION

CAZ-AVI has shown high clinical efficacy and bacterial clearance in treating CR-GNB infections. Compared with Polymyxin B, CAZ-AVI significantly improved the outcome of mechanical ventilation in patients with septic shock, agranulocytosis, Intensive Care Unit (ICU) patients, bloodstream infection, and patients with SOFA score > 6, and had a lower incidence of adverse events. We monitored the emergence of CAZ-AVI resistance and should strengthen the monitoring of drug susceptibility in clinical practice and the rational selection of antibiotic regimens to delay the onset of resistance.

Neonatal Bloodstream Infection with Ceftazidime-Avibactam-Resistant blaKPC-2-Producing Klebsiella pneumoniae Carrying blaVEB-25

BACKGROUND

Although ceftazidime/avibactam (CAZ/AVI) has become an important option for treating adults and children, no data or recommendations exist for neonates. We report a neonatal sepsis case due to CAZ/AVI-resistant blaKPC-2-harboring Klebsiella pneumoniae carrying blaVEB-25 and the use of a customized active surveillance program in conjunction with enhanced infection control measures.

METHODS

The index case was an extremely premature neonate hospitalized for 110 days that had been previously treated with multiple antibiotics. Customized molecular surveillance was implemented at hospital level and enhanced infection control measures were taken for early recognition and prevention of outbreak. Detection and identification of blaVEB-25 was performed using next-generation sequencing.

RESULTS

This was the first case of a bloodstream infection caused by KPC-producing K. pneumoniae that was resistant to CAZ/AVI without the presence of a metalo-β-lactamase in the multiplex PCR platform in a neonate. All 36 additional patients tested (12 in the same NICU and 24 from other hospital departments) carried wild-type blaVEB-1 but they did not harbor blaVEB-25.

CONCLUSION

The emergence of blaVEB-25 is signal for the horizontal transfer of plasmids at hospital facilities and it is of greatest concern for maintaining a sharp vigilance for the surveillance of novel resistance mechanisms. Molecular diagnostics can guide appropriate antimicrobial therapy and the early implementation of infection control measures against antimicrobial resistance.

Clinical Characteristics and Outcome of Ceftazidime/Avibactam-Resistant Klebsiella pneumoniae Carbapenemase-Producing Klebsiella pneumoniae Infections: A Retrospective, Observational, 2-Center Clinical Study

Background

Recently, Klebsiella pneumoniae carbapenemase (KPC)-producing Klebsiella pneumoniae (KPC-Kp) with resistance to ceftazidime/avibactam (CZA-R) has been described, including KPC variants that restore carbapenem susceptibility. The aim of the study was to analyze the clinical characteristics and outcomes of infections caused by CZA-R KPC-Kp.

Methods

From 2019 to 2021, a retrospective 2-center study including patients with infections due to CZA-R KPC-Kp hospitalized at 2 academic hospitals in Rome was conducted. Demographic and clinical characteristics were collected. Principal outcome was 30-day all-cause mortality. Statistical analyses were performed with Stata-IC17 software.

Results

Overall, 59 patients were included (mean age, 64.4 ± 14.6 years; mean Charlson comorbidity index score, 4.5 ± 2.7). Thirty-four patients (57.6%) had infections caused by CZA-R and meropenem (MEM)-susceptible strains. A previous CZA therapy was observed in 40 patients (67.8%), mostly in patients with MEM-susceptible KPC variant (79.4% vs 52%, P = .026). Primary bacteremia was observed in 28.8%, followed by urinary tract infections and pneumonia. At infection onset, septic shock was present in 15 subjects (25.4%). After adjustment for confounders, only the presence of septic shock was independently associated with mortality (P = .006).

Conclusions

Infections due to CZA-R KPC-Kp often occur in patients who had previously received CZA, especially in the presence of strains susceptible to MEM. Nevertheless, one-third of patients had never received CZA before KPC-Kp CZA-R. Since the major driver for mortality was infection severity, understanding the optimal therapy in patients with KPC-Kp CZA-R infections is of crucial importance.

Assessing the in vivo efficacy of rational antibiotics and combinations against difficult-to-treat Pseudomonas aeruginosa producing GES β-lactamases

OBJECTIVES

We evaluated the in vivo efficacy of human-simulated regimens (HSRs) of cefiderocol, ceftazidime/avibactam, meropenem and ceftazidime/avibactam/meropenem combination against Guiana-extended spectrum (GES)-producing Pseudomonas aeruginosa isolates.

METHODS

Eighteen P. aeruginosa isolates producing GES-1 (n = 5), GES-5 (n = 5) or miscellaneous GESs (combinations of GES-19, GES-20 and/or GES-26; n = 8) were evaluated. In vitro MIC testing was determined using broth microdilution. In a validated murine thigh infection model, HSRs of cefiderocol 2 g q8h as a 3 h IV infusion, ceftazidime/avibactam 2.5 g q8h as a 2 h IV infusion, meropenem 2 g q8h as a 3 h IV infusion or ceftazidime/avibactam/meropenem were administered. Change in bacterial burden relative to baseline and the proportion of isolates in each genotypic group meeting 1-log10 kill endpoint were assessed.

RESULTS

Modal MICs (mg/L) ranged from 0.125 to 1 for cefiderocol, 4 to >64 for ceftazidime/avibactam and 2 to >64 for meropenem. Cefiderocol produced >1-log10 of kill against all 18 tested isolates. Meropenem was active against all GES-1 isolates whereas activity against GES-5 and miscellaneous GESs was lacking, consistent with the MICs. Ceftazidime/avibactam was active against all GES-1 and GES-5 isolates and retained activity against 62.5% of miscellaneous GESs including isolates with elevated MICs. For isolates where ceftazidime/avibactam failed, the addition of meropenem restored the in vivo efficacy.

CONCLUSIONS

As monotherapy, cefiderocol was active in vivo against all tested isolates. The activities of meropenem or ceftazidime/avibactam alone were variable; however, a combination of both was active against all isolates. Cefiderocol and ceftazidime/avibactam/meropenem could be valuable therapeutic options for GES-producing P. aeruginosa infections. Clinical confirmatory data are warranted.

Molecular characterization of cefepime and aztreonam nonsusceptibility in Haemophilus influenzae

BACKGROUND

Cefepime and aztreonam are highly efficacious against H. influenzae, and resistant strains are rare. In this study, we isolated cefepime- and aztreonam-nonsusceptible H. influenzae strains and addressed the molecular basis of their resistance to cefepime and aztreonam.

METHODS

Two hundred and 28 specimens containing H. influenzae were screened, of which 32 isolates were enrolled and applied to antimicrobial susceptibility testing and whole-genome sequencing. Genetic variations that were detected in all nonsusceptible isolates with statistical significance by Fisher's exact tests were identified as cefepime or aztreonam nonsusceptibility related. Functional complementation assays were conducted to assess the in vitro effects of proteins with sequence substitutions on drug susceptibility.

RESULTS

Three H. influenzae isolates were nonsusceptible to cefepime, one of which was also nonsusceptible to aztreonam. Genes encoding TEM, SHV and CTX-M extended-spectrum β-lactamases were not detected in the cefepime- and aztreonam-nonsusceptible isolates. Five genetic variations in four genes and 10 genetic variations in five genes were associated with cefepime and aztreonam nonsusceptibility, respectively. Phylogenetic analyses revealed that changes in FtsI were correlated strongly with the MIC of cefepime and moderately with aztreonam. FtsI Thr532Ser-Tyr557His cosubstitution linked to cefepime nonsusceptibility and Asn305Lys-Ser385Asn-Glu416Asp cosubstitution to aztreonam nonsusceptibility. Functional complementation assays revealed that these cosubstitutions increased MICs of cefepime and aztreonam in susceptible H. influenzae isolates, respectively.

CONCLUSIONS

Genetic variations relevant to resistant phenotypes of cefepime and aztreonam nonsusceptibility in H. influenzae were identified. Moreover, the effects of FtsI cosubstitutions on increasing MICs of cefepime and aztreonam in H. influenzae were demonstrated.

The Effectiveness of Imipenem-Relebactam against Ceftazidime-Avibactam Resistant Variants of the KPC-2 β-Lactamase

BACKGROUND

Ceftazidime-avibactam was approved by the FDA to treat infections caused by Enterobacterales carrying bla_KPC-2. However, variants of KPC-2 with amino acid substitutions at position 179 have emerged and confer resistance to ceftazidime-avibactam.

METHODS

The activity of imipenem-relebactam was assessed against a panel of 19 KPC-2 D179 variants. KPC-2 and the D179N and D179Y variants were purified for biochemical analyses. Molecular models were constructed with imipenem to assess differences in kinetic profiles.

RESULTS

All strains were susceptible to imipenem-relebactam, but resistant to ceftazidime (19/19) and ceftazidime-avibactam (18/19). KPC-2 and the D179N variant hydrolyzed imipenem, but the D179N variant's rate was much slower. The D179Y variant was unable to turnover imipenem. All three β-lactamases hydrolyzed ceftazidime at varying rates. The acylation rate of relebactam for the D179N variant was ~2.5× lower than KPC-2. Poor catalytic turnover by the D179Y variant precluded the determination of inhibitory kinetic parameters. Acyl-complexes with imipenem and ceftazidime were less prevalent with the D179N variant compared to the D179Y variant, supporting the kinetic observations that the D179Y variant was not as active as the D179N variant. Relebactam was slower to form an acyl-complex with the D179Y variant compared to avibactam. The D179Y model with imipenem revealed that the catalytic water molecule was shifted, and the carbonyl of imipenem was not within the oxyanion hole. Conversely in the D179N model, imipenem was oriented favorably for deacylation.

CONCLUSIONS

Imipenem-relebactam overcame the resistance of the D179 variants, suggesting that this combination will be active against clinical isolates harboring these derivatives of KPC-2.

In vivo adaptive antimicrobial resistance in Klebsiella pneumoniae during antibiotic therapy

Klebsiella pneumoniae is one of the leading pathogens contributing to antimicrobial resistance. The emergence of carbapenem-resistant K. pneumoniae (CRKP) has put the use of clinical antimicrobial agents in a dilemma. In particular, CRKP exhibiting resistance to ceftazidime/avibactam, tigecycline and colistin have raised great clinical concern, as these are the last-resort antibiotics for the treatment of CRKP infections. Within-host evolution is a survival strategy closely related to the emergence of antimicrobial resistance, while little attention has been paid to the in vivo genetic process of conversion from antibiotic-susceptible to resistant K. pneumoniae. Here we have a literature review regarding the in vivo evolution of resistance to carbapenems, ceftazidime/avibactam, tigecycline, and colistin in K. pneumoniae during antibacterial therapy, and summarized the detailed resistance mechanisms. In general, acquiring bla KPC and bla NDM harboring-plasmid, specific mutations in bla KPC, and porin genes, such as ompK35 and ompK36, upregulation of bla KPC, contribute to the development of carbapenem and ceftazidime/avibactam resistance in vivo. Overexpression of efflux pumps, acquiring plasmid-carrying tet (A) variants, and ribosomal protein change can lead to the adaptive evolution of tigecycline resistance. Specific mutations in chromosomes result in the cationic substitution of the phosphate groups of lipid A, thus contributing to colistin resistance. The resistant plasmid might be acquired from the co-infecting or co-colonizing strains, and the internal environment and antibiotic selection pressure contribute to the emergence of resistant mutants. The internal environment within the human host could serve as an important source of resistant K. pneumoniae strains.

The primary pharmacology of ceftazidime/avibactam: resistance in vitro

This article reviews resistance to ceftazidime/avibactam as an aspect of its primary pharmacology, linked thematically with recent reviews of the basic in vitro and in vivo translational biology of the combination (J Antimicrob Chemother 2022; 77: 2321-40 and 2341-52). In Enterobacterales or Pseudomonas aeruginosa, single-step exposures to 8×  MIC of ceftazidime/avibactam yielded frequencies of resistance from <∼0.5 × 10-9 to 2-8 × 10-9, depending on the host strain and the β-lactamase harboured. β-Lactamase structural gene mutations mostly affected the avibactam binding site through changes in the Ω-loop: e.g. Asp179Tyr (D179Y) in KPC-2. Other mutations included ones proposed to reduce the permeability to ceftazidime and/or avibactam through changes in outer membrane structure, up-regulated efflux, or both. The existence, or otherwise, of cross-resistance between ceftazidime/avibactam and other antibacterial agents was also reviewed as a key element of the preclinical primary pharmacology of the new agent. Cross-resistance between ceftazidime/avibactam and other β-lactam-based antibacterial agents was caused by MBLs. Mechanism-based cross-resistance was not observed between ceftazidime/avibactam and fluoroquinolones, aminoglycosides or colistin. A low level of general co-resistance to ceftazidime/avibactam was observed in MDR Enterobacterales and P. aeruginosa. For example, among 2821 MDR Klebsiella spp., 3.4% were resistant to ceftazidime/avibactam, in contrast to 0.07% of 8177 non-MDR isolates. Much of this was caused by possession of MBLs. Among 1151 MDR, XDR and pandrug-resistant isolates of P. aeruginosa from the USA, 11.1% were resistant to ceftazidime/avibactam, in contrast to 3.0% of 7452 unselected isolates. In this case, the decreased proportion susceptible was not due to MBLs.

Comparison of the NG-Test Carba 5, Colloidal Gold Immunoassay (CGI) Test, and Xpert Carba-R for the Rapid Detection of Carbapenemases in Carbapenemase-Producing Organisms

Carbapenem-resistant Enterobacterales (CRE) are increasingly recognized as an urgent public health concern. The rapid and accurate identification of carbapenemases could provide insights into antimicrobial therapy and infection control. In this study, we evaluated the efficacy of three different methods, including the NG-test Carba 5, colloidal gold immunoassay (CGI) test, and Xpert Carba-R assay, for the rapid detection of five carbapenemases (KPC, NDM, IMP, OXA-48, and VIM). A total of 207 Gram-negative strains collected from patients and hospital sewages were tested. The presence or absence of carbapenemase genes in the whole-genome sequences was used as the gold standard for evaluating the accuracy of the above-mentioned three methods. Among the 192 strains carrying only one carbapenemase gene, the accuracies of the NG-Test Carba 5, CGI test, and Xpert Carba-R were 96.88% (95% CI, 93.01-98.72%), 96.88% (95% CI, 93.01-98.72%), and 97.92% (95% CI, 94.41-99.33%), respectively. Xpert Carba-R was able to detect all 13 types of KPC variants, including KPC-2, KPC-3, KPC-25, KPC-33, KPC-35, KPC-51, KPC-52, KPC-71, KPC-76, KPC-77, KPC-78, KPC-93, and KPC-123, with a detection sensitivity of 100.00% (95% CI, 96.50-100.00%), a specificity of 100.00% (95% CI, 92.38-100.00%), and a κ index of 1.00. For IMP, Carba 5 was superior to the other two methods, with a sensitivity of 100% (95% CI, 71.66-100.00%), a specificity of 100% (95% CI, 97.38-100.00%), and a κ index of 1.00. For the remaining 15 strains carrying two or three kinds of carbapenemase genes, Carba 5 performed the best, which accurately identified all the target genes, followed by Xpert Carba-R (12/15, 80.00%) and the CGI test (10/15, 66.67%). Therefore, all three assays demonstrated reliable performances in carbapenemase detection, and Xpert Carba-R should be recommended for the detection of KPC variants, especially for patients at a high risk of infections caused by ceftazidime/avibactam-resistant strains. IMPORTANCE: CRE was listed as one of the top three pathogens that are in critical need of new antibiotics by the WHO. The rapid and accurate identification of carbapenemases is important for antimicrobial therapy and infection control. In recent years, new beta-lactam/beta-lactamase inhibitor combinations such as ceftazidime/avibactam (CZA) have been approved by the Food and Drug Administration (FDA) to cope with CRE challenges. CZA was effective against class A, class C, and some class D enzymes such as OXA-48-like. However, CZA-resistant KPC variants emerged at an alarming speed, which posed a new challenge for the accurate identification of KPC variants. In this study, we evaluated the performance of two lateral flow immunochromatographic assays, namely, NG-test Carba 5 and the CGI test, and the automated real-time quantitative PCR Xpert Carba-R in the rapid detection of carbapenemases. Notably, 13 types of KPC variants were enrolled in this study, which covered most KPC variants discovered in China. Carba-R was superior to NG-teat Carba 5 and the CGI test; it was able to detect all of the included KPC variants, including KPC-2, KPC-3, KPC-25, KPC-33, KPC-35, KPC-51, KPC-52, KPC-71, KPC-76, KPC-77, KPC-78, KPC-93, and KPC-123.

Antimicrobial susceptibility study and molecular epidemiology of ceftazidime/avibactam against Pseudomonas aeruginosa collected from clinical patients in PR China (2004-2021)

Introduction. The increasing prevalence of multidrug-resistant (MDR) Pseudomonas aeruginosa worldwide is a significant global public health concern. Ceftazidime/avibactam (CZA) has been considered a novel promising β-lactam/β-lactamase inhibitor combination antibiotic against difficult-to-treat P. aeruginosa isolates. Big data studies on CZA susceptibility against P. aeruginosa have been limited.Gap statement. Production of metallo-β-lactamases was the most prevalent resistance mechanism for P. aeruginosa against CZA.Aim. To assess the in vitro activity of CZA against P. aeruginosa strains and the relevant resistance mechanisms.Methodology. One thousand three hundred and sixty-three P. aeruginosa isolates were collected from 2004 to 2021. Antimicrobial susceptibility testing was carried out for commonly used antipseudomonal drugs via the broth microdilution method. Polymerase chain reaction (PCR) or whole-genome sequencing were performed to analyse the most common carbapenemase genes. Molecular epidemiology was analysed by uploading the sequencing data to the Center for Genomic Epidemiology website.Results. Antimicrobial susceptibility testing showed that CZA and lipopeptides are the most active antibiotics against P. aeruginosa isolates. PCR and genome sequencing revealed that the most prevalent resistance mechanism for P. aeruginosa against CZA was the production of metallo-β-lactamases. None of the bla _PDC mutations were found to be associated with avibactam resistance.Conclusion. Our findings revealed that CZA and lipopeptides are the most active antibiotics against P. aeruginosa isolates. The most prevalent resistance mechanism for P. aeruginosa against CZA was the production of metallo-β-lactamases, and none of the bla _PDC mutations were found to be associated with avibactam resistance.

Genomic Characterization of KPC-31 and OXA-181 Klebsiella pneumoniae Resistant to New Generation of β-Lactam/β-Lactamase Inhibitor Combinations

BACKGROUND

Carbapenem resistant Klebsiella pneumoniae (cr-Kp) causes serious infections associated with a high mortality rate. The clinical efficacy of ceftazidime/avibactam (CZA), meropenem/vaborbactam (M/V), and imipenem/relebactam (I/R) against cr-Kp is challenged by the emergence of resistant strains, making the investigation and monitoring of the main resistance mechanisms crucial. In this study, we reported the genome characterization of a Klebsiella pneumoniae strain isolated from a critically ill patient and characterized by a multidrug resistant (MDR) profile, including resistance to CZA, M/V, and I/R.

METHODS

An antimicrobial susceptibility test (AST) was performed by an automated system and E-test and results were interpreted following the EUCAST guidelines. Genomic DNA was extracted using a genomic DNA extraction kit and it was sequenced using the Illumina Nova Seq 6000 platform. Final assembly was manually curated and carefully verified for detection of antimicrobial resistance genes, porins modifications, and virulence factors.

RESULTS

The K. pneumoniae isolate belonged to sequence type ST512 and harbored 23 resistance genes, conferring resistance to all antibiotic classes, including blaKPC-31 and blaOXA-181, leading to carbapenems resistance. The truncation of OmpK35 and mutation OmpK36GD were also observed.

CONCLUSIONS

The genomic characterization demonstrated the high resistant profile of new cr-Kp coharboring class A and D carbapenemases. The presence of KPC-31, as well as the detection of OXA-181 and porin modifications, further limit the therapeutic options, including the novel combinations of β-lactam/β-lactamase inhibitor antibiotics in patients with severe pneumonia caused by cr-Kp.

Outbreak of Klebsiella pneumoniae ST11 Resistant To Ceftazidime-Avibactam Producing KPC-31 and the Novel Variant KPC-115 during COVID-19 Pandemic in Argentina

We describe an outbreak of Klebsiella pneumoniae sequence type 11 (ST11) producing KPC variants resistant to ceftazidime-avibactam. Six patients hospitalized in the intensive care unit (mostly due to critical COVID pneumonia) presented infection or colonization by this bacterium. They had several comorbidities and required mechanical ventilation, central venous catheters, and urinary catheters. All 6 patients had a history of fecal colonization with KPC-producing Enterobacterales (KPC-E). Three of them had previous episodes of infection with ceftazidime-avibactam-susceptible KPC-producing K. pneumoniae, which were treated with ceftazidime-avibactam. Several phenotypic methods failed to detect carbapenemase production in these 6 ceftazidime-avibactam-resistant isolates, and they showed in vitro susceptibility to imipenem and meropenem. All of them rendered positive results for blaKPC by PCR, and amplicon sequencing identified blaKPC-31 variant in 5 isolates and a novel variant, named blaKPC-115, in the other. Moreover, matrix-assisted laser desorption ionization-time of flight mass spectrometry was able to detect KPC in all isolates. Ceftazidime-avibactam-resistant isolates, as well as those recovered from previous infection episodes (KPC-3-producing K. pneumoniae, ceftazidime-avibactam susceptible), displayed a unique pulse type and belonged to ST11. Based on whole-genome sequencing results of selected isolates, less than 7 single-nucleotide polymorphisms were identified among them, which was indicative of the presence of a unique clone. Both in vivo selection and horizontal transmission seemed to have occurred in our hospital. Detection of these strains is challenging for the laboratory. History of previous KPC-E infections or colonization and systematic testing for resistance to ceftazidime-avibactam might help raise awareness of this possibility. IMPORTANCE Klebsiella pneumoniae is one of the main bacteria that cause infections in health care settings. This pathogen has developed a high level of resistance to many antibiotics. Some K. pneumoniae isolates can produce an enzyme known as carbapenemase KPC, making carbapenems (considered the last line for therapy) not effective to treat their infections. The combination ceftazidime-avibactam, approved by FDA in 2015, is useful to treat infections caused by KPC-producing K. pneumoniae. This study describes the emergence, in one hospital in Argentina, of K. pneumoniae isolates that produce KPC variants (KPC-31 and KPC-115) resistant to ceftazidime-avibactam. The ceftazidime-avibactam-resistant bacteria were isolated in inpatients, including some that previously received this combination as treatment. Transmission of this strain to other patients also occurred in the studied period. Detection of these bacteria is challenging for the laboratory. The knowledge and awareness of the emergence of this pathogen in our region are highly valuable.

Metagenomic Next-Generation Sequencing (mNGS) for the Timely Diagnosis of Carbapenem-Resistant Klebsiella pneumoniae in Leukemia Patients

This report shows the contribution of metagenomic next-generation sequencing (mNGS) as an alternative to challenging diagnostic infection in immunosuppressed individuals. Herein, we report two leukemia patients who developed severe infections due to carbapenem-resistant Klebsiella pneumoniae (CrKP). The mNGS can be strongly recommended as an alternative investigation for patients who are at high risk of infection without positivity on body fluid culture. This can provide the opportunity for adequate therapy.

Emergence of Ceftazidime-Avibactam Resistance and Decreased Virulence in Carbapenem-Resistant ST11 Klebsiella pneumoniae During Antibiotics Treatment

Introduction

Carbapenem-resistant Klebsiella pneumoniae (CRKP) poses a serious threat to human public health. Ceftazidime-avibactam (CZA) is currently one of the few effective antibiotics for carbapenem-resistant Enterobacteriaceae (CRE).

Methods and Results

Here, we analyzed two longitudinal Klebsiella pneumoniae clinical isolates (FK8578, FK8695) that were isolated from an ICU patient during antimicrobial treatment. Broth microdilution method, whole-genome sequencing (WGS) and comparative genomic analysis were used to elucidate the dynamics and mechanisms of antibiotic resistance. String test, quantification of capsule, biofilm inhibition test and Galleria mellonella (G. mellonella) infection model were used to explore the changes in virulence of the two clinical isolates. During antibiotic treatment, CRKP FK8578 underwent a series of drug resistance and virulence changes, including CZA resistance, carbapenem susceptibility and virulence attenuation. The results of WGS showed that mutation of bla KPC-2 to bla KPC-33 was responsible for the change of drug resistance phenotype between FK8578 and FK8695. pLVPK-like virulence plasmid without siderophore synthesis operon was identified in the two strains. On the other hand, the loss of hypermucoviscosity phenotype in the FK8695 strain may be related to a single nucleotide deletion of the rmpA gene, which would further lead to a decrease in virulence. Virulence results showed that compared with FK8578, FK8695 was negative in the string test, with decreased capsular production, smaller amounts of biofilm formation and higher survival rate of G. mellonella.

Conclusion

This is the first report of CZA resistance and decreased virulence in ST11 CRKP strains during antimicrobial treatment. It is urgent to monitor CZA resistance and timely adjust anti-infective treatment strategies.

Imipenem-Relebactam Susceptibility in Enterobacterales Isolates Recovered from ICU Patients from Spain and Portugal (SUPERIOR and STEP Studies)

Imipenem-relebactam is a novel β-lactam-β-lactamase inhibitor combination. We evaluated the in vitro activity of imipenem-relebactam and comparators against Enterobacterales clinical isolates recovered in 8 Spanish and 11 Portuguese intensive care units (ICUs) (SUPERIOR, 2016-2017; STEP, 2017-2018). Overall, 747 Enterobacterales isolates (378 Escherichia coli, 252 Klebsiella spp., 64 Enterobacter spp., and 53 other species) were prospectively collected from ICU patients with complicated intraabdominal (cIAI), complicated urinary tract (cUTI), and lower respiratory tract (LRTI) infections. MICs were determined (ISO-broth microdilution), and whole-genome sequencing (WGS) was performed in a subset of isolates displaying susceptible and resistant imipenem-relebactam MICs. Imipenem-relebactam (98.7% susceptible) showed similar activity to ceftazidime-avibactam (99.5% susceptible) and higher than ceftolozane-tazobactam (86.9% susceptible). Imipenem-relebactam was inactive against 1.3% (10/747) isolates, all of them due to carbapenemase production (9 K. pneumoniae and 1 E. cloacae). Imipenem-relebactam was active against 100% of extended-spectrum β-lactamase (ESBL)-E. coli and ESBL-Klebsiella spp. isolates and 80.4% of carbapenemase-Klebsiella spp. producers. Carbapenemase genes were confirmed by WGS in 41 Klebsiella spp.: OXA-48 (20/41), KPC-3 (14/41), OXA-181 (4/41), NDM-1 (1/41), OXA-48 + VIM-2 (1/41), and KPC-3 + VIM-2 (1/41). In Klebsiella spp. isolates, relebactam restored imipenem susceptibility in all KPC-3 producers, and resistant isolates (7/41) were mostly OXA-48 + CTX-M-15-K. pneumoniae high-risk clones (7/9). Intercountry differences were detected as follows: OXA-48 (17/21) was dominant in Spain, unlike KPC-3 (14/15) in Portugal. Imipenem-relebactam was 100% active against CTX-M-15-ST131-H30Rx-E. coli high-risk clone, predominant in both countries. Our results depict the potential role of imipenem-relebactam in ICU patients with cIAIs, cUTIs, and LRTIs due to wild-type ESBL- and carbapenemase-producing Enterobacterales, particularly KPC producers. IMPORTANCE We comparatively evaluate the in vitro activity of a drug combination consisting of a carbapenem (imipenem) and a novel inhibitor of beta-lactamases (relebactam), a mechanism that destroys beta-lactam antibiotics. We assess the activity against a collection of Enterobacterales clinical isolates recovered from difficult-to-treat infections in patients admitted to different intensive care units in Portugal and Spain. Imipenem-relebactam shows excellent activity in avoiding common resistance mechanisms in this setting, such as extended-spectrum beta-lactamases and carbapenemases widely distributed, including KPCs. We show few resistant isolates (<2%). Molecular characterization by whole-genome sequencing shows that most of the resistant isolates produced specific carbapenemase, such as OXA-48 or metalo-betalactamases. Our study updates the activity of imipenem-relebactam in light of current epidemiology in a hospital setting in which the use of this combination is needed due to the presence of infections due to multidrug-resistant isolates.

Klebsiella pneumoniae Carbapenemase Variants Resistant to Ceftazidime-Avibactam: an Evolutionary Overview

First variants of the Klebsiella pneumoniae carbapenemase (KPC), KPC-2 and KPC-3, have encountered a worldwide success, particularly in K. pneumoniae isolates. These beta-lactamases conferred resistance to most beta-lactams including carbapenems but remained susceptible to new beta-lactam/beta-lactamase inhibitors, such as ceftazidime-avibactam. After the marketing of ceftazidime-avibactam, numerous variants of KPC resistant to this association have been described among isolates recovered from clinical samples or derived from experimental studies. In KPC variants resistant to ceftazidime-avibactam, point mutations, insertions and/or deletions have been described in various hot spots. Deciphering the impact of these mutations is crucial, not only from a therapeutic point of view, but also to follow the evolution in time and space of KPC variants resistant to ceftazidime-avibactam. In this review, we describe the mutational landscape of the KPC beta-lactamase toward ceftazidime-avibactam resistance based on a multidisciplinary approach including epidemiology, microbiology, enzymology, and thermodynamics. We show that resistance is associated with three hot spots, with a high representation of insertions and deletions compared with other class A beta-lactamases. Moreover, extension of resistance to ceftazidime-avibactam is associated with a trade-off in the resistance to other beta-lactams and a decrease in enzyme stability. Nevertheless, the high natural stability of KPC could underlay the propensity of this enzyme to acquire in vivo mutations conferring resistance to ceftazidime-avibactam (CAZavi), particularly via insertions and deletions.

Epidemiology, Mechanisms of Resistance and Treatment Algorithm for Infections Due to Carbapenem-Resistant Gram-Negative Bacteria: An Expert Panel Opinion

Antimicrobial resistance represents a serious threat for global health, causing an unacceptable burden in terms of morbidity, mortality and healthcare costs. In particular, in 2017, carbapenem-resistant organisms were listed by the WHO among the group of pathogens for which novel treatment strategies are urgently needed. Fortunately, several drugs and combinations have been introduced in recent years to treat multi-drug-resistant (MDR) bacteria. However, a correct use of these molecules is needed to preserve their efficacy. In the present paper, we will provide an overview on the epidemiology and mechanisms of resistance of the most common MDR Gram-negative bacteria, proposing a treatment algorithm for the management of infections due to carbapenem-resistant bacteria based on the most recent clinical evidence.

Acquisition of resistance to ceftazidime-avibactam during infection treatment in Pseudomonas aeruginosa through D179Y mutation in one of two blaKPC-2 gene copies without losing carbapenem resistance

Ceftazidime/Avibactam (CAZ/AVI) is frequently used to treat KPC-producing Pseudomonas aeruginosa (KPC-PA) and Enterobacterales. CAZ/AVI resistance is driven by several mechanisms. In P. aeruginosa this mainly occurs through alteration of AmpC, porins, and/or efflux pump overexpression, whereas in Enterobacterales it frequently occurs through D179Y substitution in the active site of KPC enzyme. This aminoacid change abolishes AVI binding to the KPC active site, hence inhibition is impaired. However, this substitution also decreases KPC-mediated resistance to carbapenems ("see-saw" effect). The goal of this work was to characterize the in vivo acquisition of CAZ/AVI resistance through D179Y substitution in a KPC-PA isolated from a hospitalized patient after CAZ/AVI treatment. Two KPC-PA isolates were obtained. The first isolate, PA-1, was obtained before CAZ/AVI treatment and was susceptible to CAZ/AVI. The second isolate, PA-2, was obtained after CAZ/AVI treatment and exhibited high-level CAZ/AVI resistance. Characterization of isolates PA-1 and PA-2 was performed through short and long-read whole genome sequencing analysis. The hybrid assembly showed that PA-1 and PA-2A had a single plasmid of 54,030 bp, named pPA-1 and pPA-2 respectively. Each plasmid harbored two copies of the bla KPC-containing Tn4401b transposon. However, while pPA-1 carried two copies of bla KPC-2, pPA-2 had one copy of bla KPC-2 and one copy of bla KPC-33, the allele with the D179Y substitution. Interestingly, isolate PA-2 did not exhibit the "see-saw" effect. The bla KPC-33 allele was detected only through hybrid assembly using a long-read-first approach. The present work describes a KPC-PA isolate harboring a plasmid-borne CAZ/AVI resistance mechanism based on two copies of bla KPC-2-Tn4401b and D179Y mutation in one of them, that is not associated with loss of resistance to carbapenems. These findings highlight the usefulness of a fine-tuned combined analysis of short and long-read data to detect similar emerging resistance mechanisms.

In vivo translational assessment of the GES genotype on the killing profile of ceftazidime, ceftazidime/avibactam and meropenem against Pseudomonas aeruginosa

Objectives

To evaluate the in vivo killing profile of human-simulated exposures of ceftazidime, ceftazidime/avibactam and meropenem against GES-harbouring Pseudomonas aeruginosa in the murine thigh infection model.

Methods

Five P. aeruginosa isolates [three isogenic (GES-1, GES-5 and GES-15) and two clinical (GES-5 and GES-15)] were evaluated. MICs were determined using broth microdilution. Human-simulated regimens (HSRs) of ceftazidime 2 g IV q8h as a 2 h infusion, ceftazidime/avibactam 2.5 g IV q8h as a 2 h infusion and meropenem 2 g IV q8h as a 3 h infusion were administered. Change in bacterial burden relative to baseline was assessed.

Results

Modal MICs ranged from 8 to >64 mg/L for ceftazidime, from 1 to 16 mg/L for ceftazidime/avibactam and from 1 to >64 mg/L for meropenem. In vivo, for the isogenic strains, avibactam augmented ceftazidime activity against the GES-1- and GES-15-harbouring isolates. Both ceftazidime and ceftazidime/avibactam resulted in significant kill against the GES-5 isogenic isolate. The meropenem HSR produced >1 log₁₀ kill against each isogenic isolate (MICs of 1–4 mg/L). Against the GES-5 clinical isolate, ceftazidime and ceftazidime/avibactam resulted in >1 log₁₀ kill compared with bacterial growth with the meropenem HSR. In the clinical isolate harbouring GES-15, the elevated MICs of ceftazidime and ceftazidime/avibactam reduced the effectiveness of both compounds, while the observed reduction in meropenem MIC translated into in vivo efficacy of the HSR regimen, predictive of clinical efficacy.

Conclusions

In GES-harbouring P. aeruginosa, quantitative reductions in bacterial density observed with the translational murine model suggest that the phenotypic profile of ceftazidime, ceftazidime/avibactam and meropenem is predictive of clinical efficacy when using the evaluated dosing regimens.

A Selective Medium for Screening Ceftazidime/Avibactam Resistance in Carbapenem-Resistant Enterobacterales

Ceftazidime/avibactam (CZA) is an alternative antibiotic used for the treatment of infections caused by carbapenem-resistant Enterobacterales (CRE). However, the CZA-resistant CRE strains have been detected worldwide. Therefore, it is critical to screen CZA-resistant CRE strains in colonized patients or a specific population so as to rapidly implement infection control measures to limit their transmission. In this study, we developed a Salmonella-Shigella (SS) CZA-selective medium and assessed its performance to screen for clinical CZA-resistant CRE isolates in both pure-strain specimens and stool samples. A total of 150 non-duplicated isolates, including 75 CZA-susceptible and 75 CZA-resistant CRE pathogens, were tested by using the broth microdilution method and the SS CZA medium, respectively. The bacterial suspensions were serially diluted in the SS CZA medium, which showed excellent screening performance in both pure CZA-resistant CRE strain and the stool samples with the lowest detection limit of 10¹-10² and 10¹-10³ CFU/ml, respectively. Notably, none of the susceptible isolates showed growth even at the highest dilution concentration of 10⁸ CFU/ml. Most importantly, the SS CZA medium demonstrated excellent performance in screening simulated clinical polymicrobial specimens. Moreover, its screening performance was unaffected by the different resistance determinants for tested isolates. Cumulatively, our data suggest that the SS CZA medium can be used as a promising selective medium to screen CZA-resistant CRE, irrespective of their resistance mechanisms.

Outbreak of IncX8 Plasmid-Mediated KPC-3-Producing Enterobacterales Infection, China

Carbapenem-resistant Enterobacterales (CRE) infection is highly endemic in China; Klebsiella pneumoniae carbapenemase (KPC) 2-producing CRE is the most common, whereas KPC-3-producing CRE is rare. We report an outbreak of KPC-3-producing Enterobacterales infection in China. During August 2020-June 2021, 25 bla_KPC-3-positive Enterobacteriale isolates were detected from 24 patients in China. Whole-genome sequencing analysis revealed that the bla_KPC-3 genes were harbored by IncX8 plasmids. The outbreak involved clonal expansion of KPC-3-producing Serratia marcescens and transmission of bla_KPC-3 plasmids across different species. The bla_KPC-3 plasmids demonstrated high conjugation frequencies (10^-3 to 10^-4). A Galleria mellonella infection model showed that 2 sequence type 65 K2 K. pneumoniae strains containing bla_KPC-3 plasmids were highly virulent. A ceftazidime/avibactam in vitro selection assay indicated that the KPC-3-producing strains can readily develop resistance. The spread of bla_KPC-3-harboring IncX8 plasmids and these KPC-3 strains should be closely monitored in China and globally.

New Perspectives on Antimicrobial Agents: Imipenem-Relebactam

Imipenem (IMI)/cilastatin/relebactam (REL) (I/R) is a novel β-lactam/β-lactamase inhibitor combination with expanded microbiologic activity against carbapenem-resistant non-Morganellaceae Enterobacterales (CR-NME) and difficult-to-treat (DTR) Pseudomonas aeruginosa. Relebactam, a bicyclic diazabicyclooctane, has no direct antimicrobial activity but provides reliable inhibition of many Ambler class A and class C enzymes. It is currently approved for the treatment of adult patients with hospital-acquired bacterial pneumonia and ventilator-associated bacterial pneumonia (HABP/VABP) and those with complicated urinary tract infections (cUTIs) and complicated intra-abdominal infections (cIAIs) when limited or no alternative treatments are available. Given the number of recently approved β-lactams with expanded activity against highly resistant Gram-negative pathogens, this review summarizes the published literature on I/R, with a focus on its similar and distinguishing characteristics relative to those of other recently approved agents. Overall, available data support its use for the treatment of patients with HABP/VABP, cUTI, and cIAI due to CR-NME and DTR P. aeruginosa. Data indicate that I/R retains some activity against CR-NME and DTR P. aeruginosa isolates that are resistant to the newer β-lactams and vice versa, suggesting that susceptibility testing be performed for all the newer agents to determine optimal treatment options for patients with CR-NME and DTR P. aeruginosa infections. Further comparative PK/PD and clinical studies are warranted to determine the optimal role of I/R, alone and in combination, for the treatment of patients with highly resistant Gram-negative infections. Until further data are available, I/R is a potential treatment for patients with CR-NME and DTR P. aeruginosa infections when the benefits outweigh the risks.

The primary pharmacology of ceftazidime/avibactam: in vitro translational biology

Previous reviews of ceftazidime/avibactam have focused on in vitro molecular enzymology and microbiology or the clinically associated properties of the combination. Here we take a different approach. We initiate a series of linked reviews that analyse research on the combination that built the primary pharmacology data required to support the clinical and business risk decisions to perform randomized controlled Phase 3 clinical trials, and the additional microbiological research that was added to the above, and the safety and chemical manufacturing and controls data, that constituted successful regulatory licensing applications for ceftazidime/avibactam in multiple countries, including the USA and the EU. The aim of the series is to provide both a source of reference for clinicians and microbiologists to be able to use ceftazidime/avibactam to its best advantage for patients, but also a case study of bringing a novel β-lactamase inhibitor (in combination with an established β-lactam) through the microbiological aspects of clinical development and regulatory applications, updated finally with a review of resistance occurring in patients under treatment. This first article reviews the biochemistry, structural biology and basic microbiology of the combination, showing that avibactam inhibits the great majority of serine-dependent β-lactamases in Enterobacterales and Pseudomonas aeruginosa to restore the in vitro antibacterial activity of ceftazidime. Translation to efficacy against infections in vivo is reviewed in the second co-published article, Nichols et al. (J Antimicrob Chemother 2022; dkac172).

Imipenem/Relebactam Resistance in Clinical Isolates of Extensively Drug Resistant Pseudomonas aeruginosa: Inhibitor-Resistant β-Lactamases and Their Increasing Importance

Multidrug-resistant (MDR) Pseudomonas aeruginosa infections are a major clinical challenge. Many isolates are carbapenem resistant, which severely limits treatment options; thus, novel therapeutic combinations, such as imipenem-relebactam (IMI/REL), ceftazidime-avibactam (CAZ/AVI), ceftolozane-tazobactam (TOL/TAZO), and meropenem-vaborbactam (MEM/VAB) were developed. Here, we studied two extensively drug-resistant (XDR) P. aeruginosa isolates, collected in the United States and Mexico, that demonstrated resistance to IMI/REL. Whole-genome sequencing (WGS) showed that both isolates contained acquired GES β-lactamases, intrinsic PDC and OXA β-lactamases, and disruptions in the genes encoding the OprD porin, thereby inhibiting uptake of carbapenems. In one isolate (ST17), the entire C terminus of OprD deviated from the expected amino acid sequence after amino acid G388. In the other (ST309), the entire oprD gene was interrupted by an ISPa1328 insertion element after amino acid D43, rendering this porin nonfunctional. The poor inhibition by REL of the GES β-lactamases (GES-2, -19, and -20; apparent Ki of 19 ± 2 μM, 23 ± 2 μM, and 21 ± 2 μM, respectively) within the isolates also contributed to the observed IMI/REL-resistant phenotype. Modeling of REL binding to the active site of GES-20 suggested that the acylated REL is positioned in an unstable conformation as a result of a constrained Ω-loop.

Resistance to Ceftazidime/Avibactam, Meropenem/Vaborbactam and Imipenem/Relebactam in Gram-Negative MDR Bacilli: Molecular Mechanisms and Susceptibility Testing

Multidrug resistance (MDR) represents a serious global threat due to the rapid global spread and limited antimicrobial options for treatment of difficult-to-treat (DTR) infections sustained by MDR pathogens. Recently, novel β-lactams/β-lactamase inhibitor combinations (βL-βLICs) have been developed for the treatment of DTR infections due to MDR Gram-negative pathogens. Although novel βL-βLICs exhibited promising in vitro and in vivo activities against MDR pathogens, emerging resistances to these novel molecules have recently been reported. Resistance to novel βL-βLICs is due to several mechanisms including porin deficiencies, increasing carbapenemase expression and/or enzyme mutations. In this review, we summarized the main mechanisms related to the resistance to ceftazidime/avibactam, meropenem/vaborbactam and imipenem/relebactam in MDR Gram-negative micro-organisms. We focused on antimicrobial activities and resistance traits with particular regard to molecular mechanisms related to resistance to novel βL-βLICs. Lastly, we described and discussed the main detection methods for antimicrobial susceptibility testing of such molecules. With increasing reports of resistance to novel βL-βLICs, continuous attention should be maintained on the monitoring of the phenotypic traits of MDR pathogens, into the characterization of related mechanisms, and on the emergence of cross-resistance to these novel antimicrobials.

Structural Characterization of the D179N and D179Y Variants of KPC-2 β-Lactamase: Ω-Loop Destabilization as a Mechanism of Resistance to Ceftazidime-Avibactam

Klebsiella pneumoniae carbapenemases (KPC-2 and KPC-3) present a global clinical threat, as these β-lactamases confer resistance to carbapenems and oxyimino-cephalosporins. Recent clinically identified KPC variants with substitutions at Ambler position D179, located in the Ω loop, are resistant to the β-lactam/β-lactamase inhibitor combination ceftazidime-avibactam, but susceptible to meropenem-vaborbactam. To gain insights into ceftazidime-avibactam resistance conferred by D179N/Y variants of KPC-2, crystal structures of these variants were determined. The D179N KPC-2 structure revealed that the change of the carboxyl to an amide moiety at position 179 disrupted the salt bridge with R164 present in wild-type KPC-2. Additional interactions were disrupted in the Ω loop, causing a decrease in the melting temperature. Shifts originating from N179 were also transmitted toward the active site, including ∼1-Å shifts of the deacylation water and interacting residue N170. The structure of the D179Y KPC-2 β-lactamase revealed more drastic changes, as this variant exhibited disorder of the Ω loop, with other flanking regions also being disordered. We postulate that the KPC-2 variants can accommodate ceftazidime because the Ω loop is displaced in D179Y or can be more readily displaced in D179N KPC-2. To understand why the β-lactamase inhibitor vaborbactam is less affected by the D179 variants than avibactam, we determined the crystal structure of D179N KPC-2 in complex with vaborbactam, which revealed wild-type KPC-2-like vaborbactam-active site interactions. Overall, the structural results regarding KPC-2 D179 variants revealed various degrees of destabilization of the Ω loop that contribute to ceftazidime-avibactam resistance, possible substrate-assisted catalysis of ceftazidime, and meropenem and meropenem-vaborbactam susceptibility.

Meropenem/Vaborbactam Plus Aztreonam as a Possible Treatment Strategy for Bloodstream Infections Caused by Ceftazidime/Avibactam-Resistant Klebsiella pneumoniae: A Retrospective Case Series and Literature Review

Objectives: The aim of this study was to describe our experience of a combination treatment including meropenem/vaborbactam (M/V) plus aztreonam (ATM) for bloodstream infections (BSIs) due to ceftazidime/avibactam-resistant Klebsiella pneumoniae (CAZ/AVI-R-Kp), for which gene typing was not available at the time the blood culture (BC) results were obtained. Methods: Between 20 July and 22 August 2021, in our hospital laboratory, the molecular test for carbapenemase gene typing was not available. All Gram-negative bloodstream infections were recorded, and characteristics of patients were analysed. Among them, three patients had positive BCs for CAZ/AVI-R-Kp, and the empirical therapy was switched to M/V plus ATM pending phenotypic testing of sensitivity to M/V. Therapy was subsequently targeted on the basis of the results of this test. Results: KPC and NDM represent the most prevalent carbapenemases in our polyclinic. Three patients with CAZ/AVI-R-Kp sepsis were treated with M/V plus ATM not knowing the carbapenemase gene. Two had an NDM-Kp infection for which, upon obtaining the result of sensitivity to M/V, combination therapy was maintained. The third had KPC-Kp infection for which ATM was discontinued, after the acquisition of an antibiogram reporting full sensitivity to M/V (MIC = 0.25 mg/L). One patient with NDM-Kp infection died due to complications of the underlying disease for which he was hospitalised. Conclusions: Meropenem/vaborbactam plus ATM and subsequent de-escalation could represent a possible therapeutic strategy in severe CAZ/AVI-R-Kp infections when carbapenemase gene typing is not rapidly available.