Purification and biochemical characterization of the VIM-1 metallo-beta-lactamase

Exploiting the aggregation propensity of beta-lactamases to design inhibitors that induce enzyme misfolding

There is an arms race between beta-lactam antibiotics development and co-evolving beta-lactamases, which provide resistance by breaking down beta-lactam rings. We have observed that certain beta-lactamases tend to aggregate, which persists throughout their evolution under the selective pressure of antibiotics on their active sites. Interestingly, we find that existing beta-lactamase active site inhibitors can act as molecular chaperones, promoting the proper folding of these resistance factors. Therefore, we have created Pept-Ins, synthetic peptides designed to exploit the structural weaknesses of beta-lactamases by causing them to misfold into intracellular inclusion bodies. This approach restores sensitivity to a wide range of beta-lactam antibiotics in resistant clinical isolates, including those with Extended Spectrum variants that pose significant challenges in medical practice. Our findings suggest that targeted aggregation of resistance factors could offer a strategy for identifying molecules that aid in addressing the global antibiotic resistance crisis.

Detection of Extended-Spectrum β-Lactamases (ESBLs) and AmpC in Class A and Class B Carbapenemase-Producing Enterobacterales

In carbapenemase-producing Enterobacterales (CPE) additional β-lactam resistance mechanisms such as extended-spectrum-β-lactamases (ESBL) and/or AmpC-β-lactamases are generally difficult to detect by phenotypical methods. Recently, a modified version of the CLSI ESBL confirmatory combination disc diffusion (CDD) test, which involves the addition of boronic acid and EDTA on discs containing ESBL and AmpC substrates ± inhibitors, has been proposed for the detection of ESBL in class A and class B CPE. Here, the performance of the modified CDD test was evaluated using 121 genotypically characterized class A and class B CPE. Also, the effectiveness of the NG-Test CTX-M-MULTI lateral flow immunoassay was evaluated for ESBL detection. For class A CPE (n = 47), the modified CDD method exhibited an equal specificity (95.7%) and a higher sensitivity (100%) compared to the standard method (91.7%). The CTX-M-MULTI test detected ESBL in all CTX-M-type ESBL producers (n = 23), whereas it was negative for all CTX-M-type ESBL-negative isolates (n = 24). For class B CPE (n = 71), the modified method significantly improved both sensitivity (95%) and specificity (100%) in detecting ESBL compared to the standard method (17.5% sensitivity and 83.9% specificity). In comparison, the CTX-M-MULTI led to identification of ESBL in all CTX-M-ESBL-producers (n = 39) and no false-positive signal was generated with the CTX-M-type-ESBL-negative isolates (n = 30). Furthermore, the modified CDD improved the robustness of the method for AmpC detection (inconclusive results were produced in 53/57 and 10/57 cases with the standard and modified method, respectively), although the sensitivity of the test was poor (23.5%). Here, we propose a practical and cost-effective approach combining the modified CDD and the CTX-M-MULTI test for detection of ESBL and/or AmpC in class A and B CPE. IMPORTANCE Antimicrobial resistance is a growing public health threat of broad concern worldwide. Timely detection of antibiotic resistance mechanisms can help to monitor and to curb the spread of resistant bacteria within the hospital setting as well as in the environment. In this work we report an accurate and affordable method to phenotypically identify difficult-to-detect resistance determinants in highly resistant (carbapenemase-producing) bacteria. This method may be implemented in any diagnostic microbiology lab and may reduce the underreporting of relevant resistance mechanisms.

Nitroxoline and its derivatives are potent inhibitors of metallo-β-lactamases

Carbapenemases such as metallo-β-lactamases (MBLs) are spreading among Gram-negative bacterial pathogens. Infections due to these multidrug-resistant bacteria constitute a major global health challenge. Therapeutic strategies against carbapenemase producing bacteria include β-lactamase inhibitor combinations. Nitroxoline is a broad-spectrum antibiotic with restricted indication for urinary tract infections. In this study, we report on nitroxoline as an inhibitor of MBLs. We investigate the structure-activity relationships of nitroxoline derivatives considering in vitro MBL inhibitory potency in a fluorescence based assay using purified recombinant MBLs, NDM-1 and VIM-1. We investigated the most potent nitroxoline derivative in combination with imipenem against clinical isolates as well as transformants producing MBL by broth microdilution and time-kill kinetics. Our findings demonstrate that nitroxoline derivatives are potent MBL inhibitors and in combination with imipenem overcome MBL-mediated carbapenem resistance.

Identification of a Novel Metallo-β-Lactamase, VAM-1, in a Foodborne Vibrio alginolyticus Isolate from China

A multidrug-resistant Vibrio alginolyticus isolate recovered from a shrimp sample with reduced carbapenem susceptibility produced a novel metallo-β-lactamase (MBL), VAM-1. That carbapenemase shared 67% to 70% amino acid identity with several VMB family subclass B1 MBLs, which were recently reported among some marine bacteria including Vibrio, Glaciecola, and Thalassomonas. The blaVAM-1 gene was located in a novel conjugative plasmid, namely, pC1579, and multiple copies of blaVAM-1 via an unusual mechanism of gene amplification were detected in pC1579. These findings underline the emergence of marine organisms acting as natural reservoirs for MBL genes and the importance of continuous bacterial antibiotic resistance surveillance.

A Novel Cooperative Metallo-β-Lactamase Fold Metallohydrolase from Pathogen Vibrio vulnificus Exhibits β-Lactam Antibiotic-Degrading Activities

Vibrio vulnificus is a pathogen that accounts for one of the highest mortality rates and is responsible for most reported seafood-related illnesses and deaths worldwide. Owing to the threats of pathogens with β-lactamase activity, it is important to identify and characterize β-lactamases with clinical significance. In this study, the protein sequence of the metallo-β-lactamase (MBL) fold metallohydrolase from V. vulnificus (designated Vmh) was analyzed, and its oligomeric state, β-lactamase activity, and metal binding ability were determined. BLASTp analysis indicated that the V. vulnificus Vmh protein showed no significant sequence identity with any experimentally identified Ambler class B MBLs or enzymes containing the MBL protein fold; it was also predicted to have a signal peptide of 19 amino acids at its N terminus and an MBL protein fold from amino acid residues 23 to 216. Recombinant V. vulnificus Vmh protein was overexpressed and purified. Analytical ultracentrifugation and electrospray ionization-mass spectrometry (MS) data demonstrated its monomeric state in an aqueous solution. Recombinant V. vulnificus Vmh protein showed broad degrading activities against β-lactam antibiotics, such as penicillins, cephalosporins, and imipenems, with k_cat/K_m values ranging from 6.23 × 10² to 1.02 × 10⁴ M^-1 s^-1. The kinetic reactions of this enzyme exhibited sigmoidal behavior, suggesting the possibility of cooperativity. Zinc ions were required for the enzyme activity, which was abolished by adding the metal chelator EDTA. Inductively coupled plasma-MS indicated that this enzyme might bind two zinc ions per molecule as a cofactor.

Detection and characterization of VIM-52, a new variant of VIM-1 from Klebsiella pneumoniae clinical isolate

Over the last two decades, antimicrobial resistance has become a global health problem. In Gram-negative bacteria, metallo-β-lactamases (MBLs), which inactivate virtually all β-lactams, increasingly contribute to this phenomenon. The aim of this study is to characterize VIM-52, a His224Arg variant of VIM-1, identified in a Klebsiella pneumoniae clinical isolate. VIM-52 conferred lower MICs to cefepime and ceftazidime as compared to VIM-1. These results were confirmed by steady state kinetic measurements, where VIM-52 yielded a lower activity towards ceftazidime and cefepime but not against carbapenems. Residue 224 is part of the L10 loop (residues 221-241), which borders the active site. As Arg 224 and Ser 228 are both playing an important and interrelated role in enzymatic activity, stability and substrate specificity for the MBLs, targeted mutagenesis at both positions were performed and further confirmed their crucial role for substrate specificity.

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.

Detection of carbapenemase producing enterobacteria using an ion sensitive field effect transistor sensor

The timely and accurate detection of carbapenemase-producing Enterobacterales (CPE) is imperative to manage this worldwide problem in an effective fashion. Herein we addressed the question of whether the protons produced during imipenem hydrolysis could be detected using an ion sensitive field effect transistor (ISFET). Application of the methodology on enzyme preparations showed that the sensor is able to detect carbapenemases of the NDM, IMP, KPC and NMC-A types at low nanomolar concentrations while VIM and OXA-48 responded at levels above 100 nM. Similar results were obtained when CPE cell suspensions were tested; NDM, IMP, NMC-A and KPC producers caused fast reductions of the output potential. Reduction rates with VIM-type and especially OXA-48 producing strains were significantly lower. Based on results with selected CPEs and carbapenemase-negative enterobacteria, a threshold of 10 mV drop at 30 min was set. Applying this threshold, the method exhibited 100% sensitivity for NDM, IMP and KPC and 77.3% for VIM producers. The OXA-48-positive strains failed to pass the detection threshold. A wide variety of carbapenemase-negative control strains were all classified as negative (100% specificity). In conclusion, an ISFET-based approach may have the potential to be routinely used for non OXA-48-like CPE detection in the clinical laboratory.

In vitro and in vivo Effect of Antimicrobial Agent Combinations Against Carbapenem-Resistant Klebsiella pneumoniae with Different Resistance Mechanisms in China

Objective

This study aimed to evaluate the in vitro and in vivo effects of different combinations of antimicrobial agents against carbapenemase-producing and non-producing Klebsiella pneumoniae from China.

Methods

A checkerboard assay of meropenem (MEM), amikacin (AK), tigecycline (TGC), colistin (COL) and their combinations was carried out against 58 clinical carbapenem-resistant K. pneumoniae (CRKp) isolates, including 11 carbapenemase-non-producing K. pneumoniae isolates and 21 isolates producing KPC-2 enzyme, 11 NDM-1, 13 IMP, one VIM-1 and one OXA-48. The checkerboard assay was analyzed by the fractional inhibitory concentration index (FICI). A time-kill assay and Galleria mellonella infection model were conducted to evaluate the in vitro and in vivo effects of the four drugs alone and in combination.

Results

In the checkerboard assay, TGC+AK and MEM+AK combinations showed the highest synergistic effect against KPC-2 and NDM-1 carbapenemase-producing isolates, with synergy+partial synergy (defined as FICI <1) rates of 76.2% and 71.4% against KPC-2 producers, and 54.5% and 81.8% against NDM-1 producers. TGC+AK and MEM+COL combinations showed the highest rate of synergistic effect against IMP-producing isolates. Against carbapenemase-non-producing isolates, TGC+COL and TGC+AK combinations showed the highest rate of synergy effect (63.6% and 54.5%). MEM+AK showed a synergistic effect against one VIM-1 producer (FICI=0.31) and an additivite effect (FICI=1) against one OXA-48 producer. In the time-kill assay, COL+AK, COL+TGC, COL+MEM and AK+TGC showed good synergistic effects against the KPC-2-producing isolate D16. COL+MEM and COL+TGC combinations showed good effects against the NDM-1-producing isolate L13 and IMP-4-producing isolate L34. Against the carbapenemase-non-producing isolate Y105, MEM+TGC and COL+AK showed high synergistic effects, with log₁₀CFU/mL decreases of 6.2 and 5.5 compared to the most active single drug. In the G. mellonella survival assay, MEM-based combinations had relatively high survival rates, especially when combined with colistin, against KPC-2 producers (90% survival rate) and with amikacin against metallo-beta-lactamase producers (95-100% survival rate).

Conclusion

Our study suggests that different antimicrobial agent combinations should be considered against CRKp infections with different resistance mechanisms.

Detection of Five mcr-9-Carrying Enterobacterales Isolates in Four Czech Hospitals

The aim of this study was to report the characterization of the first mcr-positive Enterobacterales isolated from Czech hospitals. In 2019, one Citrobacter freundii and four Enterobacter isolates were recovered from Czech hospitals. The production of carbapenemases was examined by a matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) imipenem hydrolysis assay. Additionally, bacteria were screened for the presence of carbapenemase-encoding genes and plasmid-mediated colistin resistance genes by PCR. To define the genetic units carrying mcr genes, the genomic DNAs of mcr-carrying clinical isolates were sequenced on the PacBio Sequel I platform. Results showed that all isolates carried blaVIM- and mcr-like genes. Analysis of whole-genome sequencing (WGS) data revealed that all isolates carried mcr-9-like alleles. Furthermore, the three sequence type 106 (ST106) Enterobacter hormaechei isolates harbored the blaVIM-1 gene, while the ST764 E. hormaechei and ST95 C. freundii included blaVIM-4 Analysis of plasmid sequences showed that, in all isolates, mcr-9 was carried on IncHI2 plasmids. Additionally, at least one multidrug resistance (MDR) region was identified in each mcr-9-carrying IncHI2 plasmid. The blaVIM-4 gene was found in the MDR regions of p48880_MCR_VIM and p51929_MCR_VIM. In the three remaining isolates, blaVIM-1 was localized on plasmids (∼55 kb) exhibiting repA-like sequences 99% identical to the respective gene of pKPC-CAV1193. In conclusion, to the best of our knowledge, these 5 isolates were the first mcr-9-positive bacteria of clinical origin identified in the Czech Republic. Additionally, the carriage of the blaVIM-1 on pKPC-CAV1193-like plasmids is described for the first time. Thus, our findings underline the ongoing evolution of mobile elements implicated in the dissemination of clinically important resistance determinants.IMPORTANCE Infections caused by carbapenemase-producing bacteria have led to the revival of polymyxins as the "last-resort" antibiotic. Since 2016, several reports describing the presence of plasmid-mediated colistin resistance genes, mcr, in different host species and geographic areas were published. Here, we report the first detection of Enterobacterales carrying mcr-9-like alleles isolated from Czech hospitals in 2019. Furthermore, the three ST106 Enterobacter hormaechei isolates harbored blaVIM-1, while the ST764 E. hormaechei and ST95 Citrobacter freundii isolates included blaVIM-4 Analysis of WGS data showed that, in all isolates, mcr-9 was carried on IncHI2 plasmids. blaVIM-4 was found in the MDR regions of IncHI2 plasmids, while blaVIM-1 was localized on pKPC-CAV1193-like plasmids, described here for the first time. These findings underline the ongoing evolution of mobile elements implicated in dissemination of clinically important resistance determinants. Thus, WGS characterization of MDR bacteria is crucial to unravel the mechanisms involved in dissemination of resistance mechanisms.

Inhibitory Potential of Polyclonal Camel Antibodies against New Delhi Metallo-β-lactamase-1 (NDM-1)

New Delhi Metallo-β-lactamase-1 (NDM-1) is the most prevalent type of metallo-β-lactamase, able to hydrolyze almost all antibiotics of the β-lactam group, leading to multidrug-resistant bacteria. To date, there are no clinically relevant inhibitors to fight NDM-1. The use of dromedary polyclonal antibody inhibitors against NDM-1 represents a promising new class of molecules with inhibitory activity. In the current study, immunoreactivities of dromedary Immunoglobulin G (IgG) isotypes containing heavy-chain and conventional antibodies were tested after successful immunization of dromedary using increasing amounts of the recombinant NDM-1 enzyme. Inhibition kinetic assays, performed using a spectrophotometric method with nitrocefin as a reporter substrate, demonstrated that IgG1, IgG2, and IgG3 were able to inhibit not only the hydrolytic activity of NDM-1 but also Verona integron-encoded metallo-β-lactamase (VIM-1) (subclass B1) and L1 metallo-β-lactamase (L1) (subclass B3) with inhibitory concentration (IC₅₀₎ values ranging from 100 to 0.04 μM. Investigations on the ability of IgG subclasses to reduce the growth of recombinant Escherichia coli BL21(DE3)/codon plus cells containing the recombinant plasmid expressing NDM-1, L1, or VIM-1 showed that the addition of IgGs (4 and 8 mg/L) to the cell culture was unable to restore the susceptibility of carbapenems. Interestingly, IgGs were able to interact with NDM-1, L1, and VIM-1 when tested on the periplasm extract of each cultured strain. The inhibitory concentration was in the micromolar range for all β-lactams tested. A visualization of the 3D structural basis using the three enzyme Protein Data Bank (PDB) files supports preliminarily the recorded inhibition of the three MBLs.

Comprehensive exploration of the translocation, stability and substrate recognition requirements in VIM-2 lactamase

Metallo-β-lactamases (MBLs) degrade a broad spectrum of β-lactam antibiotics, and are a major disseminating source for multidrug resistant bacteria. Despite many biochemical studies in diverse MBLs, molecular understanding of the roles of residues in the enzyme's stability and function, and especially substrate specificity, is lacking. Here, we employ deep mutational scanning (DMS) to generate comprehensive single amino acid variant data on a major clinical MBL, VIM-2, by measuring the effect of thousands of VIM-2 mutants on the degradation of three representative classes of β-lactams (ampicillin, cefotaxime, and meropenem) and at two different temperatures (25°C and 37°C). We revealed residues responsible for expression and translocation, and mutations that increase resistance and/or alter substrate specificity. The distribution of specificity-altering mutations unveiled distinct molecular recognition of the three substrates. Moreover, these function-altering mutations are frequently observed among naturally occurring variants, suggesting that the enzymes have continuously evolved to become more potent resistance genes.

A Single Salt Bridge in VIM-20 Increases Protein Stability and Antibiotic Resistance under Low-Zinc Conditions

To understand the evolution of Verona integron-encoded metallo-β-lactamase (VIM) genes (blaVIM) and their clinical impact, microbiological, biochemical, and structural studies were conducted. Forty-five clinically derived VIM variants engineered in a uniform background and expressed in Escherichia coli afforded increased resistance toward all tested antibiotics; the variants belonging to the VIM-1-like and VIM-4-like families exhibited higher MICs toward five out of six antibiotics than did variants belonging to the widely distributed and clinically important VIM-2-like family. Generally, maximal MIC increases were observed when cephalothin and imipenem were tested. Additionally, MIC determinations under conditions with low zinc availability suggested that some VIM variants are also evolving to overcome zinc deprivation. The most profound increase in resistance was observed in VIM-2-like variants (e.g., VIM-20 H229R) at low zinc availability. Biochemical analyses reveal that VIM-2 and VIM-20 exhibited similar metal binding properties and steady-state kinetic parameters under the conditions tested. Crystal structures of VIM-20 in the reduced and oxidized forms at 1.25 Å and 1.37 Å resolution, respectively, show that Arg229 forms an additional salt bridge with Glu171. Differential scanning fluorimetry of purified proteins and immunoblots of periplasmic extracts revealed that this difference increases thermostability and resistance to proteolytic degradation when zinc availability is low. Therefore, zinc scarcity appears to be a selective pressure driving the evolution of multiple metallo-β-lactamase families, although compensating mutations use different mechanisms to enhance resistance.IMPORTANCE Antibiotic resistance is a growing clinical threat. One of the most serious areas of concern is the ability of some bacteria to degrade carbapenems, drugs that are often reserved as last-resort antibiotics. Resistance to carbapenems can be conferred by a large group of related enzymes called metallo-β-lactamases that rely on zinc ions for function and for overall stability. Here, we studied an extensive panel of 45 different metallo-β-lactamases from a subfamily called VIM to discover what changes are emerging as resistance evolves in clinical settings. Enhanced resistance to some antibiotics was observed. We also found that at least one VIM variant developed a new ability to remain more stable under conditions where zinc availability is limited, and we determined the origin of this stability in atomic detail. These results suggest that zinc scarcity helps drive the evolution of this resistance determinant.

Kinetics of Sulbactam Hydrolysis by β-Lactamases, and Kinetics of β-Lactamase Inhibition by Sulbactam

Sulbactam is one of four β-lactamase inhibitors in current clinical use to counteract drug resistance caused by degradation of β-lactam antibiotics by these bacterial enzymes. As a β-lactam itself, sulbactam is susceptible to degradation by β-lactamases. I investigated the Michaelis-Menten kinetics of sulbactam hydrolysis by 14 β-lactamases, representing clinically widespread groups within all four Ambler classes, i.e., CTX-M-15, KPC-2, SHV-5, and TEM-1 for class A; IMP-1, NDM-1, and VIM-1 for class B; Acinetobacter baumannii ADC-7, Pseudomonas aeruginosa AmpC, and Enterobacter cloacae P99 for class C; and OXA-10, OXA-23, OXA-24, and OXA-48 for class D. All of the β-lactamases were able to hydrolyze sulbactam, although they varied widely in their kinetic constants for the reaction, even within each class. I also investigated the inactivation kinetics of the inhibition of these enzymes by sulbactam. The class A β-lactamases varied widely in their susceptibility to inhibition, the class C and D enzymes were very weakly inhibited, and the class B enzymes were essentially or completely unaffected. In addition, we measured the sulbactam turnover number, the sulbactam/enzyme molar ratio required for complete inhibition of each enzyme. Class C enzymes had the lowest turnover numbers, class A enzymes varied widely, and class D enzymes had very high turnover numbers. These results are valuable for understanding which β-lactamases ought to be well inhibited by sulbactam. Moreover, since sulbactam has intrinsic antibacterial activity against Acinetobacter species pathogens, these results contribute to understanding β-lactamase-mediated sulbactam resistance in Acinetobacter, especially due to the action of the widespread class D enzymes.

Genomic Characterization of VIM Metallo-β-Lactamase-Producing Alcaligenes faecalis from Gaza, Palestine

Carbapenemase-producing Gram-negative bacteria (CP-GNB) have increasingly spread worldwide, and different families of carbapenemases have been identified in various bacterial species. Here, we report the identification of five VIM metallo-β-lactamase-producing Alcaligenes faecalis isolates associated with a small outbreak in a large hospital in Gaza, Palestine. Next-generation sequencing analysis showed bla_VIM-2 is harbored by a chromosomal genomic island among three strains, while bla_VIM-4 is carried by a novel plasmid in two strains.

B1-Metallo-β-Lactamases: Where Do We Stand?

Metallo-β-Lactamases (MBLs) are class Bβ-lactamases that hydrolyze almost all clinically-availableβ-lactam antibiotics. MBLs feature the distinctive αβ/βα sandwich fold of the metallo-hydrolase/oxidoreductase superfamily and possess a shallow active-site groove containing one or two divalent zinc ions, flanked by flexible loops. According to sequence identity and zinc ion dependence, MBLs are classified into three subclasses (B1, B2 and B3), of which the B1 subclass enzymes have emerged as the most clinically significant. Differences among the active site architectures, the nature of zinc ligands, and the catalytic mechanisms have limited the development of a common inhibitor. In this review, we will describe the molecular epidemiology and structural studies of the most prominent representatives of class B1 MBLs (NDM-1, IMP-1 and VIM-2) and describe the implications for inhibitor design to counter this growing clinical threat.

Coproduction of KPC-18 and VIM-1 Carbapenemases by Enterobacter cloacae: Implications for Newer β-Lactam-β-Lactamase Inhibitor Combinations

Enterobacter cloacae strain G6809 with reduced susceptibility to carbapenems was identified from a patient in a long-term acute care hospital in Kentucky. G6809 belonged to sequence type (ST) 88 and carried two carbapenemase genes, bla(KPC-18) and bla(VIM-1). Whole-genome sequencing localized bla(KPC-18) to the chromosome and bla(VIM-1) to a 58-kb plasmid. The strain was highly resistant to ceftazidime-avibactam. Insidious coproduction of metallo-β-lactamase with KPC-type carbapenemase has implications for the use of next-generation β-lactam-β-lactamase inhibitor combinations.

Probing substrate binding to the metal binding sites in metallo-β-lactamase L1 during catalysis

Metal ions in metallo-β-lactamases (MBLs) play a major role in catalysis. In this study we investigated the role of the metal ions in the Zn₁ and Zn₂ sites of MBL L1 during catalysis. A ZnCo (with Zn(II) in the invariant Zn₁ site and Co(II) in the Zn₂ site) analog of MBL L1 was prepared by using a biological incorporation method. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic studies were used to confirm that the ZnCo analog was prepared. To study the roles of the Zn(II) and Co(II) ions during catalysis, rapid freeze quench (RFQ)-EXAFS studies were used to probe the reaction of the ZnCo-L1 analog with chromacef when quenched at 10 ms, 50 ms, and 100 ms. The L1-product complex was also analyzed with EXAFS spectroscopy. The data show that the Zn-Co distance is 3.49 Å in the resting enzyme and that this distance increases by 0.3 Å in the sample that was quenched at 10 ms. The average Zn-Co distance decreases at the other time points until reaching a distance of 3.58 Å in the L1-product complex. The data also show that a Co-S interaction is present in the 100 ms quenched sample and in the L1-product complex, which suggests that there is a significant rearrangement of product in the active site.

Biochemical Characterization of CPS-1, a Subclass B3 Metallo-β-Lactamase from a Chryseobacterium piscium Soil Isolate

CPS-1 is a subclass B3 metallo-β-lactamase from a Chryseobacterium piscium isolate collected from soil, showing 68% amino acid identity to the GOB-1 enzyme. CPS-1 was overproduced in Escherichia coli Rosetta (DE3), purified by chromatography, and biochemically characterized. This enzyme exhibits a broad-spectrum substrate profile, including penicillins, cephalosporins, and carbapenems, which overall resembles those of L1, GOB-1, and acquired subclass B3 enzymes AIM-1 and SMB-1.

Comparison of Verona Integron-Borne Metallo-β-Lactamase (VIM) Variants Reveals Differences in Stability and Inhibition Profiles

Metallo-β-lactamases (MBLs) are of increasing clinical significance; the development of clinically useful MBL inhibitors is challenged by the rapid evolution of variant MBLs. The Verona integron-borne metallo-β-lactamase (VIM) enzymes are among the most widely distributed MBLs, with >40 VIM variants having been reported. We report on the crystallographic analysis of VIM-5 and comparison of biochemical and biophysical properties of VIM-1, VIM-2, VIM-4, VIM-5, and VIM-38. Recombinant VIM variants were produced and purified, and their secondary structure and thermal stabilities were investigated by circular dichroism analyses. Steady-state kinetic analyses with a representative panel of β-lactam substrates were carried out to compare the catalytic efficiencies of the VIM variants. Furthermore, a set of metalloenzyme inhibitors were screened to compare their effects on the different VIM variants. The results reveal only small variations in the kinetic parameters of the VIM variants but substantial differences in their thermal stabilities and inhibition profiles. Overall, these results support the proposal that protein stability may be a factor in MBL evolution and highlight the importance of screening MBL variants during inhibitor development programs.

Biochemical Characterization of VIM-39, a VIM-1-Like Metallo-β-Lactamase Variant from a Multidrug-Resistant Klebsiella pneumoniae Isolate from Greece

VIM-39, a VIM-1-like metallo-β-lactamase variant (VIM-1 Thr33Ala His224Leu) was identified in a clinical isolate of Klebsiella pneumoniae belonging to sequence type 147. VIM-39 hydrolyzed ampicillin, cephalothin, and imipenem more efficiently than did VIM-1 and VIM-26 (a VIM-1 variant with the His224Leu substitution) because of higher turnover rates.

Confronting carbapenemase-producing Klebsiella pneumoniae

The ongoing spread of carbapenemase-producing (CP) multidrug-resistant enterobacteria, primarily Klebsiella pneumoniae, has undoubtedly caused a public health crisis of unprecedented dimensions. The scientific community has been struggling with these highly problematic nosocomial pathogens for more than a decade. Faced with the current situation, one cannot help but wish we could have done better, earlier. However, significant steps have been and are currently being made towards a better understanding of transmission routes of CP microorganisms and in designing strategies that could effectively curb this devastating epidemic. Most importantly, the systematic evaluation of accumulating experimental and clinical data has paved the way to a more rational management of CP-infected patients. In addition, systematic efforts of the industry have led to the development of novel antibacterial agents that are active against CP strains and expected to be introduced to clinical practice in the immediate future.

Biochemical characterization of IMP-30, a metallo-β-lactamase with enhanced activity toward ceftazidime

IMP-type enzymes constitute a clinically important family of metallo-β-lactamases that has grown dramatically in the past decade to its current 45 known members. Here, we report the biochemical characterization of IMP-30 in comparison to IMP-1, from which it deviates by a single E59K mutation. Kinetics, MIC assays, docking, and molecular dynamics simulations support a scenario in which Lys59 interacts with the ceftazidime R1 group, resulting in increased water access and enhanced turnover and MIC of ceftazidime.

Carbapenemases in Klebsiella pneumoniae and other Enterobacteriaceae: an evolving crisis of global dimensions

SUMMARY

The spread of Enterobacteriaceae, primarily Klebsiella pneumoniae, producing KPC, VIM, IMP, and NDM carbapenemases, is causing an unprecedented public health crisis. Carbapenemase-producing enterobacteria (CPE) infect mainly hospitalized patients but also have been spreading in long-term care facilities. Given their multidrug resistance, therapeutic options are limited and, as discussed here, should be reevaluated and optimized. Based on susceptibility data, colistin and tigecycline are commonly used to treat CPE infections. Nevertheless, a review of the literature revealed high failure rates in cases of monotherapy with these drugs, whilst monotherapy with either a carbapenem or an aminoglycoside appeared to be more effective. Combination therapies not including carbapenems were comparable to aminoglycoside and carbapenem monotherapies. Higher success rates have been achieved with carbapenem-containing combinations. Pharmacodynamic simulations and experimental infections indicate that modification of the current patterns of carbapenem use against CPE warrants further attention. Epidemiological data, though fragmentary in many countries, indicate CPE foci and transmission routes, to some extent, whilst also underlining the lack of international collaborative systems that could react promptly and effectively. Fortunately, there are sound studies showing successful containment of CPE by bundles of measures, among which the most important are active surveillance cultures, separation of carriers, and assignment of dedicated nursing staff.

FIM-1, a new acquired metallo-β-lactamase from a Pseudomonas aeruginosa clinical isolate from Italy

Acquired metallo-β-lactamases (MBLs) are resistance determinants of increasing clinical importance in Gram-negative bacterial pathogens, which confer a broad-spectrum β-lactam resistance, including carbapenems. Several such enzymes have been described since the 1990s. In the present study, a novel acquired MBL, named FIM-1, was identified and characterized. The bla(FIM-1) gene was cloned from a multidrug-resistant Pseudomonas aeruginosa clinical isolate (FI-14/157) cultured from a patient with a vascular graft infection in Florence, Italy. The isolate belonged in the sequence type 235 epidemic clonal lineage. The FIM-1 enzyme is a member of subclass B1 and, among acquired MBLs, exhibited the highest similarity (ca. 40% amino acid identity) with NDM-type enzymes. In P. aeruginosa FI-14/157, the bla(FIM-1) gene was apparently inserted into the chromosome and associated with ISCR19-like elements that were likely involved in the capture and mobilization of this MBL gene. Transfer experiments of the bla(FIM-1) gene to an Escherichia coli strain or another P. aeruginosa strain by conjugation or electrotransformation were not successful. The FIM-1 protein was produced in E. coli and purified by two chromatography steps. Analysis of the kinetic parameters, carried out with the purified enzyme, revealed that FIM-1 has a broad substrate specificity, with a preference for penicillins (except the 6α-methoxy derivative temocillin) and carbapenems. Aztreonam was not hydrolyzed. Detection of this novel type of acquired MBL in a P. aeruginosa clinical isolate underscores the increasing diversity of such enzymes that can be encountered in the clinical setting.

Activity of carbapenems with ME1071 (disodium 2,3-diethylmaleate) against Enterobacteriaceae and Acinetobacter spp. with carbapenemases, including NDM enzymes

OBJECTIVES

ME1071 is a maleic acid that inhibits metallo-β-lactamases (MBLs). We examined its ability to potentiate different carbapenems against MBL-producing Enterobacteriaceae in relation to its inhibition kinetics.

METHODS

Enterobacteriaceae and Acinetobacter isolates with IMP, VIM and NDM MBLs were tested; bacteria with other types of carbapenem resistance were used as controls. Chequerboard titrations were performed by CLSI agar dilution, carbapenemases were cloned into pET-28a(+) and purified by column chromatography, and kinetic parameters were determined by spectrophotometry.

RESULTS

The key findings were: (i) the MICs of carbapenems varied widely among isolates with the same carbapenemase, but those with the NDM types were generally the most resistant; (ii) biapenem was the carbapenem least compromised by all MBL types, owing to weaker kinetic efficiency (k(cat)/K(m)) for hydrolysis, contingent on lower affinity (higher K(m)); (iii) MBLs were the only carbapenemases inhibited by ME1071, confirming its specificity of action; and (iv) irrespective of the partner carbapenem, synergy with ME1071 was least for organisms with NDM MBLs and most for those with IMP types, correlating with ME1071 having weakest affinity (highest K(i)) for NDM-1 and strongest affinity for IMP-1.

CONCLUSIONS

ME1071 reduced the MICs of carbapenems for bacteria with NDM-1 enzyme though synergy was weaker than for bacteria with IMP and VIM metallo-enzymes; this correlated with ME1071 having weaker affinity for NDM-1 than IMP-1 and VIM-2. As the weakest MBL substrate carbapenem, biapenem was the easiest to protect.

Biochemical and structural characterization of the subclass B1 metallo-β-lactamase VIM-4

The metallo-β-lactamase VIM-4, mainly found in Pseudomonas aeruginosa or Acinetobacter baumannii, was produced in Escherichia coli and characterized by biochemical and X-ray techniques. A detailed kinetic study performed in the presence of Zn²+ at concentrations ranging from 0.4 to 100 μM showed that VIM-4 exhibits a kinetic profile similar to the profiles of VIM-2 and VIM-1. However, VIM-4 is more active than VIM-1 against benzylpenicillin, cephalothin, nitrocefin, and imipenem and is less active than VIM-2 against ampicillin and meropenem. The crystal structure of the dizinc form of VIM-4 was solved at 1.9 Å. The sole difference between VIM-4 and VIM-1 is found at residue 228, which is Ser in VIM-1 and Arg in VIM-4. This substitution has a major impact on the VIM-4 catalytic efficiency compared to that of VIM-1. In contrast, the differences between VIM-2 and VIM-4 seem to be due to a different position of the flapping loop and two substitutions in loop 2. Study of the thermal stability and the activity of the holo- and apo-VIM-4 enzymes revealed that Zn²+ ions have a pronounced stabilizing effect on the enzyme and are necessary for preserving the structure.

VIM-19, a metallo-beta-lactamase with increased carbapenemase activity from Escherichia coli and Klebsiella pneumoniae

Two carbapenem-resistant isolates, one Escherichia coli isolate and one Klebsiella pneumoniae isolate, recovered from an Algerian patient expressed a novel VIM-type metallo-beta-lactamase (MBL). The identified bla(VIM-19) gene was located on a ca. 160-kb plasmid and located inside a class 1 integron in both isolates. VIM-19 differed from VIM-1 by the Asn215Lys and Ser228Arg substitutions, increasing its hydrolytic activity toward carbapenems. Site-directed mutagenesis experiments showed that both substitutions were necessary for the increased carbapenemase activity of VIM-19. This study indicates that MBLs with enhanced activity toward carbapenems may be obtained as a result of very few amino acid substitutions.

Novel VIM metallo-beta-lactamase variant from clinical isolates of Enterobacteriaceae from Algeria

Five different strains of bacteria belonging to the family Enterobacteriaceae were isolated from two patients hospitalized in the intensive care unit of the Central Military Hospital of Algiers, Algeria. All five strains, one Providencia stuartii strain, two Escherichia coli strains, and two Klebsiella pneumoniae strains, were intermediate or resistant to all beta-lactams, including carbapenems. Synergy between imipenem and EDTA was observed for all five strains. The results of the PCR experiment confirmed the presence of a bla(VIM) gene in all five strains. The bla(VIM) genes were located as part of a class 1 integron on a 180-kb conjugative plasmid. They encoded a novel metallo-beta-lactamase designated VIM-19, which differed from the parental enzyme VIM-1 by only two substitutions: Ser228Arg, previously observed in the closely related enzyme VIM-4, and Asn215Lys, not previously observed in other VIM-type carbapenemases. VIM-19 was further characterized after purification through determination of its kinetic constants. This enzyme was inhibited by EDTA and hydrolyzed penicillins, cephalosporins, and carbapenems, as observed for other VIM-type carbapenemases but with greater catalytic efficiency against penicillins than VIM-1. VIM-19 is the first carbapenemase enzyme identified from an isolate from Algeria. These results confirm the emergence of VIM-4-like enzymes in members of the family Enterobacteriaceae from Mediterranean countries.

Identification of bla(IMP-22) in Pseudomonas spp. in urban wastewater and nosocomial environments: biochemical characterization of a new IMP metallo-enzyme variant and its genetic location

OBJECTIVES

The aim of the study was the biochemical characterization of a new variant of the metallo-beta-lactamase, IMP-22. Moreover, the genetic environment of the bla(IMP-22) gene was investigated in Pseudomonas fluorescens and Pseudomonas aeruginosa collected from urban wastewater and a teaching hospital in L'Aquila, Italy.

METHODS

Molecular characterization of genetic elements was carried out by PCR and DNA sequencing methods. The new enzyme was purified from recombinant Escherichia coli BL21(DE)Rosetta/pBC-SK/IMP-22. Steady-state kinetic parameters (K(m) and V(max)) were determined for a large pattern of substrates.

RESULTS

A new IMP metallo-beta-lactamase gene was found in a class 1 integron and in one case, in a plasmid of Pseudomonas spp. The bla(IMP-22) encodes for a pre-protein of 246 amino acids and the N-terminus of the mature beta-lactamase (NH(2)-PDLK) was also determined. The molecular mass and pI were 24 930 Da and 6.2, respectively. On the basis of the kinetic parameters calculated (K(m) and V(max)), IMP-22 was found to hydrolyse narrow- and extended-spectrum beta-lactams. Enzyme activity was found to be inhibited by metal chelators such as EDTA, 1,10-o-phenathroline and dipicolinic acid with an IC(50) of 800, 750 and 300 microM, respectively.

CONCLUSIONS

The finding of the bla(IMP-22) gene in P. fluorescens environmental strains and P. aeruginosa clinical isolate suggests the ongoing spread of bla(MBL) genes in several bacterial species and in different environments.

KHM-1, a novel plasmid-mediated metallo-beta-lactamase from a Citrobacter freundii clinical isolate

A novel gene, bla(KHM-1), encoding a metallo-beta-lactamase, KHM-1, was cloned from a clinical isolate of Citrobacter freundii resistant to most beta-lactam antibiotics. Escherichia coli expressing bla(KHM-1) was resistant to all broad-spectrum beta-lactams except for monobactams and showed reduced susceptibility to carbapenems. Recombinant KHM-1 exhibited EDTA-inhibitable hydrolytic activity against most beta-lactams, with an overall preference for cephalosporins.

Characterization of the new metallo-beta-lactamase VIM-13 and its integron-borne gene from a Pseudomonas aeruginosa clinical isolate in Spain

During a survey conducted to evaluate the incidence of class B carbapenemase (metallo-beta-lactamase [MBL])-producing Pseudomonas aeruginosa strains from hospitals in Majorca, Spain, five clinical isolates showed a positive Etest MBL screening test result. In one of them, strain PA-SL2, the presence of a new bla(VIM) derivative (bla(VIM-13)) was detected by PCR amplification with bla(VIM-1)-specific primers followed by sequencing. The bla(VIM-13)-producing isolate showed resistance to all beta-lactams (except aztreonam), gentamicin, tobramycin, and ciprofloxacin. VIM-13 exhibited 93% and 88% amino acid sequence identities with VIM-1 and VIM-2, respectively. bla(VIM-13) was cloned in parallel with bla(VIM-1), and the resistance profile conferred was analyzed both in Escherichia coli and in P. aeruginosa backgrounds. Compared to VIM-1, VIM-13 conferred slightly higher levels of resistance to piperacillin and lower levels of resistance to ceftazidime and cefepime. VIM-13 and VIM-1 were purified in parallel as well, and their kinetic parameters were compared. The k(cat)/K(m) ratios for the antibiotics mentioned above were in good agreement with the MIC data. Furthermore, EDTA inhibited the activity of VIM-13 approximately 25 times less than it inhibited the activity of VIM-1. VIM-13 was harbored in a class 1 integron, along with a new variant (Ala108Thr) of the aminoglycoside-modifying enzyme encoding gene aacA4, which confers resistance to gentamicin and tobramycin. Finally, the VIM-13 integron was apparently located in the chromosome, since transformation and conjugation experiments consistently yielded negative results and the bla(VIM-13) probe hybridized only with the genomic DNA.

Kinetic characterization of VIM-7, a divergent member of the VIM metallo-beta-lactamase family

Purified recombinant VIM-7 possesses efficient penicillinase and carbapenemase activities comparable to those of VIM-2. Cephalosporinase activity was variable and generally lower than those of VIM-1 and VIM-2. A homology model suggests that the VIM-7 Tyr-218 Phe substitution may be responsible for the reduced catalytic efficiency against certain cephalosporins, including ceftazidime and cefepime.

Carbapenemases: molecular diversity and clinical consequences

Carbapenemases are beta-lactamases that hydrolyze most beta-lactams including carbapenems. Carbapenemases are classified in four molecular classes; those belonging to class A are the chromosomally-encoded and clavulanic acid-inhibited IMI, NMC-A and SME, identified in Enterobacter cloacae and Serratia marcescens; the plasmid-encoded KPC enzymes identified in Enterobacteriaceae (and rarely in Pseudomonas aeruginosa); and the GES-type enzymes identified in Enterobacteriaceae and P. aeruginosa. The class B enzymes are the most clinically-significant carbapenemases; they are metallo-beta-lactamases, mostly of the IMP and the VIM series. They have been reported worldwide and their genes are plasmid- and integron-located, hydrolyzing all beta-lactams with the exception of aztreonam. One single plasmid-mediated AmpC beta-lactamase, CMY-10, identified in an Enterobacter aerogenes isolate, has been shown to be a cephaslosporinase with some carbapenemase properties. Finally, the class D carbapenemases are being increasingly reported, mostly in Acinetobacter baumannii, and they compromise the efficacy of imipenem and meropenem significantly.

The three-dimensional structure of VIM-2, a Zn-beta-lactamase from Pseudomonas aeruginosa in its reduced and oxidised form

The crystal structures of the universally widespread metallo-beta-lactamase (MBL) Verona integron-encoded MBL (VIM)-2 from Pseudomonas aeruginosa have been solved in their native form as well as in an unexpected oxidised form. This carbapenem-hydrolysing enzyme belongs to the so-called B1 subfamily of MBLs and shares the folding of alpha beta/beta alpha sandwich, consisting of a core of beta-sheet surrounded by alpha-helices. Surprisingly, it showed a high tendency to be strongly oxidised at the catalytic cysteine located in the Cys site, Cys221, which, in the oxidised structure, becomes a cysteinesulfonic residue. Its native structure was obtained only in the presence of Tris(2-carboxyethyl)phosphine. This oxidation might be a consequence of a lower affinity for the second Zn located in the Cys site that would also explain the observed susceptibility of VIM-2 to chelating agents. This modification, if present in nature, might play a role in catalytic down-regulation. Comparison between native and oxidised VIM-2 and a predicted model of VIM-1 (which shows one residue different in the Cys site compared with VIM-2) is performed to explain the different activities and antibiotic specificities.

Competitive inhibitors of the CphA metallo-beta-lactamase from Aeromonas hydrophila

Various inhibitors of metallo-beta-lactamases have been reported; however, none are effective for all subgroups. Those that have been found to inhibit the enzymes of subclass B2 (catalytically active with one zinc) either contain a thiol (and show less inhibition towards this subgroup than towards the dizinc members of B1 and B3) or are inactivators behaving as substrates for the dizinc family members. The present work reveals that certain pyridine carboxylates are competitive inhibitors of CphA, a subclass B2 enzyme. X-ray crystallographic analyses demonstrate that pyridine-2,4-dicarboxylic acid chelates the zinc ion in a bidentate manner within the active site. Salts of these compounds are already available and undergoing biomedical testing for various nonrelated purposes. Pyridine carboxylates appear to be useful templates for the development of more-complex, selective, nontoxic inhibitors of subclass B2 metallo-beta-lactamases.

Emergence of Proteus mirabilis carrying the bla metallo-beta-lactamase gene

Seven genetically related Proteus mirabilis clinical isolates from a hospital in Thessaloniki, Greece, exhibited decreased susceptibility to imipenem and carried a bla(VIM-1) metallo-beta-lactamase gene. PCR mapping revealed that bla(VIM-1) was part of a class 1 integron that was probably located in the chromosome and also included the aacA7, dhfr and aadA genes. This is the first description of the bla(VIM-1) metallo-beta-lactamase gene in P. mirabilis.

Genetic and enzymatic properties of metallo-beta-lactamase VIM-5 from a clinical isolate of Enterobacter cloacae

A VIM-5-producing Enterobacter cloacae isolate (EDV/1) was identified in a collection of clinical strains stored before 2002. The gene, bla(VIM-5), was located on a 2,712-bp BamHI-HindIII fragment of a 23-kbp (approximately) nonconjugative plasmid (pEDV5) in a class 1 integron as a single gene cassette.

Risk factors for acquisition of multidrug-resistant Pseudomonas aeruginosa producing SPM metallo-beta-lactamase

To evaluate risk factors for colonization or infection due to multidrug-resistant Pseudomonas aeruginosa (MDRPa) carrying the bla(SPM) gene (SPM-MRDPa) among hospitalized patients, we undertook a case control study at a 480-bed, tertiary-care university hospital. Two different case definitions were used. In the first definition, a case patient (SPM case patient) was defined as a patient who had at least one isolate of SPM-MDRPa (14 patients). In the second, a case patient (non-SPM case patient) was defined as a patient who had at least one isolate of non-SPM-MDRPa (18 patients). For each case patient, we selected two controls, defined as a patient colonized and/or infected by a non-MDRPa isolate during the same study period and with the closest duration of hospitalization until the isolation of P. aeruginosa as cases. The use of quinolones was the single independent predictor of colonization and/or infection by bla(SPM) MDRPa (odds ratio [OR] = 14.70, 95% confidence interval [95% CI] = 1.70 to 127.34, P = 0.01), whereas the use of cefepime was the single predictor of colonization and/or infection by non-bla(SPM) MDRPa (OR = 8.50, 95% CI = 1.51 to 47.96, P = 0.01). The main risk factor for MDRPa was a history of antibiotics usage. Stratification of risk factor analysis by a precise mechanism of resistance led us to identify a specific antibiotic, a quinolone, as a predictor for SPM-MDRPa.

Evaluation of dipicolinic acid for detection of IMP- or VIM- type metallo-β-lactamase–producing Pseudomonas aeruginosa clinical isolates

We evaluated dipicolinic acid (DPA) as a chelating agent for detection of IMP- or VIM-type metallo-beta-lactamase (MBL)-producing Pseudomonas aeruginosa clinical isolates. Using the broth microdilution testing in the presence or absence of DPA, MBL producers exhibited 100%, 92%, or 100% of >or=8 times (media, 32 times) reduction of MICs in presence of DPA for ceftazidime, imipenem, or meropenem, respectively. In disk diffusion testing, expansion of growth inhibitory zone of these clinical isolates was clearly observed. Thus, DPA could be useful in the detection for MBL-producing P. aeruginosa clinical isolates.

OXA-46, a new class D beta-lactamase of narrow substrate specificity encoded by a blaVIM-1-containing integron from a Pseudomonas aeruginosa clinical isolate

A novel OXA-type enzyme, named OXA-46, was found to be encoded by a gene cassette inserted into a class 1 integron from a multidrug-resistant Pseudomonas aeruginosa clinical isolate. The variable region of the integron also contained a bla(VIM-1) metallo-beta-lactamase cassette and a duplicated aacA4 aminoglycoside acetyltransferase cassette. OXA-46 belongs to the OXA-2 lineage of class D beta-lactamases. It exhibits 78% sequence identity with OXA-2 and the highest similarity (around 92% identity) with another OXA-type enzyme detected in clinical isolates of Burkholderia cepacia and in unidentified bacteria from a wastewater plant. Expression of bla(OXA-46) in Escherichia coli decreased susceptibility to penicillins and narrow-spectrum cephalosporins but not to extended-spectrum cephalosporins, cefsulodin, aztreonam, or carbapenems. The enzyme was overproduced in E. coli and purified by two anion-exchange chromatography steps (approximate yield, 6 mg/liter). OXA-46 was made of a 28.5-kDa polypeptide and exhibited an alkaline pI (7.8). In its native form OXA-46 appeared to be dimeric, and the oligomerization state was not affected by EDTA. Kinetic analysis of OXA-46 revealed a specificity for narrow-spectrum substrates, including oxacillin, other penicillins (but not temocillin), and narrow-spectrum cephalosporins. The enzyme apparently did not interact with temocillin, oxyimino-cephalosporins, or aztreonam. OXA-46 was inactivated by tazobactam and carbapenems and, although less efficiently, also by clavulanic acid. Enzyme activity was not affected either by EDTA or by divalent cations and exhibited low susceptibility to NaCl. These findings underscore the functional and structural diversity that can be encountered among class D beta-lactamases.

Impact of remote mutations on metallo-beta-lactamase substrate specificity: implications for the evolution of antibiotic resistance

Metallo-beta-lactamases have raised concerns due to their ability to hydrolyze a broad spectrum of beta-lactam antibiotics. The G262S point mutation distinguishing the metallo-beta-lactamase IMP-1 from IMP-6 has no effect on the hydrolysis of the drugs cephalothin and cefotaxime, but significantly improves catalytic efficiency toward cephaloridine, ceftazidime, benzylpenicillin, ampicillin, and imipenem. This change in specificity occurs even though residue 262 is remote from the active site. We investigated the substrate specificities of five other point mutants resulting from single-nucleotide substitutions at positions near residue 262: G262A, G262V, S121G, F218Y, and F218I. The results suggest two types of substrates: type I (nitrocefin, cephalothin, and cefotaxime), which are converted equally well by IMP-6, IMP-1, and G262A, but even more efficiently by the other mutants, and type II (ceftazidime, benzylpenicillin, ampicillin, and imipenem), which are hydrolyzed much less efficiently by all the mutants. G262V, S121G, F218Y, and F218I improve conversion of type I substrates, whereas G262A and IMP-1 improve conversion of type II substrates, indicating two distinct evolutionary adaptations from IMP-6. Substrate structure may explain the catalytic efficiencies observed. Type I substrates have R2 electron donors, which may stabilize the substrate intermediate in the binding pocket. In contrast, the absence of these stabilizing interactions with type II substrates may result in poor conversion. This observation may assist future drug design. As the G262A and F218Y mutants confer effective resistance to Escherichia coli BL21(DE3) cells (high minimal inhibitory concentrations), they are likely to evolve naturally.