Multiyear, Multinational Survey of the Incidence and Global Distribution of Metallo-β-Lactamase-Producing Enterobacteriaceae and Pseudomonas aeruginosa

In vitro activity of imipenem/relebactam plus aztreonam against metallo-β-lactamase producing, OprD-deficient Pseudomonas aeruginosa with varying levels of Pseudomonas-derived cephalosporinase production

BACKGROUND

Limited treatment options exist for metallo-β-lactamase (MBL)-producing Pseudomonas aeruginosa infections. Imipenem/relebactam plus aztreonam may be an option.

METHODS

Ten OprD(-) P. aeruginosa isolates (3 parent strains; 7 MBL-producers) were evaluated using checkerboard methodology and Fractional Inhibitory Concentration Index (FICI). Isolates exhibiting synergy in checkerboard studies (FICI ≤0.5) were evaluated using 24-hour static concentration time-kill. Bacteria in late log-phase growth were diluted to 1 × 10⁶ cfu/mL and incubated at 37°C for 24 hours. Samples were drawn at 0, 2, 4, 6 and 24 hours. Physiologic fC_max, fC_ss,avg and fC_min of imipenem (26.7, 5.6, 0.5 mg/L), relebactam (13.1, 4, 0.8 mg/L) and aztreonam (62, 29, 8 mg/L) were used. Synergy in time-kill studies was defined as >2 log₁₀ cfu/mL reduction compared to the most active individual agent.

RESULTS

Synergy was observed in five isolates in checkerboard studies, including three of seven MBL-producing isolates. Isolates which were OprD(-) and harboured inducible Pseudomonas-derived cephalosporinases (PDCs) did not show synergy as defined by FICI, however aztreonam MICs were significantly reduced with the combination. In time-kill studies, ATM alone was as active as combination regimens for MBL-producing isolates with deleted or inducible PDC production. For strains exhibiting constitutive PDC production, I/R plus ATM was synergistic at fC_ss,avg concentrations but exhibited similar activity to ATM at fC_min and fC_max concentrations.

CONCLUSIONS

Imipenem/relebactam plus aztreonam appears to exhibit synergy for some MBL-producing P. aeruginosa at physiologic concentrations. Further study of the effect of dynamic concentrations is needed to understand fully the utility of this combination.

Activity of ceftolozane/tazobactam against Gram-negative isolates among different infections in Hong Kong: SMART 2017-2019

Introduction. Ceftolozane/tazobactam was approved by the Drug Office, Department of Health, Government of the Hong Kong Special Administrative Region in 2017.Hypothesis/Gap Statement. Currently the in vitro activity of ceftolozane/tazobactam against Gram-negative pathogens isolated from patients in Hong Kong is undocumented. It would be prudent to document the activity of ceftolozane/tazobactam against Pseudomonas aeruginosa and Enterobacterales isolated from hospitalized patients in Hong Kong.Aim. To describe the in vitro susceptibility of recent clinical isolates of P. aeruginosa and the two most common Enterobacterales species (Klebsiella pneumoniae, Escherichia coli) cultured from respiratory tract, intra-abdominal, urinary tract and bloodstream infection samples to ceftolozane/tazobactam and other commonly used antimicrobial agents.Methodology. CLSI-defined broth microdilution MICs were determined and interpreted for Gram-negative isolates collected in Hong Kong from 2017 to 2019 by the SMART surveillance programme.Results. For P. aeruginosa, 96.7 % of isolates (n=210) were susceptible to ceftolozane/tazobactam, while susceptibility rates were ≥14 % lower to meropenem (82.9 % susceptible), cefepime (82.4 %), ceftazidime (81.4 %), piperacillin/tazobactam (76.7 %) and levofloxacin (79.5 %). Ceftolozane/tazobactam inhibited 85.7 % of piperacillin/tazobactam-nonsusceptible isolates, 80.6-82.1 % of cefepime-, ceftazidime- or meropenem-nonsusceptible isolates, and 75.9 % of multidrug-resistant (MDR) isolates of P. aeruginosa. For K. pneumoniae, 96.1 % of isolates (n=308) were susceptible to ceftolozane/tazobactam compared with meropenem (99.0 % susceptible), piperacillin/tazobactam (93.8 %), cefepime (85.7 %) and ceftazidime (85.4 %). The majority (88.3 %) of ESBL (extended-spectrum β-lactamase) non-CRE (carbapenem-resistant Enterobacterales) phenotype isolates of K. pneumoniae were susceptible to ceftolozane/tazobactam, comparable to piperacillin/tazobactam (85.0 %) but lower than meropenem (100 %). For E. coli, 98.5 % of isolates (n=609) were susceptible to ceftolozane/tazobactam compared to meropenem (99.3 % susceptible), piperacillin/tazobactam (96.7 %), ceftazidime (82.3 %) and cefepime (76.5 %). The majority (96.7 %) of ESBL non-CRE phenotype isolates of E. coli were susceptible to ceftolozane/tazobactam, similar to both meropenem (100 %) and piperacillin/tazobactam (94.5 %).Conclusions. Overall, >96 % of clinical isolates of P. aeruginosa, K. pneumoniae and E. coli collected in Hong Kong in 2017-2019 were susceptible to ceftolozane/tazobactam, while the activity of several commonly prescribed β-lactams was reduced, especially for P. aeruginosa. Continued surveillance of ceftolozane/tazobactam and other agents is warranted.

Antimicrobial Susceptibility Testing of Clinical Isolates of Gram-Negative Bacilli Collected in Morocco by the ATLAS Global Surveillance Program from 2018 to 2020

OBJECTIVE

To report reference method antimicrobial susceptibility testing results for recent clinical isolates of Gram-negative bacilli from Morocco.

METHODS

CLSI broth microdilution antimicrobial susceptibility testing was performed by a central laboratory for isolates of Enterobacterales (n=810), Pseudomonas aeruginosa (n=321), and Acinetobacter baumannii (n=191) collected in 2018-2020 by three hospital laboratories in Morocco. MICs were interpreted using both CLSI (2021) and EUCAST (2021) breakpoints. Molecular testing for β-lactamase genes was performed on isolates meeting defined screening criteria.

RESULTS

Most isolates of Enterobacterales were susceptible (CLSI/EUCAST breakpoints) to amikacin (98.0%/96.2%), ceftazidime-avibactam (94.8%/94.8%), and meropenem (92.5%/94.2%). Of Enterobacterales isolates eligible for β-lactamase gene screening (n=210), 174 were ESBL-positive, 40 were metallo-β-lactamase-positive (all NDM), 39 were serine carbapenemase-positive (all OXA); and 7 isolates carried both OXA-48 and NDM-1. Amikacin (89.1%/89.1%) and ceftazidime-avibactam (88.2%/88.2%) were the most active agents tested against P. aeruginosa. Applying CLSI and EUCAST breakpoints, MDR rates were 21.9% and 29.3% for Enterobacterales and 18.4% and 21.8% for P. aeruginosa. Susceptible rates for amikacin, ceftazidime-avibactam, and meropenem were 93.2%/89.5%, 77.4%/82.3%, and 67.8%/80.2% for MDR Enterobacterales and 50.8%/57.1%, 40.7%/45.7%, and 27.1/32.9% for MDR P. aeruginosa. ≥70% of A. baumannii isolates were resistant to all agents tested (except colistin, EUCAST breakpoints only) including amikacin and meropenem.

CONCLUSION

Newer β-lactam/β-lactamase inhibitor combinations such as ceftazidime-avibactam warrant testing and reporting for Enterobacterales and P. aeruginosa in Morocco given the presence of significant resistance to first-line β-lactams and fluoroquinolones, pervasive ESBLs and carbapenemases, and toxicity concerns associated with some second-line agents.

Consensus on β-Lactamase Nomenclature

Assigning names to β-lactamase variants has been inconsistent and has led to confusion in the published literature. The common availability of whole genome sequencing has resulted in an exponential growth in the number of new β-lactamase genes. In November 2021 an international group of β-lactamase experts met virtually to develop a consensus for the way naturally-occurring β-lactamase genes should be named. This document formalizes the process for naming novel β-lactamases, followed by their subsequent publication.

Emergence of Carbapenem-Resistant ST244, ST292, and ST2446 Pseudomonas aeruginosa Clones in Burn Patients in Yunnan Province

INTRODUCTION

The prevalence of carbapenem-resistant Pseudomonas aeruginosa is increasing persistently, particularly in burn ward isolates. Here, we investigate the prevalence of carbapenem-resistant Pseudomonas aeruginosa in a burn ward of a provincial-level hospital at Kunming, Yunnan province, China.

METHODS

A total of 118 P. aeruginosa strains were isolated from 57 hospitalized patients, and their MICs were measured. Carbapenem-resistant isolates were selected for multilocus sequence typing (MLST). Carbapenem-resistance mechanisms were identified by examining carbapenemase genes and OprD protein and Carba-NP testing. Representative isolates were further characterized by de novo sequencing for carbapenemase molecular background.

RESULTS

Among 118 P. aeruginosa isolates, 54 (54/118,45.8%) were carbapenem-resistant Pseudomonas aeruginosa, and 3 genotypes were found (ST292, ST244, and ST2446). Non-carbapenemase-producing ST292 was the most prevalent ST, followed by ST2446 and ST244. A novel 13-bp oprD deletion was found in the ST292 clone, which formed the truncated outer membrane protein and may cause carbapenem resistance. ST244 and ST2446 harbored blaIMP-45 and blaIMP-87, respectively. blaIMP-45 is located in a megaplasmid, together with aac(6')-Ib3, blaOXA-1, catB3, qnrVC6, armA, msr(E), mph(E), aph(3')-Ia, tetC/tetR, aac(6')-Ib3, floR, mexC-mexD-oprJ, fosA and lead to extensive drug resistance. ST2446 contains a carbapenem-resistant gene blaIMP-87 on the chromosome and is acquired by a novel gene cassette array (blaIMP-87-ant(2")-Ia-blaOXA-10-aac(6')-Ib3) of class 1 integron.

DISCUSSION

For the first time, ST244, ST292 and ST2446 are reported emerging in burn patients, with distinctive carbapenem-resistance mechanisms, respectively. The obtained results highlight the need to surveillance carbapenem-resistant isolates in burn patients.

Overcoming the rising incidence and evolving mechanisms of antibiotic resistance by novel drug delivery approaches - An overview

Antimicrobial resistance is a normal evolutionary process for microorganisms. Antibiotics exerted accelerated selective pressure that hasten bacterial resistance through mutation, and acquisition external genes. These genes often carry multiple antibiotic resistant determinants allowing the recipient microbe an instant "super-bug" status. The extent of Antimicrobial Resistance (AMR) has reached a level of global crisis, existing antimicrobials are no long effective in treating infections caused by AMR pathogens. The great majority of clinically available antimicrobial agents are administered through oral and intra-venous routes. Overcoming antibacterial resistance by novel drug delivery approach offered new hopes, particularly in the treatment of AMR pathogens in sites less assessible through systemic circulation such as the lung and skin. In the current review, we will revisit the mechanism and incidence of important AMR pathogens. Finally, we will discuss novel drug delivery approaches including novel local antibiotic delivery systems, hybrid antibiotics, and nanoparticle-based antibiotic delivery systems.

Pseudomonas aeruginosa Pneumonia: Evolution of Antimicrobial Resistance and Implications for Therapy

Pseudomonas aeruginosa (PA), a non-lactose-fermenting gram-negative bacillus, is a common cause of nosocomial infections in critically ill or debilitated patients, particularly ventilator-associated pneumonia (VAP), and infections of urinary tract, intra-abdominal, wounds, skin/soft tissue, and bloodstream. PA rarely affects healthy individuals, but may cause serious infections in patients with chronic structural lung disease, comorbidities, advanced age, impaired immune defenses, or with medical devices (e.g., urinary or intravascular catheters, foreign bodies). Treatment of pseudomonal infections is difficult, as PA is intrinsically resistant to multiple antimicrobials, and may acquire new resistance determinants even while on antimicrobial therapy. Mortality associated with pseudomonal VAP or bacteremias is high (> 35%) and optimal therapy is controversial. Over the past three decades, antimicrobial resistance (AMR) among PA has escalated globally, via dissemination of several international multidrug resistant "epidemic" clones. We discuss the importance of PA as a cause of pneumonia including health care-associated pneumonia, hospital-acquired pneumonia, VAP, the emergence of AMR to this pathogen, and approaches to therapy (both empirical and definitive).

New β-Lactam Antibiotics and Ceragenins - A Study to Assess Their Potential in Treatment of Infections Caused by Multidrug-Resistant Strains of Pseudomonas aeruginosa

Background

The increasing number of infections caused by antibiotic resistant strains of Pseudomonas aeruginosa posed a very serious challenge for clinical practice. This standing is driving scientists to develop new antibiotics against these microorganisms.

Methods

In this study, we measured the MIC/MBC values and estimated the ability of tested molecules to prevent bacterial biofilm formation to explore the effectiveness of β-lactam antibiotics ceftolozane/tazobactam, ceftazidime/avibactam, meropenem/vaborbactam, and ceragenins CSA-13, CSA-44, and CSA-131 against 150 clinical isolates of Pseudomonas aeruginosa that were divided into five groups, based on their antibiotic resistance profiles to beta-lactams. Selected strains of microorganisms from each group were also subjected to prolonged incubations (20 passages) with ceragenins to probe the development of resistance towards those molecules. Cytotoxicity of tested ceragenins was evaluated using human red blood cell (RBCs) hemolysis and microscopy observations of human lung epithelial A549 cells after ceragenin treatment. Poloxamer 407 (pluronic F-127) at concentrations ranging from 0.5% to 5% was tested as a potential drug delivery substrate to reduce ceragenin toxicity.

Results

Collected data proved that ceragenins at low concentrations are highly active against clinical strains of Pseudomonas aeruginosa regardless of their resistance mechanisms to conventional antibiotics. Ceragenins also show low potential for resistance development, high antibiofilm activity, and controlled toxicity when used together with poloxamer 407.

Conclusion

This data strongly supports the need for further study directed to develop this group of molecules as new antibiotics to fighting infections caused by antibiotic resistant strains of Pseudomonas aeruginosa.

Epidemiology of resistance of carbapenemase-producing Klebsiella pneumoniae to ceftazidime-avibactam in a Chinese hospital

AIMS

Klebsiella pneumoniae has been reported to develop increased antibiotic resistance. Ceftazidime-avibactam (CZA) is a novel antibiotic with activity against serine-lactamase. Here, we investigated the sensitivity of carbapenem-resistant K. pneumoniae (CRKP) to CZA and the mechanisms of drug resistance in our hospital.

METHODS AND RESULTS

Patient characteristics were obtained from medical records. K. pneumoniae and its antibiotic susceptibility were determined using the Vitek-2 Compact instrument. The antibiotic resistance genes KPC, NDM, OXA-48, VIM, IMP, CIM, SPM, TMB, SMB, SIM, AIM and DIM were detected using real-time PCR. Multilocus sequence typing was used for genetic RELATEDNESS analysis. In total, 121 CRKP strains were isolated from patients in the intensive care unit (51·2%), senior ward (12·4%) and neurosurgery department (10%). With an average age of 72·5 years, most patients were in care for respiratory (34·7%), brain (20·7%), digestive tract (13·2%) and cardiovascular (8·3%) diseases. Specimens were predominantly obtained from sputum (39·67%), urine (29·75%) and blood (6·61%).

CONCLUSION

Of 23 CZA-resistant CRKP strains (19·01%), ST11 being the most common at 56·52%, 11 NDM-1-positive (47·83%) and four NDM-5-positive (17·39%) strains were detected.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study indicates that CZA resistance occurs in ~19·01% CRKP strains and that blaNDM-1 and blaNDM-5 might be critical for resistance.

The ERACE-PA Global Surveillance Program: Ceftolozane/tazobactam and Ceftazidime/avibactam in vitro Activity against a Global Collection of Carbapenem-resistant Pseudomonas aeruginosa

The cephalosporin-β-lactamase-inhibitor-combinations, ceftolozane/tazobactam and ceftazidime/avibactam, have revolutionized treatment of carbapenem-resistant Pseudomonas aeruginosa (CR-PA). A contemporary assessment of their in vitro potency against a global CR-PA collection and an assessment of carbapenemase diversity are warranted. Isolates determined as CR-PA by the submitting site were collected from 2019-2021 (17 centers in 12 countries) during the ERACE-PA Global Surveillance Program. Broth microdilution MICs were assessed per CLSI standards for ceftolozane/tazobactam, ceftazidime/avibactam, ceftazidime, and cefepime. Phenotypic carbapenemase testing was conducted (modified carbapenem inactivation method (mCIM)). mCIM positive isolates underwent genotypic carbapenemase testing using the CarbaR, the CarbaR NxG, or whole genome sequencing. The MIC50/90 was reported as well as percent susceptible (CLSI and EUCAST interpretation). Of the 807 isolates, 265 (33%) tested carbapenemase-positive phenotypically. Of these, 228 (86%) were genotypically positive for a carbapenemase with the most common being VIM followed by GES. In the entire cohort of CR-PA, ceftolozane/tazobactam and ceftazidime/avibactam had MIC50/90 values of 2/ > 64 and 4/64 mg/L, respectively. Ceftazidime/avibactam was the most active agent with 72% susceptibility per CLSI compared with 63% for ceftolozane/tazobactam. For comparison, 46% of CR-PA were susceptible to ceftazidime and cefepime. Against carbapenemase-negative isolates, 88 and 91% of isolates were susceptible to ceftolozane/tazobactam and ceftazidime/avibactam, respectively. Ceftolozane/tazobactam and ceftazidime/avibactam remained highly active against carbapenem-resistant P. aeruginosa, particularly in the absence of carbapenemases. The contemporary ERACE-PA Global Program cohort with 33% carbapenemase positivity including diverse enzymology will be useful to assess therapeutic options in these clinically challenging organisms with limited therapies.

Epidemiology and high incidence of metallo-β-lactamase and AmpC-β-lactamases in nosocomial Pseudomonas aeruginosa

OBJECTIVES

Isolates producing metallo-β-lactamase (MBL) have a significant impact on therapeutic and diagnostic layouts, plus their increased frequency has been reported globally. Determination of incidence of clinical isolates of Pseudomonas aeruginosa that are capable of producing MBL and AmpC-β-lactamases making them resistant to imipenem and cefoxitin.

MATERIALS AND METHODS

Out of 1159 collected samples of urine, wound swabs, blood, tissue, and pus, the isolation rate of P. aeruginosa in the period of March 2020 to February 2021 was 22.0% (255/1159). Bacterial strains that were resistant towards imipenem were further processed for detecting the β-lactamase group of genes followed by statistical analysis of risk factors done based on clinical sample, gender, plus department of sample collection.

RESULTS

The percentage of resistance against imipenem was found to be 53%. Out of 135 strains, phenotypic tests revealed MBLs incidence to be 61.5% by combination disc test and 81.5% by Modified Hodge test (MHT). Frequencies of blaIMP-1, blaVIM, blaSHV, blaTEM, and blaOXA genes were calculated to be 13%, 15%, 32%, 43%, and 21%, respectively. Co-expressions of blaMBLs (blaVIM and blaIMP-1) plus blaESBL (blaSHV, blaOXA, blaTEM) were detected using simplex and multiplex PCR. blaTEM, blaSHV, and blaOXA co-existed in 7.5% of clinical isolates. 5.5% of the isolates exhibited simultaneous expression of MBL/ESBL genes. 15% of the isolates resistant to cefoxitin were positive for the blaAmpC gene (17/114).

CONCLUSION

This is a pioneer report from Pakistan that concomitantly presents expression of blaVIM and blaIMP-1 with blaTEM, blaOXA, blaSHV, and blaAmpC in isolates of P. aeruginosa.

In vitro activity of ceftazidime/avibactam and comparators against carbapenemase-producing Enterobacterales and Pseudomonas aeruginosa isolates collected globally between 2016 and 2018

OBJECTIVES

This study reports the antimicrobial activity of ceftazidime/avibactam (CZA) and comparators against carbapenemase-producing Enterobacterales (N = 1992) and carbapenemase-producing Pseudomonas aeruginosa (N = 784) collected in Africa/Middle East, Asia/South Pacific, Europe and Latin America (2016-2018).

METHODS

Minimum inhibitory concentrations (MICs) and susceptibility were determined using broth microdilution methodology and EUCAST breakpoints. Carbapenemase-encoding genes were detected using multiplex PCR.

RESULTS

No isolates of carbapenemase-producing, metallo-β-lactamase (MBL)-negative Enterobacterales from Africa/Middle East or Latin America were resistant to CZA; resistance rates in Europe and Asia/South Pacific were ≤4.5%. Colistin had the lowest resistance rate among MBL-positive isolates (6.0-11.4%). Enterobacterales isolates collected in Latin America predominantly carried a KPC carbapenemase (77.6%), whereas in Africa/Middle East OXA-48-like carbapenemases were most frequently detected (55.9%), and in Asia/South Pacific most isolates carried NDM carbapenemases (56.2%). Among all Enterobacterales carrying KPC carbapenemases, the lowest rate of resistance was to CZA (1.5%), and among isolates carrying NDM carbapenemases it was to colistin (10.8%). Among carbapenemase-producing, MBL-negative P. aeruginosa, resistance rates to CZA were 8.6% for isolates collected in Europe and 53.2% in Latin America. Isolates in each region most frequently carried VIM carbapenemases, ranging from 41.7% of isolates in Asia/South Pacific to 86.2% in Africa/Middle East. No P. aeruginosa carrying KPC or NDM carbapenemases and 1.0% of isolates carrying GES carbapenemases were resistant to colistin.

CONCLUSION

Given the limited therapeutic options to treat infections caused by carbapenemase-positive Enterobacterales and P. aeruginosa, continued surveillance of CZA activity as well as agents such as colistin is crucial.

Emergence and Genetic Characterization of Plasmid-Encoded VIM-2-Producing Pseudomonas stutzeri with Novel Integron In1998 Isolated from Cerebrospinal Fluid

Purpose

To investigate the genomic and plasmid characteristics of a newly discovered Pseudomonas stutzeri strain with a bla _VIM-2-carrying plasmid and novel integron In1998 isolated from a cerebrospinal fluid specimen in a teaching hospital.

Methods

Species identification was performed by MALDI-TOF MS, and bla _VIM-2 was identified by PCR and Sanger sequencing. Whole-genome sequencing analysis was conducted using the Illumina NovaSeq 6000 and Oxford Nanopore platforms. Integron detection was performed using INTEGRALL. The phylogenetic tree was constructed by using kSNP3.0. Plasmid characteristics were assessed by S1-pulsed-field gel electrophoresis (S1-PFGE), Southern blotting, conjugation experiments, and whole-genome sequencing analysis. Comparative genomics analysis of the plasmid and genetic context of bla _VIM-2 were conducted by using BLAST Ring Image Generator (BRIG) and Easyfig 2.3, respectively.

Results

ZDHY95, an MDR strain of P. stutzeri harboring bla _VIM-2, was identified. It was sensitive only to amikacin and was resistant to carbapenems, β-lactams, aztreonam, fluoroquinolones, and aminoglycosides. Joint S1-PFGE, Southern blot, conjugation assay, and whole-genome sequencing experiments confirmed that the bla _VIM-2 gene was located within class I integron In1722 of the plasmid and that the surrounding genetic environment was 5'CS-aacA4'-30-bla _VIM-2-aacA4'-3'CS. The novel class I integron In1998 was detected on the chromosome of P. stutzeri ZDHY95, and the gene cassette array was 5'CS-aacA3-aadA13-cmlA8-bla _OXA-246-arr3-dfrA27-3'CS. Phylogenetic analysis showed that antimicrobial resistance gene-carrying P. stutzeri isolates were divided into two clusters, mainly containing isolates from the USA and Pakistan.

Conclusion

A novel bla _VIM-2-carrying conjugative plasmid, pZDHY95-VIM-2, was reported for the first time in P. stutzeri, elucidating the genetic environment and transfer mechanism. The gene structure of the novel class I integron In1998 was also clarified. We explored the phylogenetic relationship of P. stutzeri with drug resistance genes and suggested that Pseudomonas with metallo-β-lactamases (MBLs) in the hospital environment may cause infection in patients with long-term intubation or after interventional surgery.

The Revival of Aztreonam in Combination with Avibactam against Metallo-β-Lactamase-Producing Gram-Negatives: A Systematic Review of In Vitro Studies and Clinical Cases

Infections caused by metallo-β-lactamase (MBL)-producing Enterobacterales and Pseudomonas are increasingly reported worldwide and are usually associated with high mortality rates (>30%). Neither standard therapy nor consensus for the management of these infections exist. Aztreonam, an old β-lactam antibiotic, is not hydrolyzed by MBLs. However, since many MBL-producing strains co-produce enzymes that could hydrolyze aztreonam (e.g., AmpC, ESBL), a robust β-lactamase inhibitor such as avibactam could be given as a partner drug. We performed a systematic review including 35 in vitro and 18 in vivo studies on the combination aztreonam + avibactam for infections sustained by MBL-producing Gram-negatives. In vitro data on 2209 Gram-negatives were available, showing the high antimicrobial activity of aztreonam (MIC ≤ 4 mg/L when combined with avibactam) in 80% of MBL-producing Enterobacterales, 85% of Stenotrophomonas and 6% of MBL-producing Pseudomonas. Clinical data were available for 94 patients: 83% of them had bloodstream infections. Clinical resolution within 30 days was reported in 80% of infected patients. Analyzing only patients with bloodstream infections (64 patients), death occurred in 19% of patients treated with aztreonam + ceftazidime/avibactam. The combination aztreonam + avibactam appears to be a promising option against MBL-producing bacteria (especially Enterobacterales, much less for Pseudomonas) while waiting for new antimicrobials.

Nationwide Surveillance and Molecular Characterization of Critically Drug-Resistant Gram-Negative Bacteria: Results of the Research University Network Thailand Study

A large-scale surveillance is an important measure to monitor the regional spread of antimicrobial resistance. We prospectively studied the prevalence and molecular characteristics of clinically important Gram-negative bacilli, including Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii complex (ABC), and Pseudomonas aeruginosa, from blood, respiratory tract, urine, and sterile sites at 47 hospitals across Thailand. Among 187,619 isolates, 93,810 isolates (50.0%) were critically drug resistant, of which 12,915 isolates (13.8%) were randomly selected for molecular characterization. E. coli was most commonly isolated from all specimens, except the respiratory tract, in which ABC was predominant. Prevalence of extended-spectrum cephalosporin resistance (ESCR) was higher in E. coli (42.5%) than K. pneumoniae (32.0%), but carbapenem-resistant (CR)-K. pneumoniae (17.2%) was 4.5-fold higher than CR-E. coli (3.8%). The majority of ESCR/CR-E. coli and K. pneumoniae isolates carried bla_CTX-M (64.6% to 82.1%). bla_NDM and bla_OXA-48-like were the most prevalent carbapenemase genes in CR-E. coli/CR-K. pneumoniae (74.9%/52.9% and 22.4%/54.1%, respectively). In addition, 12.9%/23.0% of CR-E. coli/CR-K. pneumoniae cocarried bla_NDM and bla_OXA-48-like. Among ABC isolates, 41.9% were extensively drug resistant (XDR) and 35.7% were multidrug resistant (MDR), while P. aeruginosa showed XDR/MDR at 6.3%/16.5%. A. baumannii was the most common species among ABC isolates. The major carbapenemase gene in MDR-A. baumannii/XDR-A. baumannii was bla_OXA-23-like (85.8%/93.0%), which had much higher rates than other ABC species. bla_IMP, bla_VIM, bla_OXA-40-like, and bla_OXA-58-like were also detected in ABC at lower rates. The most common carbapenemase gene in MDR/XDR-P. aeruginosa was bla_IMP (29.0%/30.6%), followed by bla_VIM (9.5%/25.3%). The findings reiterate an alarming situation of drug resistance that requires serious control measures.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Determining the optimal dosing of a novel combination regimen of ceftazidime/avibactam with aztreonam against NDM-1-producing Enterobacteriaceae using a hollow-fibre infection model

BACKGROUND

MBL-producing strains of Enterobacteriaceae are a major public health concern. We sought to define optimal combination regimens of ceftazidime/avibactam with aztreonam in a hollow-fibre infection model (HFIM) of MBL-producing strains of Escherichia coli and Klebsiella pneumoniae.

METHODS

E. coli ARLG-1013 (blaNDM-1, blaCTX-M, blaCMY, blaTEM) and K. pneumoniae ARLG-1002 (blaNDM-1, blaCTXM-15, blaDHA, blaSHV, blaTEM) were studied in the HFIM using simulated human dosing regimens of ceftazidime/avibactam and aztreonam. Experiments were designed to evaluate the effect of staggered versus simultaneous administration, infusion duration and aztreonam daily dose (6 g/day versus 8 g/day) on bacterial killing and resistance suppression. Prospective validation experiments for the most active combination regimens were performed in triplicate to ensure reproducibility.

RESULTS

Staggered administration of the combination (ceftazidime/avibactam followed by aztreonam) was found to be inferior to simultaneous administration. Longer infusion durations (2 h and continuous infusion) also resulted in enhanced bacterial killing relative to 30 min infusions. The rate of killing was more pronounced with 8 g/day versus 6 g/day aztreonam combination regimens for both tested strains. In the prospective validation experiments, ceftazidime/avibactam with aztreonam dosed every 8 and 6 h, respectively (ceftazidime/avibactam 2/0.5 g every 8 h + aztreonam 2 g every 6 h), or ceftazidime/avibactam with aztreonam as continuous infusions resulted in maximal bacterial killing and resistance suppression over 7 days.

CONCLUSIONS

Simultaneous administration of aztreonam 8 g/day given as a continuous or 2 h infusion with ceftazidime/avibactam resulted in complete bacterial eradication and resistance suppression. Further study of this combination is needed with additional MBL-producing Gram-negative pathogens. The safety of this double β-lactam strategy also warrants further study in Phase 1 clinical trials.

Molecular epidemiology of imipenem resistance in invasive Haemophilus influenzae infections in Germany in 2016

BACKGROUND

The carbapenems imipenem and meropenem play an important role in the empirical anti-infective treatment of critically ill patients. Carbapenem resistance in Haemophilus influenzae (Hi) has rarely been reported.

OBJECTIVES

We provide prevalence data for resistance to carbapenems from laboratory surveillance of invasive Hi infections in Germany in 2016.

METHODS

Phenotypic susceptibility testing against ampicillin, amoxicillin/clavulanate, cefotaxime and imipenem was carried out on 474 isolates from blood and CSF. The isolates were collected as part of the national laboratory surveillance programme. Imipenem-resistant strains were further tested for meropenem susceptibility. Molecular analysis was done by ftsI sequencing to detect mutations in PBP3, by acrR sequencing to detect alterations in the regulatory protein of the AcrAB-TolC efflux pump and by MLST.

RESULTS

No resistance to meropenem was detected. Cefotaxime resistance was rare (n = 3; 0.6%). Imipenem resistance was found in 64 strains (13.5%) using gradient agar diffusion and was confirmed in 26 isolates by broth microdilution (5.5%). Imipenem resistance occurred predominantly in Hi that were β-lactamase negative but ampicillin resistant and in those that were β-lactamase positive but nevertheless amoxicillin/clavulanate resistant. This finding suggested a β-lactamase-independent mechanism. Accordingly, sequence analysis of PBP3 identified previously described mutations. MLST of the imipenem-resistant strains, which were all non-typeable Hi, revealed a high diversity.

CONCLUSIONS

We conclude that imipenem, but not meropenem, resistance is frequent in Hi. It is likely to be supported by PBP3 mutations.

Pan-Resistome Insights into the Multidrug Resistance of Acinetobacter baumannii

Acinetobacter baumannii is an important Gram-negative opportunistic pathogen that is responsible for many nosocomial infections. This etiologic agent has acquired, over the years, multiple mechanisms of resistance to a wide range of antimicrobials and the ability to survive in different environments. In this context, our study aims to elucidate the resistome from the A. baumannii strains based on phylogenetic, phylogenomic, and comparative genomics analyses. In silico analysis of the complete genomes of A. baumannii strains was carried out to identify genes involved in the resistance mechanisms and the phylogenetic relationships and grouping of the strains based on the sequence type. The presence of genomic islands containing most of the resistance gene repertoire indicated high genomic plasticity, which probably enabled the acquisition of resistance genes and the formation of a robust resistome. A. baumannii displayed an open pan-genome and revealed a still constant genetic permutation among their strains. Furthermore, the resistance genes suggest a specific profile within the species throughout its evolutionary history. Moreover, the current study performed screening and characterization of the main genes present in the resistome, which can be used in applied research to develop new therapeutic methods to control this important bacterial pathogen.

Activity of β-Lactam Antibiotics against Metallo-β-Lactamase-Producing Enterobacterales in Animal Infection Models: a Current State of Affairs

Metallo-β-lactamases (MBLs) result in resistance to nearly all β-lactam antimicrobial agents, as determined by currently employed susceptibility testing methods. However, recently reported data demonstrate that variable and supraphysiologic zinc concentrations in conventional susceptibility testing media compared with physiologic (bioactive) zinc concentrations may be mediating discordant in vitro-in vivo MBL resistance. While treatment outcomes in patients appear suggestive of this discordance, these limited data are confounded by comorbidities and combination therapy. To that end, the goal of this review is to evaluate the extent of β-lactam activity against MBL-harboring Enterobacterales in published animal infection model studies and provide contemporary considerations to facilitate the optimization of current antimicrobials and development of novel therapeutics.

Epidemiology of extended-spectrum β-lactamase and metallo-β-lactamase-producing Escherichia coli in South Asia

Aim: To determine the prevalence of extended-spectrum β-lactamase (ESBL) and metallo-β-lactamase (MBL)-producing Escherichia coli in South Asia. Methodology: A systematic review and meta-analysis of data published in PubMed, EMBASE, Web of Science and Scopus. Results: The pooled prevalence of ESBL and MBL-producing E. coli in South Asia were 33% (95% CI: 27-40%) and 17% (95% CI: 12-24%), respectively. The prevalence of bla_CTX-M type was 58% (95% CI: 49-66%) with bla_CTX-M-15 being the most prevalent (51%, 95% CI: 40-62%) variant. The most prevalent MBL variant was bla_NDM-1 (33%, 95% CI: 20-50%). Conclusion: This study suggests a high prevalence of ESBLs and MBLs among E. coli clinical isolates. Comprehensive resistance surveillance is required to guide clinicians prescribing antibiotics in South Asia.

Abundance of New Delhi Metallo-β-Lactamase-Producing Acinetobacter, Escherichia, Proteus, and Pseudomonas spp. in Mahananda and Karala Rivers of India

In this study, we report a high incidence of New Delhi metallo-β-lactamase (NDM)-producing and ampicillin-catabolizing bacteria within carbapenem-resistant bacterial populations in the waters of two important rivers, Mahananda and Karala, bisecting two most populous towns, Siliguri and Jalpaiguri, respectively, in the northern West Bengal, India. Isolates producing NDM belonged to four genera, Acinetobacter, Escherichia, Proteus, and Pseudomonas; among which few were phylogenetically determined as putatively novel species. Class 1 integrons with the frequent presence of aadA and aac(6')-Ib gene cassettes in 50% of NDM-bearing isolates are indicative of possible selective pressures generated out of unregulated use of streptomycin, in agriculture practiced by the cultivators and tea planters living in locales drained by these two rivers, in their up- and downstream, and amikacin in the most crowded government-sponsored "sadar" and district hospitals of Siliguri and Jalpaiguri. NDM-delivering bacteria in rivers have genuine consequences for city inhabitants who are dependent on public water and sanitation facilities. Standard reconnaissance of antibiotic resistance, consolidating ecological sampling just as the assessment of clinical isolates, should be set up as a need.

Carbapenem-Resistant Enterobacterales, Carbapenem Resistant Organisms, Carbapenemase-Producing Enterobacterales, and Carbapenemase-Producing Organisms: Terminology Past its "Sell-By Date" in an Era of New Antibiotics and Regional Carbapenemase Epidemiology

Carbapenem resistance in Gram-negative bacteria is a public health concern. Consequently, numerous government and agency reports discuss carbapenem-resistant Enterobacterales (CRE) and carbapenem-resistant organisms (CROs). Unfortunately, these terms are fuzzy. Do they include (1) Proteeae with inherent imipenem resistance; (2) porin-deficient Enterobacterales resistant to ertapenem but not other carbapenems; (3) Enterobacterales with OXA-48-like enzymes that remain "carbapenem susceptible" at breakpoint; and (4) Pseudomonas aeruginosa that merely lack porin OprD? Counting CPE or CPOs is better but still insufficient, because different carbapenemases have differing treatment implications, particularly for new β-lactam/β-lactamase inhibitor combinations. At the least, it is essential for authors, journals, and regulatory agencies to specify the carbapenemases meant. The future may demand even greater precision, for mutations can alter hydrolytic activity, and the ability to confer resistance, within carbapenemase families.

Activity of ceftolozane/tazobactam against Gram-negative isolates from patients with lower respiratory tract infections - SMART United States 2018-2019

BACKGROUND

Ceftolozane/tazobactam (C/T) is approved in 70 countries, including the United States, for the treatment of patients with hospital-acquired and ventilator-associated bacterial pneumonia caused by susceptible Gram-negative pathogens. C/T is of particular importance as an agent for the treatment of multidrug-resistant (MDR) Pseudomonas aeruginosa infections. The current study summarizes 2018-2019 data from the United States on lower respiratory tract isolates of Gram-negative bacilli from the SMART global surveillance program. The CLSI reference broth microdilution method was used to determine in vitro susceptibility of C/T and comparators against isolates of P. aeruginosa and Enterobacterales.

RESULTS

C/T inhibited 96.0% of P. aeruginosa (n = 1237) at its susceptible MIC breakpoint (≤4 μg/ml), including > 85% of meropenem-nonsusceptible and piperacillin/tazobactam (P/T)-nonsusceptible isolates and 76.2% of MDR isolates. Comparator agents demonstrated lower activity than C/T against P. aeruginosa: meropenem (74.8% susceptible), cefepime (79.2%), ceftazidime (78.5%), P/T (74.4%), and levofloxacin (63.1%). C/T was equally active against ICU (96.0% susceptible) and non-ICU (96.7%) isolates of P. aeruginosa. C/T inhibited 91.8% of Enterobacterales (n = 1938) at its susceptible MIC breakpoint (≤2 μg/ml); 89.5% of isolates were susceptible to cefepime and 88.0% susceptible to P/T. 67.1 and 86.5% of extended-spectrum β-lactamase (ESBL) screen-positive isolates of Klebsiella pneumoniae (n = 85) and Escherichia coli (n = 74) and 49.6% of MDR Enterobacterales were susceptible to C/T. C/T was equally active against ICU (91.3% susceptible) and non-ICU (92.6%) Enterobacterales isolates.

CONCLUSION

Data from the current study support the use of C/T as an important treatment option for lower respiratory tract infections including those caused by MDR P. aeruginosa.

Molecular characterization of two novel NDM-1-producing atypical enteroaggregative Escherichia coli isolates from patients

To investigate NDM-1-producing atypical Enteroaggregative Escherichia coli (aEAEC) of sequence type 349 from hospitalized patients, the isolates 13ZX28 and 13ZX36 were subjected to antimicrobial susceptibility testing, conjugation and whole genome sequencing. Only one single nucleotide mutation was detected in chromosomes despite different plasmid profiles. Both isolates were positive for bla_NDM-1 mediating resistance to carbapenem. A novel plasmid p13ZX28-272 (~272-kb) from 13ZX28 encodes bla_NDM-1. Interestingly, its sequence was identical to the two plasmids p13ZX36-200 (~200-kb) and p13ZX36-70 (~70-kb) from 13ZX36. Formation of the former episome possibly involved homologous recombination through a 4948-bp large fragment located on each of the two latter plasmids. Furthermore, plasmid p13ZX28-272 could be resolved into a ~ 98-kb daughter plasmid by IS26 rearrangement following conjugation. The plasticity of the plasmids is recognized, which warrants further investigation to evaluate the underlying public health risk and understand how antibiotic selection pressure drives this process.

Mobile Carbapenemase Genes in Pseudomonas aeruginosa

Carbapenem-resistant Pseudomonas aeruginosa is one of the major concerns in clinical settings impelling a great challenge to antimicrobial therapy for patients with infections caused by the pathogen. While membrane permeability, together with derepression of the intrinsic beta-lactamase gene, is the global prevailing mechanism of carbapenem resistance in P. aeruginosa, the acquired genes for carbapenemases need special attention because horizontal gene transfer through mobile genetic elements, such as integrons, transposons, plasmids, and integrative and conjugative elements, could accelerate the dissemination of the carbapenem-resistant P. aeruginosa. This review aimed to illustrate epidemiologically the carbapenem resistance in P. aeruginosa, including the resistance rates worldwide and the carbapenemase-encoding genes along with the mobile genetic elements responsible for the horizontal dissemination of the drug resistance determinants. Moreover, the modular mobile elements including the carbapenemase-encoding gene, also known as the P. aeruginosa resistance islands, are scrutinized mostly for their structures.

Co-infections of two carbapenemase-producing Enterobacter hormaechei clinical strains isolated from the same diabetes individual in China

Introduction. Since mcr-1 was first reported in China, there have been ten variants of MCR appearing nationwide so far. Multidrug-resistant Enterobacteriaceae bacteria carrying both NDM and MCR have become a serious threat to global public health.Hypothesis/Gap Statement. The genetic structure of mcr-9 needs to be better understood in order to better prevent and control the transmission of drug-resistant genes.Aims. The aim of this study was to characterize the presence of two Enterobacter hormaechei isolates, which carries bla _NDM-5 CME2 and the coexistence of mcr-9 and bla _NDM-1 strain CMD2, which were isolated from a patient with diabetes in Sichuan, China.Methodology. The microbroth dilution method was used for antibiotic susceptibility. Conjugation experiment was used to investigate the transferability of bla _NDM-1, bla _NDM-5 and mcr-9. Whole-genome sequencing was performed on Illumina HiSeq platform. The ability of biofilm formation was detected by crystal-violet staining, the virulence of the bacteria was measured by Galleria mellonella killing assay.Results. bla _NDM-5 carrier CME2 and CMD2 with bla _NDM-1 and mcr-9 were resistant to carbapenems, β-lactam, aminoglycoside, quinolone and tetracycline, while CMD2 was also resistant to colistin. Conjugation assay and plasmid replicon typing further demonstrated that both bla _NDM-1 and bla _NDM-5 were respectively present on the self-transferrable IncX3 plasmid, mcr-9 was located on the self-transferrable IncHI2 plasmid. Through the analysis of mcr-9 gene context, the structure was DUF4942-rcnR-rcnA-copS-IS903-mcr-9-wbuC-qseC-qseB-IS1R-ΔsilR-IS903, bla _NDM-1 context was IS3000-ΔISAba125-IS5-bla _NDM-1-ble-trpF-groS-groL-insE-ΔIS26 structure, bla _NDM-5 structure was IS3000-bla _NDM-5-ble-trpF-dsbC-ΔIS26-umuD-ISKox3-tnpR-parA. Biofilm formation of CME2 was stronger than CMD2. There was no significant difference in virulence between the two strains.Conclusion. This study reveals two multiple drug-resistant E. hormaechei isolates from diabetes patient samples. E. hormaechei carrying two NDM-resistant genes is already a serious threat, where MCR is an important cause of treatment failure in bacterial infections. This study is a reminder not only to prevent infection in patients with diabetes, but also to constantly monitor the epidemic and spread of the drug-resistant gene.

Therapeutic Options for Metallo-β-Lactamase-Producing Enterobacterales

The spread of metallo-β-lactamase (MBL)-producing Enterobacterales worldwide without the simultaneous increase in active antibiotics makes these organisms an urgent public health threat. This review summarizes recent advancements in diagnostic and treatment strategies for infections caused by MBL-producing Enterobacterales. Adequate treatment of patients infected with MBL-producing Enterobacterales relies on detection of the β-lactamase in the clinic. There are several molecular platforms that are currently available to identify clinically relevant MBLs as well as other important serine-β-lactamases. Once detected, there are several antibiotics that have historically been used for the treatment of MBL-producing Enterobacterales. Antimicrobials such as aminoglycosides, tetracyclines, fosfomycin, and polymyxins often show promising in vitro activity though clinical data are currently lacking to support their widespread use. Ceftazidime-avibactam combined with aztreonam is promising for treatment of infections caused by MBL-producing Enterobacterales and currently has the most clinical data of any available antibiotic to support its use. While cefiderocol has displayed promising activity against MBL-producing Enterobacterales in vitro and in preliminary clinical studies, further clinical studies will better shed light on its place in treatment. Lastly, there are several promising MBL inhibitors in the pipeline, which may further improve the treatment of MBL-producing Enterobacterales.

Evaluation of a Phenotypic Algorithm to Direct Carbapenemase Testing in Pseudomonas aeruginosa: Validation in a Multicenter German Cohort

Pseudomonas aeruginosa remains a prominent nosocomial pathogen. Detection of carbapenemase-producing P. aeruginosa is vital to dictate antimicrobial therapy and infection control measures. A pragmatic, minimum inhibitory concentration-based algorithm using imipenem AND meropenem-resistant plus ceftazidime-, cefepime-, and piperacillin/tazobactam-nonsusceptible criterion was derived to guide carbapenemase testing in P. aeruginosa. This study was an assessment of the algorithm's test performance in a cohort of 985 nonduplicate P. aeruginosa isolates collected from 20 German medical laboratories. Susceptibility data were assessed in the algorithm using both Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) interpretations. Sensitivity and specificity were calculated to evaluate algorithm test performance. The original algorithm criteria resulted in high specificity (95-97%) using both CLSI and EUCAST criteria; however, it failed to capture five carbapenemase-harboring isolates testing piperacillin/tazobactam susceptibility (CLSI/EUCAST). Two carbapenemase-producing isolates were also meropenem susceptible per EUCAST. A modified algorithm utilizing imipenem OR meropenem-resistant plus ceftazidime and cefepime nonsusceptible, improved the sensitivity of the criteria without significantly compromising specificity (CLSI sensitivity/specificity: 96%/94% and EUCAST sensitivity/specificity: 96%/95%). Application of the modified algorithm criteria resulted in high sensitivity and specificity using both CLSI and EUCAST interpretations in a large cohort of clinical P. aeruginosa. Utilization of this algorithm can improve the efficiency of carbapenemase testing in the clinical laboratory.

Evaluation of the Xpert Carba-R NxG Assay for Detection of Carbapenemase Genes in a Global Challenge Set of Pseudomonas aeruginosa Isolates

The growing prevalence and diversity of carbapenemase producers among carbapenem-resistant Pseudomonas aeruginosa (CRPA) isolates warrants an expansion of detection capabilities. The purpose of this study was to evaluate the performance of the commercially available Xpert Carba-R (Carba-R) and the research-use-only Xpert Carba-R NxG (Carba-R NxG) in a global collection of P. aeruginosa The challenge set included 123 P. aeruginosa clinical isolates from 12 countries. Isolates were previously categorized via PCR or whole-genome sequencing. Carbapenemase classes tested include VIM, IMP, NDM, SPM, KPC, and GES. Non-carbapenemase (non-CP)-harboring isolates were also tested (negative control). Isolates were tested using the Carba-R NxG and the Carba-R tests per the manufacturer's instructions. Carba-R NxG testing was completed by Cepheid (Sunnyvale, CA), blinded to genotype. Both assays gave negative results for all non-CP isolates and positive results for all VIM, NDM, and KPC isolates. An improvement in IMP detection among isolates was observed (100% detection by Carba-R NxG versus 58% by Carba-R). All SPM and GES isolates, targets not present in commercially available Carba-R, were positive by Carba-R NxG. Two isolates harbored both VIM and GES, while a third isolate contained VIM and NDM. The Carba-R NxG identified both targets in all 3 isolates, while the Carba-R was negative for both GES-containing isolates. Overall, the Carba-R NxG successfully categorized 100% of isolates tested compared with 68% for its predecessor. The Carba-R NxG will expand the detection spectrum of the current Carba-R assay to include SPM, GES, and expanded IMP variants, increasing the global utility of the test.

Treatment of severe infections due to metallo-β-lactamases-producing Gram-negative bacteria

In the last decades, there was an important paucity of agents for adequately treating infections due to metallo-β-lactamases-producing Gram-negative bacteria (MBL-GNB). Cefiderocol, a novel siderophore cephalosporin showing in vitro activity against MBL-GNB, has been recently marketed, and a combination of aztreonam and ceftazidime/avibactam has shown a possible favorable effect on survival of patients with severe MBL-GNB infections in observational studies. Other agents showing in vitro activity against MBL-GNB are currently in clinical development (e.g., cefepime/taniborbactam, LYS228, cefepime/zidebactam) that could be an important addition to our future armamentarium for severe MBL-GNB infections. Nonetheless, we should not discontinue our efforts to optimize the use of non-β-lactams agents, since they could remain an essential last-resort or alternative option in selected cases.

NDM-1-Positive K. pneumoniae at a Teaching Hospital in Southwestern China: Clinical Characteristics, Antimicrobial Resistance, Molecular Characterization, Biofilm Assay, and Virulence

Background

The emergence of the NDM-1-positive Klebsiella pneumoniae (K. pneumoniae) strains has led to limited therapeutic options for clinical treatment. Understanding the clinical characteristics, antimicrobial resistance, biofilm assay, and the virulence genes of these isolated strains is of great significance.

Methods

The polymerase chain reaction (PCR) was used to screen isolated NDM-1-positive K. pneumoniae. The clinical information of the patients was collected from medical records. The NDM-1-positive K. pneumoniae isolates were subjected to antimicrobial susceptibility testing and multilocus sequence typing. Sixty strains of NDM-1-negative K. pneumoniae isolated during the same period were collected as the control group for the virulence analysis. The virulence phenotype of the strains was preliminarily evaluated by the string test and crystal violet semiquantitative biofilm formation experiment. PCR combined with gene sequencing was used to detect common high toxicity capsule genes (K1, K2, K5, K20, K54, and K57) and common virulence-related genes (entB, ybtS, ureA, ycf, WabG, FimH, uge, iutA, KfuB, aerobactin, rmpA, magA, Alls, IrnN, and VatD).

Results

In the 30 nonduplicated NDM-1-positive K. pneumoniae isolates, 43.33% (13/30) of the patients had a history of a stay in the neonatal intensive care unit (NICU). All of the isolates exhibited multidrug resistance. Nine STs were identified, 77% (10/13) strains from the NICU were ST11. The NDM-1-positive K. pneumoniae string tests were all negative, and 35% (21/60) NDM-1-negative K. pneumoniae were positive. The ratios of NDM-1-positive K. pneumoniae isolates biofilm formation ability according to strong, medium, and weak classification were 67%, 23%, and 10%, respectively. NDM-1-negative K. pneumoniae isolates were 60%, 25%, and 15%, respectively. There was no statistical difference between the two groups (t = 0.61, P=0.2723). The virulence-associated genes with more than 80% of detection rates among the 30 NDM-1-positive K. pneumoniae isolates included entB (100%, 30/30), ybtS (93.33%, 28/30), ureA (90%, 27/30), ycf (83.33%, 25/30), and wabG (90%, 27/30). KfuB and iutA were detected at prevalence of 3.33% and 13.33%. vatD, allS, iroN, aerobactin, and rmpA were not detected. In the NDM-1-negative K. pneumoniae, all other 14 virulence genes except VatD were detected. After statistical analysis, FimH, WabG, ycf, iutA, kfuB, aerobactin, rmpA, and Alls virulence genes, P < 0.005, there was a statistical difference.

Conclusion

NDM-1-positive K. pneumoniae exhibited multidrug resistance, MLST typing is mainly ST11, there is small clonal dissemination in the NICU in the hospital, and the NDM-1-positive K. pneumoniae virulence genes carrier rate is lower than the NDM-1-negative K. pneumoniae virulence genes carrier rate.

Development and Application of a Pragmatic Algorithm to Guide Definitive Carbapenemase Testing to Identify Carbapenemase-Producing Pseudomonas aeruginosa

A minimum inhibitory concentration (MIC) derived algorithm, predictive of carbapenemase production, was developed using a challenge set (n = 92) of Pseudomonas aeruginosa (PA), including carbapenemase-producing (CP), cephalosporinase and/or efflux/porin mutation, and wild-type isolates. Broth microdilution MICs to clinically relevant anti-pseudomonal agents were utilized. The algorithm was applied to 1209 clinical PA isolates from a US surveillance program. Confirmatory genotypic (Xpert^® Carba-R assay) and phenotypic (mCIM/eCIM) testing for carbapenemases was conducted on algorithm-derived isolates. With the algorithm, carbapenem resistance alone resulted in poor specificity to identify CP-PA (54%) within the challenge set of isolates. Inclusion of cefepime, ceftazidime, and piperacillin/tazobactam non-susceptibility resulted in a specificity of 66%. Ceftolozane/tazobactam resistance further improved specificity (89%). Of the 1209 isolates, 116 met criteria (carbapenem-resistant and non-susceptibility to cefepime, ceftazidime, and piperacillin/tazobactam) for confirmatory testing. Carba-R and mCIM/eCIM identified five (all bla_VIM-positive) and seven carbapenemase-producing isolates, respectively. This MIC algorithm combined with genotypic/phenotypic carbapenemase testing is a pragmatic and streamlined approach to identify CP-PA.

Is it time to move away from polymyxins?: evidence and alternatives

Increasing burden of carbapenem resistance and resultant difficult-to-treat infections are of particular concern due to the lack of effective and safe treatment options. More recently, several new agents with activity against certain multidrug-resistant (MDR) and extensive drug-resistant (XDR) Gram-negative pathogens have been approved for clinical use. These include ceftazidime-avibactam, meropenem-vaborbactam, imipenem-cilastatin-relebactam, plazomicin, and cefiderocol. For the management of MBL infections, clinically used triple combination comprising ceftazidime-avibactam and aztreonam is hindered due to non-availability of antimicrobial susceptibility testing methods and lack of information on potential drug-drug interaction leading to PK changes impacting its safety and efficacy. Moreover, in several countries including Indian subcontinent and developing countries, these new agents are yet to be made available. Under these circumstances, polymyxins are the only last resort for the treatment of carbapenem-resistant infections. With the recent evidence of suboptimal PK/PD particularly in lung environment, limited efficacy and increased nephrotoxicity associated with polymyxin use, the Clinical and Laboratory Standards Institute (CLSI) has revised both colistin and polymyxin B breakpoints. Thus, polymyxins 'intermediate' breakpoint for Enterobacterales, P. aeruginosa, and Acinetobacter spp. are now set at ≤ 2 mg/L, implying limited clinical efficacy even for isolates with the MIC value 2 mg/L. This change has questioned the dependency on polymyxins in treating XDR infections. In this context, recently approved cefiderocol and phase 3 stage combination drug cefepime-zidebactam assume greater significance due to their potential to act as polymyxin-supplanting therapies.

The prolonged disruption of a single-course amoxicillin on mice gut microbiota and resistome, and recovery by inulin, Bifidobacterium longum and fecal microbiota transplantation

The usages of antibiotics in treating the pathogenic infections could alter the gut microbiome and associated resistome, causing long term adverse impact on human health. In this study, mice were treated with human-simulated regimen 25.0 mg kg^-1 of amoxicillin for seven days, and their gut microbiota and resistome were characterized using the 16S rRNA amplicons sequencing and the high-throughput qPCR, respectively. Meanwhile, the flora restorations after individual applications of inulin, Bifidobacterium longum (B. longum), and fecal microbiota transplantation (FMT) were analyzed for up to 35 days. The results revealed the prolonged negative impact of single course AMX exposure on mice gut microbiota and resistome. To be specific, pathobionts of Klebsiella and Escherichia-Shigella were significantly enriched, while prebiotics of Bifidobacterium and Lactobacillus were dramatically depleted. Furthermore, β-lactam resistance genes and efflux resistance genes were obviously enriched after amoxicillin exposure. Compared to B. longum, FMT and inulin were demonstrated to preferably restore the gut microbiota via reconstituting microbial community and stimulating specific prebiotic respectively. Such variation of microbiome caused their distinct alleviations on resistome alteration. Inulin earned the greatest elimination on AMX induced ARG abundance and diversity enrichment. FMT and B. longum caused remove of particular ARGs such as ndm-1, blaPER. Network analysis revealed that most of the ARGs were prone to be harbored by Firmicutes and Proteobacteria. In general, gut resistome shift was partly associated with the changing bacterial community structures and transposase and integron. Taken together, these results demonstrated the profound disruption of gut microbiota and resistome after single-course amoxicillin treatment and different restoration by inulin, B. longum and FMT.

Activity of aztreonam in combination with ceftazidime-avibactam against serine- and metallo-β-lactamase-producing Pseudomonas aeruginosa

Existing data support the combination of aztreonam and ceftazidime-avibactam against serine-β-lactamase (SBL)- and metallo-β-lactamase (MBL)-producing Enterobacterales, although there is a paucity of data against SBL- and MBL-producing Pseudomonas aeruginosa. In this study, 5 SBL- and MBL-producing P. aeruginosa (1 IMP, 4 VIM) were evaluated against aztreonam and ceftazidime-avibactam alone and in combination via broth microdilution and time-kill analyses. All 5 isolates were nonsusceptible to aztreonam, aztreonam-avibactam, and ceftazidime-avibactam. Combining aztreonam with ceftazidime-avibactam at subinhibitory concentrations produced synergy and restored bactericidal activity in 4/5 (80%) isolates tested. These results suggest that the combination of aztreonam and ceftazidime-avibactam may be a viable treatment option against SBL- and MBL-producing P. aeruginosa.

β-Lactamase triggered visual detection of bacteria using cephalosporin functionalized biomaterials

We report the conjugation of a chromogenic cephalosporin β-lactamase (βL) substrate to polymers and integration into biomaterials for facile, visual βL detection. Identification of these bacterial enzymes, which are a leading cause of antibiotic resistance, is critical in the treatment of infectious diseases. The βL substrate polymer conjugate undergoes a clear to deep yellow color change upon incubation with common pathogenic Gram-positive and Gram-negative bacteria species. We have demonstrated the feasibility of formulating hydrogels with the βL substrate covalently tethered to a poly(ethylene glycol) (PEG) polymer matrix, exhibiting a visible color change in the presence of βLs. This approach has the potential to be used in diagnostic biomaterials for point-of-care detection of βL-producing bacteria, helping combat the spread of drug resistant microbes.

Characteristics of dual carbapenemase-producing Klebsiella pneumoniae strains from an outbreak in Venezuela: a retrospective study

Objective.

To characterize carbapenemase-producing Klebsiella pneumoniae isolated from patients treated at a hospital in Cumaná, Sucre, Venezuela.

Methods.

This was a retrospective study conducted at the general hospital in Cumaná where 58 K. pneumoniae strains were analyzed for resistance to antimicrobials, specifically carbapenems, in January – June 2015. Production of metallo-β-lactamases and serine carbapenemases was determined by the double-disc synergy test, using EDTA-sodium mercaptoacetic acid and 3-aminophenyl boronic acid discs, respectively. Multiplex-PCR was used to detect genes coding for carbapenemases. Molecular typing using ERIC-PCR determined the presence of clones.

Results.

Four strains of K. pneumoniae resistant to carbapenems were identified. Phenotypic methods for detection of metallo-β-lactamases and serine carbapenemases were positive, and PCR demonstrated the co-presence of bla_NDM and bla_KPC genes in all four strains. ERIC-PCR identified two clones circulating in the hospital.

Conclusions.

Infection control strategies are needed at the central hospital in Cumaná and its surrounding areas to prevent the spread of these pathogens, especially given the high levels of migration from Venezuela to other countries in South America.

Metallo-β-Lactamases: Structure, Function, Epidemiology, Treatment Options, and the Development Pipeline

Modern medicine is threatened by the global rise of antibiotic resistance, especially among Gram-negative bacteria. Metallo-β-lactamase (MBL) enzymes are a particular concern and are increasingly disseminated worldwide, though particularly in Asia. Many MBL producers have multiple further drug resistances, leaving few obvious treatment options. Nonetheless, and more encouragingly, MBLs may be less effective agents of carbapenem resistance in vivo, under zinc limitation, than in vitro Owing to their unique structure and function and their diversity, MBLs pose a particular challenge for drug development. They evade all recently licensed β-lactam-β-lactamase inhibitor combinations, although several stable agents and inhibitor combinations are at various stages in the development pipeline. These potential therapies, along with the epidemiology of producers and current treatment options, are the focus of this review.

Global travel and Gram-negative bacterial resistance; implications on clinical management

INTRODUCTION

Antimicrobial resistance (AR) is escalating worldwide with the potential for dire consequences, global travel contributes to the dissemination of resistant pathogens from one region to another. The World Health Organization identified the rapid emergence and prevalence of carbapenem-resistant Gram-negative species, including Enterobacterales, Acinetobacter baumannii, and Pseudomonas aeruginosa, as an international crisis due to treatment challenges, poor health outcomes, increased mortality, and high economic costs caused by these pathogens.

AREAS COVERED

This review describes key carbapenem-resistant (CR) Gram-negative species, changes in current global and regional trends, AR surveillance and reporting, and identifies drivers of change, specifically travel. Finally, we review clinical implications and challenges of treating CR infections which exist due to widespread dissemination of CR bacteria. A literature search was conducted using PubMed, Google Scholar, Ebsco, and ProQuest (from 2000 to December 2019).

EXPERT OPINION

The level of global travel is increasing, and antimicrobial resistance continues to disseminate worldwide. Healthcare providers risk assessment for AR needs to consider a patient's recent travel history, including pre-travel and intra-travel antimicrobial prescription, and potential exposure based on geography. Patient education, healthcare provider awareness, and access to data and surveillance resources are critical to inform antimicrobial selection and improve health outcomes.

Antibiotic Susceptibility of NDM-Producing Enterobacterales Collected in the United States in 2017 and 2018

The treatment of infections caused by carbapenem-resistant Enterobacterales, especially New Delhi metallo-β-lactamase (NDM)-producing bacteria, is challenging. Although less common in the United States than some other carbapenemase producers, NDM-producing bacteria are a public health threat due to the limited treatment options available. Here, we report on the antibiotic susceptibility of 275 contemporary NDM-producing Enterobacterales collected from 30 U.S. states through the Centers for Disease Control and Prevention's Antibiotic Resistance Laboratory Network. The aims of the study were to determine the susceptibility of these isolates to 32 currently available antibiotics using reference broth microdilution and to explore the in vitro activity of 3 combination agents that are not yet available. Categorical interpretations were determined using Clinical and Laboratory Standards Institute (CLSI) interpretive criteria. For agents without CLSI criteria, Food and Drug Administration (FDA) interpretive criteria were used. The percentage of susceptible isolates did not exceed 90% for any of the FDA-approved antibiotics tested. The antibiotics with breakpoints that had the highest in vitro activity were tigecycline (86.5% susceptible), eravacycline (66.2% susceptible), and omadacycline (59.6% susceptible); 18.2% of isolates were susceptible to aztreonam. All NDM-producing isolates tested were multidrug resistant, and 116 isolates were extensively drug resistant (42.2%); 207 (75.3%) isolates displayed difficult-to-treat resistance. The difficulty in treating infections caused by NDM-producing Enterobacterales highlights the need for containment and prevention efforts to keep these infections from becoming more common.

Approach to the Treatment of Patients with Serious Multidrug-Resistant Pseudomonas aeruginosa Infections

Multidrug resistance(MDR) among Pseudomonas aeruginosa (PSA) isolates presents a significant clinical challenge and can substantially complicate the approach to selection of optimal antibiotic therapy. This review addresses major considerations in antibiotic selection for patients with suspected or documented serious MDR-PSA infections. Common mechanisms contributing to MDR among clinical PSA isolates are summarized. Empiric and definitive therapy considerations are addressed including the potential role of combination therapy. Newer agents with in vitro activity against MDR-PSA (e.g., ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, and cefiderocol) and their potential roles in clinical settings are discussed. Although these newer agents are promising options for the treatment of MDR-PSA, clinical data remain generally limited. Future studies are needed to determine optimal agents for the empiric and definitive treatment of MDR-PSA.

Intravenous Compatibility of Ceftazidime-Avibactam and Aztreonam Using Simulated and Actual Y-site Administration

PURPOSE

The objective of this communication was to determine the intravenous compatibility of ceftazidime/avibactam and aztreonam using simulated and actual Y-site administration.

METHODS

Ceftazidime-avibactam was reconstituted and diluted to concentrations of 8, 25, and 50 mg/mL in 0.9% sodium chloride. Aztreonam was reconstituted and diluted to concentrations of 10 and 20 mg/mL. Each combination of concentrations was tested for compatibility using visual, Tyndall beam, microscopy, turbidity, and pH assessments. Microscopy results were compared to those from sodium chloride 0.9% in water, pH was compared to that at time 0, and turbidity of combinations was compared to that of individual agents. Actual Y-site mixing was conducted over 2-h infusions with samples collected at 0, 1, and 2 h. Test results were evaluated at 0, 1, 2, 4, 8, and 12 h after mixing. All experiments were completed in triplicate.

FINDINGS

Across simulated and actual Y-site experiments, no evidence of incompatibility between combinations of ceftazidime-avibactam + aztreonam was observed. Visual and microscopic tests revealed no particulate matter, color changes, or turbidity. Tyndall beam tests were negative with all combinations. No evidence of incompatibility was observed in turbidity testing. The pH values were consistent across each of the 6 combinations, from immediately after mixing until 12 h after mixing. When the addition of agents was reversed in simulated Y-site experiments, no differences in compatibility were observed. No differences in compatibility between actual and simulated Y-site administration were observed, and there was minimal variability across all replicate experiments.

IMPLICATIONS

Ceftazidime-avibactam, at concentrations of 8, 25, and 50 mg/mL, appeared compatible with aztreonam at concentrations of 10 and 20 mg/mL.

EDTA-modified carbapenem inactivation method (eCIM) for detecting IMP Metallo-β-lactamase-producing Pseudomonas aeruginosa: an assessment of increasing EDTA concentrations

Background

Prompt identification of carbapenemase-harboring organisms is valuable in informing therapeutic and infection-control measures. The modified carbapenem inactivation method (mCIM) and EDTA-modified carbapenem inactivation method (eCIM) are inexpensive and easy to interpret phenotypic tests endorsed by the Clinical and Laboratory Standards Institute (CLSI) for the detection of carbapenemase-harboring Enterobacterales. Only mCIM is endorsed by CLSI for detecting carbapenemase-harboring Pseudomonas aeruginosa. eCIM’s ability to delineate serine and metallo-β-lactamases (MBL) could be advantageous in areas prevalent with carbapenemase-harboring P. aeruginosa. A recent assessment of mCIM/eCIM on MBL-harboring P. aeruginosa demonstrated high eCIM sensitivity for NDMs and VIMs but not for IMP-producers. Therefore, this study aimed to determine whether increasing EDTA concentrations would enhance eCIM sensitivity for a collection of IMP-harboring P. aeruginosa isolates.

Twenty-six IMP-harboring P. aeruginosa isolates were utilized. For test validation, additional P. aeruginosa isolates harboring NDM (n = 3), VIM (n = 3), KPC (n = 8), wild-type (n = 1), and Enterobacterales isolates harboring IMP (n = 6) and NDM (n = 1) were assessed. The mCIM test was conducted as outlined by CLSI. Simultaneously, the eCIM test was performed with the standard 5 mM EDTA concentration and doubling EDTA concentrations: 10 mM, 20 mM, and 40 mM.

Results

Concentration-dependent improvement was observed among the IMP-harboring P. aeruginosa with eCIM sensitivities at 0, 31, 85, and 100% respectively. Remaining Enterobacterales and P. aeruginosa responded concordantly with their genotype at the standard 5 mM eCIM concentration, with doubling EDTA concentrations providing no greater sensitivity.

Conclusion

Combination of mCIM and an eCIM with a 40 mM EDTA concentration appropriately capture IMP-harboring P. aeruginosa without sacrificing test utility for other carbapenemase-harboring isolates.

Activity of nacubactam (RG6080/OP0595) combinations against MBL-producing Enterobacteriaceae

BACKGROUND

Diazabicyclooctanes (DBOs) are promising β-lactamase inhibitors. Some, including nacubactam (OP0595/RG6080), also bind PBP2 and have an enhancer effect, allowing activity against Enterobacteriaceae with MBLs, which DBOs do not inhibit. We tested the activity of nacubactam/β-lactam combinations against MBL-producing Enterobacteriaceae.

METHODS

Test panels comprised (i) 210 consecutive Enterobacteriaceae with NDM or VIM MBLs, as referred by UK diagnostic laboratories, and (ii) 99 supplementary MBL-producing Enterobacteriaceae, representing less prevalent phenotypes, species and enzymes. MICs were determined by CLSI agar dilution.

RESULTS

MICs of nacubactam alone were bimodal, clustering at 1-8 mg/L or >32 mg/L; >85% of values for Escherichia coli and Enterobacter spp. fell into the low MIC cluster, whereas Proteeae were universally resistant and the Klebsiella spp. were divided between the two groups. Depending on the prospective breakpoint (4 + 4 or 8 + 4 mg/L), and on whether all isolates were considered or solely the Consecutive Collection, meropenem/nacubactam and cefepime/nacubactam inhibited 80.3%-93.3% of MBL producers, with substantial gains over nacubactam alone. Against the most resistant isolates (comprising 57 organisms with MICs of nacubactam >32 mg/L, cefepime ≥128 mg/L and meropenem ≥128 mg/L), cefepime/nacubactam at 8 + 4 mg/L inhibited 63.2% and meropenem/nacubactam at 8 + 4 mg/L inhibited 43.9%. Aztreonam/nacubactam, incorporating an MBL-stable β-lactam partner, was almost universally active against the MBL producers and, unlike aztreonam/avibactam, had an enhancer effect.

CONCLUSIONS

Nacubactam combinations, including those using MBL-labile β-lactams, e.g. meropenem and cefepime, can overcome most MBL-mediated resistance. This behaviour reflects nacubactam's direct antibacterial and enhancer activity.

Multidrug Resistance (MDR) and Collateral Sensitivity in Bacteria, with Special Attention to Genetic and Evolutionary Aspects and to the Perspectives of Antimicrobial Peptides-A Review

Antibiotic poly-resistance (multidrug-, extreme-, and pan-drug resistance) is controlled by adaptive evolution. Darwinian and Lamarckian interpretations of resistance evolution are discussed. Arguments for, and against, pessimistic forecasts on a fatal “post-antibiotic era” are evaluated. In commensal niches, the appearance of a new antibiotic resistance often reduces fitness, but compensatory mutations may counteract this tendency. The appearance of new antibiotic resistance is frequently accompanied by a collateral sensitivity to other resistances. Organisms with an expanding open pan-genome, such as Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae, can withstand an increased number of resistances by exploiting their evolutionary plasticity and disseminating clonally or poly-clonally. Multidrug-resistant pathogen clones can become predominant under antibiotic stress conditions but, under the influence of negative frequency-dependent selection, are prevented from rising to dominance in a population in a commensal niche. Antimicrobial peptides have a great potential to combat multidrug resistance, since antibiotic-resistant bacteria have shown a high frequency of collateral sensitivity to antimicrobial peptides. In addition, the mobility patterns of antibiotic resistance, and antimicrobial peptide resistance, genes are completely different. The integron trade in commensal niches is fortunately limited by the species-specificity of resistance genes. Hence, we theorize that the suggested post-antibiotic era has not yet come, and indeed might never come.

Ceftazidime-Avibactam in Combination With Fosfomycin: A Novel Therapeutic Strategy Against Multidrug-Resistant Pseudomonas aeruginosa

Previously, by targeting penicillin-binding protein 3, Pseudomonas-derived cephalosporinase (PDC), and MurA with ceftazidime-avibactam-fosfomycin, antimicrobial susceptibility was restored among multidrug-resistant (MDR) Pseudomonas aeruginosa. Herein, ceftazidime-avibactam-fosfomycin combination therapy against MDR P. aeruginosa clinical isolate CL232 was further evaluated. Checkerboard susceptibility analysis revealed synergy between ceftazidime-avibactam and fosfomycin. Accordingly, the resistance elements present and expressed in P. aeruginosa were analyzed using whole-genome sequencing and transcriptome profiling. Mutations in genes that are known to contribute to β-lactam resistance were identified. Moreover, expression of blaPDC, the mexAB-oprM efflux pump, and murA were upregulated. When fosfomycin was administered alone, the frequency of mutations conferring resistance was high; however, coadministration of fosfomycin with ceftazidime-avibactam yielded a lower frequency of resistance mutations. In a murine infection model using a high bacterial burden, ceftazidime-avibactam-fosfomycin significantly reduced the P. aeruginosa colony-forming units (CFUs), by approximately 2 and 5 logs, compared with stasis and in the vehicle-treated control, respectively. Administration of ceftazidime-avibactam and fosfomycin separately significantly increased CFUs, by approximately 3 logs and 1 log, respectively, compared with the number at stasis, and only reduced CFUs by approximately 1 log and 2 logs, respectively, compared with the number in the vehicle-treated control. Thus, the combination of ceftazidime-avibactam-fosfomycin was superior to either drug alone. By employing a "mechanism-based approach" to combination chemotherapy, we show that ceftazidime-avibactam-fosfomycin has the potential to offer infected patients with high bacterial burdens a therapeutic hope against infection with MDR P. aeruginosa that lack metallo-β-lactamases.