In vitro antimicrobial activity of six novel β-lactam and β-lactamase inhibitor combinations and cefiderocol against NDM-producing Enterobacterales in China

INTRODUCTION

To date, the global prevalence of New Delhi metallo-β-lactamase (NDM) in carbapenem-resistant Enterobacterales (CRE) has been of concern, which is not inhibited by classical β-lactamase inhibitors (BLIs). In this study, we investigated the newly developed antimicrobial agents or inhibitors against NDM-producing Enterobacterales (NPEs).

METHODS

The in vitro activities of cefiderocol, cefepime/taniborbactam, meropenem/taniborbactam, cefepime/zidebactam, meropenem/nacubactam, aztreonam/nacubactam and aztreonam/avibactam were analyzed in 204 NPE strains collected in China. The potential resistance mechanisms were identified by whole genome sequencing.

RESULTS

Of 204 NPE strains, 18.1% (37/204) were resistant to cefiderocol, in which cirA deleterious alteration, PBP3 insertion and NDM production were taken as potential resistance mechanisms; 28.9% (59/204) were resistant to cefepime/zidebactam, involving K. pneumoniae with ompK35 deleterious alteration; 22.5% (46/204) were resistant to cefepime/taniborbactam, in which YRIN or YRIK inserted in PBP3 and altered ompC are more frequently detected in the resistant E. coli isolates; 27.9% (57/204) were resistant to meropenem/taniborbactam. Aztreonam/avibactam and aztreonam/nacubactam exhibited excellent activity against NPE. However, meropenem/nacubactam had the lowest activity, with only 49.0% (100/204) of all isolates having MICs of <4/4 mg/L.

CONCLUSIONS

Aztreonam/avibactam and aztreonam/nacubactam showed the highest activity against NPE. The potential resistance mechanisms of novel antimicrobial agents against NPE should be under active surveillance.

The Growing Threat of NDM-Producing Escherichia coli With Penicillin-Binding Protein 3 Mutations in the United States-Is There a Potential Role for Durlobactam?

We report identification of 5 patients with infections caused by NDM-5-producing Escherichia coli harboring PBP3 mutations that showed reduced susceptibility to aztreonam-avibactam and cefiderocol. Durlobactam, a novel diazabicyclooctane β-lactamase inhibitor, demonstrated minimum inhibitory concentrations ranging from 0.5 to 2 µg/mL supporting future investigations into a potential role in clinical management.

Rapid Emergence of Resistance to Broad-Spectrum Direct Antimicrobial Activity of Avibactam

Avibactam (AVI) is a diazabicyclooctane (DBO) β-lactamase inhibitor used clinically in combination with ceftazidime. At concentrations higher than those typically achieved in vivo, it also has broad-spectrum direct antibacterial activity against Enterobacterales strains, including metallo-β-lactamase-producing isolates, mediated by inhibition of penicillin-binding protein 2 (PBP2). This activity is mechanistically similar to that of more potent novel DBOs (zidebactam, nacubactam) in late clinical development. We found that resistance to AVI emerged readily, with a mutation frequency of 2×10-6 to 8×10-5. Whole genome sequencing of resistant isolates revealed a heterogeneous mutational target that permitted bacterial survival and replication despite PBP2 inhibition, in line with prior studies of PBP2-targeting drugs. While such mutations are believed to act by upregulating the bacterial stringent response, we found a similarly high mutation frequency in bacteria deficient in components of the stringent response, although we observed a different set of mutations in these strains. Although avibactam-resistant strains had increased lag time, suggesting a fitness cost that might render them less problematic in clinical infections, there was no statistically significant difference in growth rates between susceptible and resistant strains. The finding of rapid emergence of resistance to avibactam as the result of a large mutational target has important implications for novel DBOs with potent direct antibacterial activity, which are being developed with the goal of expanding cell wall-active treatment options for multidrug-resistant gram-negative infections but may be vulnerable to treatment-emergent resistance.

A role for the stringent response in ciprofloxacin resistance in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a major cause of nosocomial infections and the leading cause of chronic lung infections in cystic fibrosis and chronic obstructive pulmonary disease patients. Antibiotic treatment remains challenging because P. aeruginosa is resistant to high concentrations of antibiotics and has a remarkable ability to acquire mutations conferring resistance to multiple groups of antimicrobial agents. Here we report that when P. aeruginosa is plated on ciprofloxacin (cipro) plates, the majority of cipro-resistant (ciproR) colonies observed at and after 48 h of incubation carry mutations in genes related to the Stringent Response (SR). Mutations in one of the major SR components, spoT, were present in approximately 40% of the ciproR isolates. Compared to the wild-type strain, most of these isolates had decreased growth rate, longer lag phase and altered intracellular ppGpp content. Also, 75% of all sequenced mutations were insertions and deletions, with short deletions being the most frequently occurring mutation type. We present evidence that most of the observed mutations are induced on the selective plates in a subpopulation of cells that are not instantly killed by cipro. Our results suggests that the SR may be an important contributor to antibiotic resistance acquisition in P. aeruginosa.

In vitro synergy of the combination of sulbactam-durlobactam and cefepime at clinically relevant concentrations against A. baumannii, P. aeruginosa and Enterobacterales

BACKGROUND

Sulbactam-durlobactam is a potent combination active against Acinetobacter baumannii; however, it lacks activity against other nosocomial pathogens. Cefepime is a common first-line therapy for hospital/ventilator-associated pneumonia caused by Gram-negative pathogens including Pseudomonas aeruginosa and Enterobacterales. With increasing resistance to cefepime, and the significant proportion of polymicrobial nosocomial infections, effective therapy for infections caused by Acinetobacter baumannii, P. aeruginosa and Enterobacterales is needed. This study investigated the in vitro synergy of sulbactam-durlobactam plus cefepime against relevant pathogens.

METHODS

Static time-kills assays were performed in duplicate against 14 cefepime-resistant isolates (A. baumannii, n = 4; P. aeruginosa, n = 4; Escherichia coli, n = 3; Klebsiella pneumoniae, n = 3). One WT K. pneumoniae isolate was included. Antibiotic concentrations simulated the free-steady state average concentration of clinically administered doses in patients.

RESULTS

Sulbactam-durlobactam alone showed significant activity against A. baumannii consistent with the MIC values. Sulbactam-durlobactam plus cefepime showed synergy against one A. baumannii isolate with an elevated MIC to sulbactam-durlobactam (32 mg/L). Against all P. aeruginosa isolates, synergy was observed with sulbactam-durlobactam plus cefepime. For the Enterobacterales, one E. coli isolate demonstrated synergy while the others were indifferent due to significant kill from sulbactam-durlobactam alone. The combination of sulbactam-durlobactam plus cefepime showed synergy against one of the K. pneumoniae and additive effects against the other two K. pneumoniae tested. No antagonism was observed in any isolates including the WT strain.

CONCLUSIONS

Synergy and no antagonism was observed with a combination of sulbactam-durlobactam and cefepime; further in vivo pharmacokinetic/pharmacodynamics data and clinical correlation are necessary to support our findings.

Penicillin-binding protein (PBP) inhibitor development: A 10-year chemical perspective

Bacterial cell wall formation is essential for cellular survival and morphogenesis. The peptidoglycan (PG), a heteropolymer that surrounds the bacterial membrane, is a key component of the cell wall, and its multistep biosynthetic process is an attractive antibacterial development target. Penicillin-binding proteins (PBPs) are responsible for cross-linking PG stem peptides, and their central role in bacterial cell wall synthesis has made them the target of successful antibiotics, including β-lactams, that have been used worldwide for decades. Following the discovery of penicillin, several other compounds with antibiotic activity have been discovered and, since then, have saved millions of lives. However, since pathogens inevitably become resistant to antibiotics, the search for new active compounds is continuous. The present review highlights the ongoing development of inhibitors acting mainly in the transpeptidase domain of PBPs with potential therapeutic applications for the development of new antibiotic agents. Both the critical aspects of the strategy, design, and structure-activity relationships (SAR) are discussed, covering the main published articles over the last 10 years. Some of the molecules described display activities against main bacterial pathogens and could open avenues toward the development of new, efficient antibacterial drugs.

Targeting Essential Hypothetical Proteins of Pseudomonas aeruginosa PAO1 for Mining of Novel Therapeutics: An In Silico Approach

As an omnipresent opportunistic bacterium, Pseudomonas aeruginosa PAO1 is responsible for acute and chronic infection in immunocompromised individuals. Currently, this bacterium is on WHO's red list where new antibiotics are urgently required for the treatment. Finding essential genes and essential hypothetical proteins (EHP) can be crucial in identifying novel druggable targets and therapeutics. This study is aimed at characterizing these EHPs and analyzing subcellular and physiochemical properties, PPI network, nonhomologous analysis against humans, virulence factor and novel drug target prediction, and finally structural analysis of the identified target employing around 42 robust bioinformatics tools/databases, the output of which was evaluated using the ROC analysis. The study discovered 18 EHPs from 336 essential genes, with domain and functional annotation revealing that 50% of these proteins belong to the enzyme category. The majority are cytoplasmic and cytoplasmic membrane proteins, with half being stable proteins subjected to PPIs network analysis. The network contains 261 nodes and 269 edges for 9 proteins of interest, with 11 hubs containing at least three nodes each. Finally, a pipeline builder predicts 7 proteins with novel drug targets, 5 nonhomologous proteins against human proteome, human antitargets, and human gut flora, and 3 virulent proteins. Among these, homology modeling of NP_249450 and NP_251676 was done, and the Ramachandran plot analysis revealed that more than 94% of the residues were in the preferred region. By analyzing functional attributes and virulence characteristics, the findings of this study may facilitate the development of innovative antibacterial drug targets and drugs of Pseudomonas aeruginosa PAO1.

Activity of aztreonam in combination with novel β-lactamase inhibitors against metallo-β-lactamase-producing Enterobacterales from Spain

Metallo-β-lactamase (MBL)-producing Enterobacterales are of particular concern because they are widely disseminated and difficult to treat, being resistant to almost all β-lactam antibiotics. Aztreonam is not hydrolysed by MBLs but is labile to serine β-lactamases (SBLs), which are usually co-produced by MBL-producing Enterobacterales. This study investigated the activity of aztreonam in combination with novel β-lactamase inhibitors (BLIs) against a national multi-centre study collection of strains co-producing MBLs and SBLs. Fifty-five clinical isolates co-producing MBLs (41 VIM producers, 10 NDM producers and 4 IMP producers) and SBLs were selected, and whole-genome sequencing (WGS) was performed. The minimum inhibitory concentration (MIC) values of aztreonam, aztreonam/avibactam, aztreonam/relebactam, aztreonam/zidebactam, aztreonam/taniborbactam, aztreonam/vaborbactam and aztreonam/enmetazobactam were determined. β-lactam/BLI resistance mechanisms were analysed by WGS. All BLIs decreased the MIC values of aztreonam for strains that were not susceptible to aztreonam. Aztreonam/zidebactam (MIC ≤1 mg/L for 96.4% of isolates), aztreonam/avibactam (MIC ≤1 mg/L for 92.7% of isolates) and aztreonam/taniborbactam (MIC ≤1 mg/L for 87.3 % of isolates) were the most active combinations. For other aztreonam/BLI combinations, 50-70% of the isolates yielded MIC values ≤1 mg/L. WGS data revealed that mutations in PBP3, defective OmpE35/OmpK35 porins, and the presence of extended-spectrum β-lactamases and class C β-lactamases were some of the resistance mechanisms involved in reduced susceptibility to aztreonam/BLIs. Combinations of aztreonam with new BLIs show promising activity against Enterobacterales co-producing MBLs and SBLs, particularly aztreonam/zidebactam, aztreonam/avibactam and aztreonam/taniborbactam. The present results show that these novel drugs may represent innovative therapeutic strategies by their use in yet-unexplored combinations as solutions for difficult-to-treat infections.

Intrinsic Antibacterial Activity of Xeruborbactam In Vitro : Assessing Spectrum and Mode of Action

Xeruborbactam (formerly QPX7728) is a cyclic boronate inhibitor of numerous serine and metallo-beta-lactamases. At concentrations generally higher than those required for beta-lactamase inhibition, xeruborbactam has direct antibacterial activity against some Gram-negative bacteria, with MIC₅₀/MIC₉₀ values of 16/32 μg/mL and 16/64 μg/mL against carbapenem-resistant Enterobacterales and carbapenem-resistant Acinetobacter baumannii, respectively (the MIC₅₀/MIC₉₀ values against Pseudomonas aeruginosa are >64 μg/mL). In Klebsiella pneumoniae, inactivation of OmpK36 alone or in combination with OmpK35 resulted in 2- to 4-fold increases in the xeruborbactam MIC. In A. baumannii and P. aeruginosa, AdeIJK and MexAB-OprM, respectively, affected xeruborbactam’s antibacterial potency (the MICs were 4- to 16-fold higher in efflux-proficient strains). In Escherichia coli and K. pneumoniae, the 50% inhibitory concentrations (IC₅₀s) of xeruborbactam’s binding to penicillin-binding proteins (PBPs) PBP1a/PBP1b, PBP2, and PBP3 were in the 40 to 70 μM range; in A. baumannii, xeruborbactam bound to PBP1a, PBP2, and PBP3 with IC₅₀s of 1.4 μM, 23 μM, and 140 μM, respectively. Treating K. pneumoniae and P. aeruginosa with xeruborbactam at 1× and 2× MIC resulted in changes of cellular morphology similar to those observed with meropenem; the morphological changes observed after treatment of A. baumannii were consistent with inhibition of multiple PBPs but were unique to xeruborbactam compared to the results for control beta-lactams. No single-step xeruborbactam resistance mutants were obtained after selection at 4× MIC of xeruborbactam using wild-type strains of E. coli, K. pneumoniae, and A. baumannii; mutations selected at 2× MIC in K. pneumoniae did not affect antibiotic potentiation by xeruborbactam through beta-lactamase inhibition. Consistent with inhibition of PBPs, xeruborbactam enhanced the potencies of beta-lactam antibiotics even against strains that lacked beta-lactamase. In a large panel of KPC-producing clinical isolates, the MIC₉₀ values of meropenem tested with xeruborbactam (8 μg/mL) were at least 4-fold lower than those in combination with vaborbactam at 64 μg/mL, the concentration of vaborbactam that is associated with complete inhibition of KPC. The additional enhancement of the potency of beta-lactam antibiotics beyond beta-lactamase inhibition may contribute to the potentiation of beta-lactam antibiotics by xeruborbactam.

Rational design of a new antibiotic class for drug-resistant infections

The development of new antibiotics to treat infections caused by drug-resistant Gram-negative pathogens is of paramount importance as antibiotic resistance continues to increase worldwide¹. Here we describe a strategy for the rational design of diazabicyclooctane inhibitors of penicillin-binding proteins from Gram-negative bacteria to overcome multiple mechanisms of resistance, including β-lactamase enzymes, stringent response and outer membrane permeation. Diazabicyclooctane inhibitors retain activity in the presence of β-lactamases, the primary resistance mechanism associated with β-lactam therapy in Gram-negative bacteria^2,3. Although the target spectrum of an initial lead was successfully re-engineered to gain in vivo efficacy, its ability to permeate across bacterial outer membranes was insufficient for further development. Notably, the features that enhanced target potency were found to preclude compound uptake. An improved optimization strategy leveraged porin permeation properties concomitant with biochemical potency in the lead-optimization stage. This resulted in ETX0462, which has potent in vitro and in vivo activity against Pseudomonas aeruginosa plus all other Gram-negative ESKAPE pathogens, Stenotrophomonas maltophilia and biothreat pathogens. These attributes, along with a favourable preclinical safety profile, hold promise for the successful clinical development of the first novel Gram-negative chemotype to treat life-threatening antibiotic-resistant infections in more than 25 years.

Activity of cefepime/zidebactam (WCK 5222) against 'problem' antibiotic-resistant Gram-negative bacteria sent to a national reference laboratory

BACKGROUND

Triple-action diazabicyclooctanes, e.g. zidebactam, combine β-lactamase inhibition, antibacterial activity, and 'enhancement' of PBP3-targeted β-lactams.

OBJECTIVES

To examine the activity of cefepime/zidebactam against consecutive 'problem' Gram-negative bacteria referred to the UK national reference laboratory.

METHODS

MICs were determined by BSAC agar dilution for 1632 Enterobacterales, 745 Pseudomonas aeruginosa and 450 other non-fermenters, categorized by carbapenemase detection and interpretive reading.

RESULTS

Universal susceptibility to cefepime/zidebactam 8 + 8 mg/L was seen for otherwise multidrug-resistant Enterobacterales with AmpC, extended-spectrum, K1, KPC and OXA-48-like β-lactamases, or with impermeability and 'unassigned' mechanisms. Unlike ceftazidime/avibactam and all other comparators, cefepime/zidebactam 8 + 8 mg/L also inhibited most (190/210, 90.5%) Enterobacterales with MBLs. Resistance in the remaining minority of MBL producers, and in 13/24 with both NDM MBLs and OXA-48-like enzymes, was associated with Klebsiella pneumoniae ST14. For Pseudomonas aeruginosa, MICs of cefepime/zidebactam rose with efflux grade, but exceeded 8 + 8 mg/L for only 11/85 isolates even in the highly-raised efflux group. Among 103 P. aeruginosa with ESBLs or MBLs, 97 (94.5%) were inhibited by cefepime/zidebactam 8 + 8 mg/L whereas fewer than 15% were susceptible to any comparator. MICs for Acinetobacter baumannii with acquired OXA carbapenemases clustered around 8 + 8 to 32 + 32 mg/L, with higher values for MBL producers. A strong enhancer effect augmented activity against many isolates that were highly resistant to cefepime and zidebactam alone and which had mechanisms not inhibited by zidebactam.

CONCLUSIONS

Assuming successful clinical trials, cefepime/zidebactam has scope to widely overcome critical resistances in both Enterobacterales and non-fermenters.

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.

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.

Interplay between β-lactamases and new β-lactamase inhibitors

Resistance to β-lactam antibiotics in Gram-negative bacteria is commonly associated with production of β-lactamases, including extended-spectrum β-lactamases (ESBLs) and carbapenemases belonging to different molecular classes: those with a catalytically active serine and those with at least one active-site Zn²⁺ to facilitate hydrolysis. To counteract the hydrolytic activity of these enzymes, combinations of a β-lactam with a β-lactamase inhibitor (BLI) have been clinically successful. However, some β-lactam-BLI combinations have lost their effectiveness against prevalent Gram-negative pathogens that produce ESBLs, carbapenemases or multiple β-lactamases in the same organism. In this Review, descriptions are provided for medically relevant β-lactamase families and various BLI combinations that have been developed or are under development. Recently approved inhibitor combinations include the inhibitors avibactam and vaborbactam of the diazabicyclooctanone and boronic acid inhibitor classes, respectively, as new scaffolds for future inhibitor design.

The latest advances in β-lactam/β-lactamase inhibitor combinations for the treatment of Gram-negative bacterial infections

Introduction: Antimicrobial resistance in Gram-negative pathogens is a significant threat to global health. β-Lactams (BL) are one of the safest and most-prescribed classes of antibiotics on the market today. The acquisition of β-lactamases, especially those which hydrolyze carbapenems, is eroding the efficacy of BLs for the treatment of serious infections. During the past decade, significant advances were made in the development of novel BL-β-lactamase inhibitor (BLI) combinations to target β-lactamase-mediated resistant Gram-negatives.Areas covered: The latest progress in 20 different approved, developing, and preclinical BL-BLI combinations to target serine β-lactamases produced by Gram-negatives are reviewed based on primary literature, conference abstracts (when available), and US clinical trial searches within the last 5 years. The majority of the compounds that are discussed are being evaluated as part of a BL-BLI combination.Expert opinion: The current trajectory in BLI development is promising; however, a significant challenge resides in the selection of an appropriate BL partner as well as the development of resistance linked to the BL partner. In addition, dosing regimens for these BL-BLI combinations need to be critically evaluated. A revolution in bacterial diagnostics is essential to aid clinicians in the appropriate selection of novel BL-BLI combinations for the treatment of serious infections.

Selection of mutants with resistance or diminished susceptibility to ceftazidime/avibactam from ESBL- and AmpC-producing Enterobacteriaceae

Introduction

Difficult Gram-negative infections are increasingly treated with new β-lactamase inhibitor combinations, e.g. ceftazidime/avibactam. Disturbingly, mutations in KPC carbapenemases can confer ceftazidime/avibactam resistance, which is sometimes selected during therapy. We explored whether this risk extended to AmpC and ESBL enzymes.

Methods

Mutants were selected by plating AmpC-derepressed strains, ESBL producers and ceftazidime-susceptible controls on agar containing ceftazidime + avibactam (1 or 4 mg/L). MICs were determined by CLSI agar dilution; WGS was by Illumina methodology.

Results

Using 2× MIC of ceftazidime + 1 mg/L avibactam, mutants were selected from all strain types at frequencies of 10-7-10-9. Rates diminished to <10-9 with 4 mg/L avibactam or higher MIC multiples, except with AmpC-derepressed Enterobacteriaceae. Characterized mutants (n = 10; MICs 4-64 mg/L) of AmpC-derepressed strains had modifications in ampC, variously giving Arg168Pro/His, Gly176Arg/Asp, Asn366Tyr or small deletions around positions 309-314. Mutants of ESBL producers (n = 19; MICs 0.5-16 mg/L) mostly had changes affecting permeability, efflux or β-lactamase quantity; only one had an altered β-lactamase, with an Asp182Tyr substitution in CTX-M-15, raising the ceftazidime/avibactam MIC, but abrogating other cephalosporin resistance. Mutants of ceftazidime-susceptible strains were not sequenced, but phenotypes suggested altered drug accumulation or, for Enterobacter cloacae only, AmpC derepression. In further experiments, avibactam reduced, but did not abolish, selection of AmpC-derepressed Enterobacteriaceae by ceftazidime.

Conclusions

Most mutants of AmpC-derepressed Enterobacteriaceae had structural mutations in ampC; those of ESBL producers mostly had genetic modifications outside β-lactamase genes, commonly affecting uptake, efflux, or β-lactamase quantity. The clinical significance of these observations remains to be determined.

Antibiotic Killing of Diversely Generated Populations of Nonreplicating Bacteria

Nonreplicating bacteria are known to be (or at least commonly thought to be) refractory to antibiotics to which they are genetically susceptible. Here, we explore the sensitivity to killing by bactericidal antibiotics of three classes of nonreplicating populations of planktonic bacteria: (i) stationary phase, when the concentration of resources and/or nutrients are too low to allow for population growth; (ii) persisters, minority subpopulations of susceptible bacteria surviving exposure to bactericidal antibiotics; and (iii) antibiotic-static cells, bacteria exposed to antibiotics that prevent their replication but kill them slowly if at all, the so-called bacteriostatic drugs. Using experimental populations of Staphylococcus aureus Newman and Escherichia coli K-12 (MG1655) and, respectively, nine and seven different bactericidal antibiotics, we estimated the rates at which these drugs kill these different types of nonreplicating bacteria. In contrast to the common belief that bacteria that are nonreplicating are refractory to antibiotic-mediated killing, all three types of nonreplicating populations of these Gram-positive and Gram-negative bacteria are consistently killed by aminoglycosides and the peptide antibiotics daptomycin and colistin, respectively. This result indicates that nonreplicating cells, irrespectively of why they do not replicate, have an almost identical response to bactericidal antibiotics. We discuss the implications of these results to our understanding of the mechanisms of action of antibiotics and the possibility of adding a short-course of aminoglycosides or peptide antibiotics to conventional therapy of bacterial infections.

Structural Basis for E. coli Penicillin Binding Protein (PBP) 2 Inhibition, a Platform for Drug Design

Penicillin-binding proteins (PBPs) are the targets of the β-lactams, the most successful class of antibiotics ever developed against bacterial infections. Unfortunately, the worldwide and rapid spread of large spectrum β-lactam resistance genes such as carbapenemases is detrimental to the use of antibiotics in this class. New potent PBP inhibitors are needed, especially compounds that resist β-lactamase hydrolysis. Here we describe the structure of the E. coli PBP2 in its Apo form and upon its reaction with 2 diazabicyclo derivatives, avibactam and CPD4, a new potent PBP2 inhibitor. Examination of these structures shows that unlike avibactam, CPD4 can perform a hydrophobic stacking on Trp370 in the active site of E. coli PBP2. This result, together with sequence analysis, homology modeling, and SAR, allows us to propose CPD4 as potential starting scaffold to develop molecules active against a broad range of bacterial species at the top of the WHO priority list.

Frequency and Mechanism of Spontaneous Resistance to Sulbactam Combined with the Novel β-Lactamase Inhibitor ETX2514 in Clinical Isolates of Acinetobacter baumannii

The novel diazabicyclooctenone ETX2514 is a potent, broad-spectrum serine β-lactamase inhibitor that restores sulbactam activity against resistant Acinetobacter baumannii The frequency of spontaneous resistance to sulbactam-ETX2514 in clinical isolates was found to be 7.6 × 10^-10 to <9.0 × 10^-10 at 4× MIC and mapped to residues near the active site of penicillin binding protein 3 (PBP3). Purified mutant PBP3 proteins demonstrated reduced affinity for sulbactam. In a sulbactam-sensitive isolate, resistance also mapped to stringent response genes associated with resistance to PBP2 inhibitors, suggesting that in addition to β-lactamase inhibition, ETX2514 may enhance sulbactam activity in A. baumannii via inhibition of PBP2.

In vitro activity of cefepime/zidebactam (WCK 5222) against Gram-negative bacteria

Objectives

Diazabicyclooctanes (DBOs) inhibit class A, class C and some class D β-lactamases. A few also bind PBP2, conferring direct antibacterial activity and a β-lactamase-independent 'enhancer' effect, potentiating β-lactams targeting PBP3. We tested a novel DBO, zidebactam, combined with cefepime.

Methods

CLSI agar dilution MICs were determined with cefepime/zidebactam in a chequerboard format. Bactericidal activity was also measured.

Results

Zidebactam MICs were ≤2 mg/L (mostly 0.12-0.5 mg/L) for most Escherichia coli , Klebsiella , Citrobacter and Enterobacter spp., but were >32 mg/L for Proteeae, most Serratia and a few E. coli , Klebsiella and Enterobacter/Citrobacter . The antibacterial activity of zidebactam dominated chequerboard studies for Enterobacteriaceae, but potentiation of cefepime was apparent for zidebactam-resistant isolates with class A and C enzymes, illustrating β-lactamase inhibition. Overall, cefepime/zidebactam inhibited almost all Enterobacteriaceae with AmpC, ESBL, K1, KPC and OXA-48-like β-lactamases at 1 + 1 mg/L and also 29 of 35 isolates with metallo-carbapenemases, including several resistant to zidebactam alone. Zidebactam MICs for 36 of 50 Pseudomonas aeruginosa were 4-16 mg/L, and the majority of AmpC, metallo-β-lactamase-producing and cystic fibrosis isolates were susceptible to cefepime/zidebactam at 8 + 8 mg/L. Zidebactam MICs for Acinetobacter baumannii and Stenotrophomonas maltophilia were >32 mg/L; potentiation of cefepime was frequent for S. maltophilia , but minimal for A. baumannii . Kill curve results largely supported MICs.

Conclusions

Zidebactam represents a second triple-action DBO following RG6080, with lower MICs for Enterobacteriaceae and P. aeruginosa . Clinical evaluation of cefepime/zidebactam must critically evaluate the reliance that can be placed on this direct antibacterial activity and on the enhancer effect as well as β-lactamase inhibition.

Clinical isolates of Escherichia coli solely resistant to mecillinam: prevalence and epidemiology

In routine susceptibility testing of Gram-negative bacteria, a particular resistance phenotype was observed: an Escherichia coli isolate from a urine sample exhibited resistance solely to mecillinam (MEC) but was fully susceptible to other β-lactam antibiotics (MEC-R-BL-S). The objectives as this study were to determine the prevalence of this phenotype and to describe the phenotype, molecular epidemiology and genetic background. Between 1 January 2014 and 31 January 2016, MEC-R-BL-S E. coli isolates from urine were collected and genes previously reported as mostly involved in MEC resistance were analysed. The genetic relatedness among isolates was investigated by repetitive element sequence-based PCR (rep-PCR) and multilocus sequence typing (MLST). Ten MEC-R-BL-S isolates were collected, accounting for 0.4% (10/2547) of all E. coli obtained from urine samples, 0.9% (10/1135) of ampicillin-susceptible E. coli isolates and 9.6% (10/104) of MEC-R E. coli isolates. The isolates appeared as small colonies with round morphology and had impaired fitness. The isolates were not clonal and belonged to various extraintestinal or commensal E. coli phylogroups. Mutations in the cysB gene were evidenced in all clinical isolates. In conclusion, microbiologists should be aware of these isolates with a particular susceptibility phenotype, which is not due to error in disk diffusion but is a real non-enzymatic antibiotic resistance pattern.

An update on β-lactamase inhibitor discovery and development

Antibiotic resistance, and the emergence of pan-resistant clinical isolates, seriously threatens our capability to treat bacterial diseases, including potentially deadly hospital-acquired infections. This growing issue certainly requires multiple adequate responses, including the improvement of both diagnosis methods and use of antibacterial agents, and obviously the development of novel antibacterial drugs, especially active against Gram-negative pathogens, which represent an urgent medical need. Considering the clinical relevance of both β-lactam antibiotics and β-lactamase-mediated resistance, the discovery and development of combinations including a β-lactamase inhibitor seems to be particularly attractive, despite being extremely challenging due to the enormous diversity, both structurally and mechanistically, of the potential β-lactamase targets. This review will cover the evolution of currently available β-lactamase inhibitors along with the most recent research leading to new β-lactamase inhibitors of potential clinical interest or already in the stage of clinical development.

Efficacy and Pharmacokinetics of the Combination of OP0595 and Cefepime in a Mouse Model of Pneumonia Caused by Extended-Spectrum-Beta-Lactamase-Producing Klebsiella pneumoniae

OP0595 (RG6080) is a novel diazabicyclooctane that inhibits class A and C serine beta-lactamases. Although the combination of OP0595 and cefepime (FEP) showed good in vitro activity against extended-spectrum-beta-lactamase (ESBL)-producing pathogens, the effect of the combination therapy against severe infections, such as pneumonia or bacteremia, remains unknown in vivo In this study, we investigated the efficacy and pharmacokinetics of the combination therapy of OP0595 and FEP in a mouse model of pneumonia caused by Klebsiella pneumoniae harboring SHV- and CTX-M-9-type ESBLs. The infected BALB/c mice were intraperitoneally administered saline (control), 100 mg/kg of body weight of FEP, 20 mg/kg of OP0595, or both FEP and OP0595, twice a day. The MIC of FEP against the bacteria was 8 mg/liter and markedly improved to 0.06 mg/liter with the addition of 0.5 mg/ml of OP0595. In the survival study, the combination of FEP and OP0595 significantly improved the survival rate compared with that reported with either OP0595 or FEP alone (P < 0.001). The number of bacteria in the lungs and blood significantly decreased in the combination therapy group compared to that reported for the monotherapy groups (P < 0.001). In addition, the in vivo effect depended on the dose of FEP. However, pharmacokinetic analysis revealed that the percentage of time above MIC remained constant when increasing the dose of FEP in combination with 20 mg/kg of OP0595. The results of our study demonstrated the in vivo effectiveness of the combination of OP0595 and FEP.