Cloning, nucleotide sequence, and expression of the Bacillus cereus 5/B/6 beta-lactamase II structural gene

Characterization and Identification of a novel chromosome-encoded metallo-β-lactamase WUS-1 in Myroides albus P34

In this study, we identified and characterized a novel chromosomally-encoded class B metallo-β-lactamase (MBL) gene designated bla WUS-1 in a carbapenem-resistant isolate Myroides albus P34 isolated from sewage discharged from an animal farm. Comparative analysis of the deduced amino acid sequence revealed that WUS-1 shares the highest amino acid similarities with the function-characterized MBLs MUS-1 (AAN63647.1; 70.73%) and TUS-1 (AAN63648.1; 70.32%). The recombinant carrying bla WUS-1 exhibited increased MICs levels against a number of β-lactam antimicrobials such as carbenicillin, ampicillin and imipenem, and β-lactamase inhibitors (clavulanic acid and tazobactam). The metallo-β-lactamase WUS-1 could also hydrolyze these antimicrobials and the hydrolytic activities could be inhibited by EDTA. Genetic context analysis of bla WUS-1 revealed that no mobile genetic element was found in its surrounding region. The plasmid pMA84474 of Myroides albus P34 harbored 6 resistance genes (bla OXA-347, aadS, bla MYO-1, ereD, sul2 and ermF) within an approximately 17 kb multidrug resistance (MDR) region. These genes, however, were all related to mobile genetic elements.

Bacillus cereus Invasive Infections in Preterm Neonates: an Up-to-Date Review of the Literature

Bacillus cereus group species are widespread, Gram-positive, spore-forming environmental bacteria. B. cereus sensu stricto is one of the major causes of food poisoning worldwide. In high-risk individuals, such as preterm neonates, B. cereus infections can cause fatal infections. It is important to note that the phenotypic identification methods commonly used in clinical microbiology laboratories make no distinction between B. cereus sensu stricto and the other members of the group (Bacillus anthracis excluded). As a result, all the invasive infections attributed to B. cereus are not necessarily due to B. cereus sensu stricto but likely to other closely related species of the B. cereus group. Next-generation sequencing (NGS) should be used to characterize the whole genome of the strains belonging to the B. cereus group. This could confirm whether the strains involved in previously reported B. cereus invasive infections preferentially belong to formerly known or emerging individual species. Moreover, infections related to B. cereus group species have probably been overlooked, since their isolation in human bacteriological samples has for a long time been regarded as an environmental contaminant of the cultures. Recent studies have questioned the emergence or reemergence of B. cereus invasive infections in preterm infants. This review reports our current understanding of B. cereus infections in neonates, including taxonomical updates, microbiological characteristics, bacterial identification, clinical features, host-pathogen interactions, environmental sources of contamination, and antimicrobial resistance.

Variations in the SDN Loop of Class A Beta-Lactamases: A Study of the Molecular Mechanism of BlaC (Mycobacterium tuberculosis) to Alter the Stability and Catalytic Activity Towards Antibiotic Resistance of MBIs

The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis calls for an immediate search for novel treatment strategies. Recently, BlaC, the principal beta-lactamase of Mycobacterium tuberculosis, was recognized as a potential therapeutic target. BlaC belongs to Ambler class A, which is generally susceptible to the beta-lactamase inhibitors currently used in clinics: tazobactam, sulbactam, and clavulanate. Alterations at Ser130 in conserved SDN loop confer resistance to mechanism-based inhibitors (MBIs) commonly observed in various clinical isolates. The absence of clinical evidence of S130G conversion in M. tuberculosis draws our attention to build laboratory mutants of S130G and S130A of BlaC. The study involving steady state, inhibition kinetics, and fluorescence microscopy shows the emergence of resistance against MBIs to the mutants expressing S130G and S130A. To understand the molecular reasoning behind the unavailability of such mutation in real life, we have used circular dichroism (CD) spectroscopy, differential scanning calorimetry (DSC), molecular dynamics (MD) simulation, and stability-based enzyme activity to compare the stability and dynamic behaviors of native and S130G/A mutant form of BlaC. A significant decrease in melting temperature (BlaC T_M 60°C, S130A T_M 50°C, and S130G T_M 45°C), kinetic instability at higher temperature, and comparative dynamic instability correlate the fact that resistance to beta-lactam/beta-lactamase inhibitor combinations will likely not arise from the structural alteration of BlaC, therefore establishing confidence that this therapeutic modality can be potentially applied as a part of a successful treatment regimen against M. tuberculosis.

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.

A comprehensive characterization of novel CYP-BM3 homolog (CYP-BA) from Bacillus aryabhattai

Functionalizing C-H bond poses one of the most significant challenges for chemists providing them with very few substrate-specific synthetic routes. Despite being incredibly plastic in their enzymatic ability, they are confined with deficient enzymatic action and limited explicitness of the substrates. In this study, we have endeavored to characterize novel cytochrome P450 from Bacillus aryabhattai (CYP-BA), a homolog of CYP P450-BM3, by taking interdisciplinary approaches. We conducted structure and sequence comparison to understand the conservation pattern for active site residues, conserved fold, evolutionary relationships among others. Molecular dynamics simulations were performed to understand the dynamic nature and interaction with the substrates. CYP-BA was successfully cloned, purified, and characterized. The enzyme's stability toward various physicochemical parameters was evaluated by UV-vis spectroscopy and Circular Dichroism (CD) spectroscopy. Various saturated fatty acids being the natural cytochrome P450 substrates were evaluated as catalytic efficiency of substrate oxidation by CYP-BA. The binding affinity of these natural substrates was monitored against CYP-BA by isothermal titration calorimetry (ITC). The catalytic performance of CYP-BA was satisfactory enough to proceed to the next step, that is, engineering to expand the substrate range to include polycyclic aromatic hydrocarbons (PAH). This is the first evidence of cloning, purifying and characterizing a novel homolog of CYP-BM3 to enable a better understanding of this novel biocatalyst and to provide a platform toward expanding its catalytic process through enzyme engineering.

The Hydric Environment: A Hub for Clinically Relevant Carbapenemase Encoding Genes

Carbapenems are β-lactams antimicrobials presenting a broad activity spectrum and are considered as last-resort antibiotic. Since the 2000s, carbapenemase producing Enterobacterales (CPE) have emerged and are been quickly globally spreading. The global dissemination of carbapenemase encoding genes (CEG) within clinical relevant bacteria is attributed in part to its location onto mobile genetic elements. During the last decade, carbapenemase producing bacteria have been isolated from non-human sources including the aquatic environment. Aquatic ecosystems are particularly impacted by anthropic activities, which conduce to a bidirectional exchange between aquatic environments and human beings and therefore the aquatic environment may constitute a hub for CPE and CEG. More recently, the isolation of autochtonous aquatic bacteria carrying acquired CEG have been reported and suggest that CEG exchange by horizontal gene transfer occurred between allochtonous and autochtonous bacteria. Hence, aquatic environment plays a central role in persistence, dissemination and emergence of CEG both within environmental ecosystem and human beings, and deserves to be studied with particular attention.

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.

Epidemiology of β-Lactamase-Producing Pathogens

β-Lactam antibiotics have been widely used as therapeutic agents for the past 70 years, resulting in emergence of an abundance of β-lactam-inactivating β-lactamases. Although penicillinases in Staphylococcus aureus challenged the initial uses of penicillin, β-lactamases are most important in Gram-negative bacteria, particularly in enteric and nonfermentative pathogens, where collectively they confer resistance to all β-lactam-containing antibiotics. Critical β-lactamases are those enzymes whose genes are encoded on mobile elements that are transferable among species. Major β-lactamase families include plasmid-mediated extended-spectrum β-lactamases (ESBLs), AmpC cephalosporinases, and carbapenemases now appearing globally, with geographic preferences for specific variants. CTX-M enzymes include the most common ESBLs that are prevalent in all areas of the world. In contrast, KPC serine carbapenemases are present more frequently in the Americas, the Mediterranean countries, and China, whereas NDM metallo-β-lactamases are more prevalent in the Indian subcontinent and Eastern Europe. As selective pressure from β-lactam use continues, multiple β-lactamases per organism are increasingly common, including pathogens carrying three different carbapenemase genes. These organisms may be spread throughout health care facilities as well as in the community, warranting close attention to increased infection control measures and stewardship of the β-lactam-containing drugs in an effort to control selection of even more deleterious pathogens.

Diversity and Proliferation of Metallo-β-Lactamases: a Clarion Call for Clinically Effective Metallo-β-Lactamase Inhibitors

The worldwide proliferation of life-threatening metallo-β-lactamase (MBL)-producing Gram-negative bacteria is a serious concern to public health. MBLs are compromising the therapeutic efficacies of β-lactams, particularly carbapenems, which are last-resort antibiotics indicated for various multidrug-resistant bacterial infections. Inhibition of enzymes mediating antibiotic resistance in bacteria is one of the major promising means for overcoming bacterial resistance. Compounds having potential MBL-inhibitory activity have been reported, but none are currently under clinical trials. The need for developing safe and efficient MBL inhibitors (MBLIs) is obvious, particularly with the continuous spread of MBLs worldwide. In this review, the emergence and escalation of MBLs in Gram-negative bacteria are discussed. The relationships between different class B β-lactamases identified up to 2017 are represented by a phylogenetic tree and summarized. In addition, approved and/or clinical-phase serine β-lactamase inhibitors are recapitulated to reflect the successful advances made in developing class A β-lactamase inhibitors. Reported MBLIs, their inhibitory properties, and their purported modes of inhibition are delineated. Insights into structural variations of MBLs and the challenges involved in developing potent MBLIs are also elucidated and discussed. Currently, natural products and MBL-resistant β-lactam analogues are the most promising agents that can become clinically efficient MBLIs. A deeper comprehension of the mechanisms of action and activity spectra of the various MBLs and their inhibitors will serve as a bedrock for further investigations that can result in clinically useful MBLIs to curb this global menace.

Antimicrobial susceptibility and characterization of metallo-β-lactamases, extended-spectrum β-lactamases, and carbapenemases of Bacillus cereus isolates

The susceptibility of 66 clinical and environmental B. cereus isolates were tested to selected antimicrobials by a broth microdilution method. All strains were resistant to β-lactams and susceptible to gentamicin and imipenem. Sixty-five (98.5%) isolates were susceptible to meropenem and ciprofloxacin and 74.2% to azithromycin. Significant differences in MIC values between environmental and clinical isolates were not demonstrated (p > 0.05). According to the disc diffusion method, 80.3%-98.5% of the strains were resistant to one or more of four cephalosporins. The presence of genes for B. cereus β-lactamases BCI, BCII, BCIII, extended-spectrum β-lactamases from the CTX and TEM family and the carbapenemases belonging to IMP and VIM family was studied. BlaII genes were expressed in all isolates; the PCR products for blaIII were also detected in two strains, but none of them was positive for blaI. The amplicon of the family bla_CTX-M, mostly M-1 and M-15, was confirmed among 68.2% of the isolates, while were bla_VIM-like genes determined in 21.2% of the samples.

High prevalence of blaVIM-1 gene in bacteria from Brazilian soil

This study investigated bacteria from soil samples to (i) determine the main bacterial genera and species having resistance to carbapenem and other β-lactams and (ii) establish if the mechanism of resistance was due to the production of metallo-β-lactamases. The isolates were characterized by PCR for metallo-β-lactamases and integrons, by antimicrobial susceptibility testing, and by sequencing. The antimicrobial profile of 40 imipenem-resistant Gram-positive soil isolates from all Brazilian regions demonstrated that 31 (77.5%) of them were multidrug resistant. Among the 40 isolates, 19 presented the bla_VIM gene and class 1 integrons by PCR. Six of the 19 isolates were identified as Paenibacillus sp., 12 as Bacillus sp., and just 1 was classified as Staphylococcus sp., by sequencing of the 16S rRNA gene. These results suggest that bacteria from soil can act as a source of bla_VIM-1 genes, representing a threat to public health.

Evolution of Metallo-β-lactamases: Trends Revealed by Natural Diversity and in vitro Evolution

The production of β-lactamase enzymes is one of the most distributed resistance mechanisms towards β-lactam antibiotics. Metallo-β-lactamases constitute a worrisome group of these kinds of enzymes, since they present a broad spectrum profile, being able to hydrolyze not only penicillins, but also the latest generation of cephalosporins and carbapenems, which constitute at present the last resource antibiotics. The VIM, IMP, and NDM enzymes comprise the main groups of clinically relevant metallo-β-lactamases. Here we present an update of the features of the natural variants that have emerged and of the ones that have been engineered in the laboratory, in an effort to find sequence and structural determinants of substrate preferences. This knowledge is of upmost importance in novel drug design efforts. We also discuss the advances in knowledge achieved by means of in vitro directed evolution experiments, and the potential of this approach to predict natural evolution of metallo-β-lactamases.

Overcoming differences: The catalytic mechanism of metallo-β-lactamases

Metallo-β-lactamases are the latest resistance mechanism of pathogenic and opportunistic bacteria against carbapenems, considered as last resort drugs. The worldwide spread of genes coding for these enzymes, together with the lack of a clinically useful inhibitor, have raised a sign of alarm. Inhibitor design has been mostly impeded by the structural diversity of these enzymes. Here we provide a critical review of mechanistic studies of the three known subclasses of metallo-β-lactamases, analyzed at the light of structural and mutagenesis investigations. We propose that these enzymes present a modular structure in their active sites that can be dissected into two halves: one providing the attacking nucleophile, and the second one stabilizing a negatively charged reaction intermediate. These are common mechanistic elements in all metallo-β-lactamases. Nucleophile activation does not necessarily requires a Zn(II) ion, but a Zn(II) center is essential for stabilization of the anionic intermediate. Design of a common inhibitor could be therefore approached based in these convergent mechanistic features despite the structural differences.

Carbapenemase-producing Gram-negative bacteria: current epidemics, antimicrobial susceptibility and treatment options

ABSTRACT 

Carbapenemases, with versatile hydrolytic capacity against β-lactams, are now an important cause of resistance of Gram-negative bacteria. The genes encoding for the acquired carbapenemases are associated with a high potential for dissemination. In addition, infections due to Gram-negative bacteria with acquired carbapenemase production would lead to high clinical mortality rates. Of the acquired carbapenemases, Klebsiella pneumoniae carbapenemase (Ambler class A), Verona integron-encoded metallo-β-lactamase (Ambler class B), New Delhi metallo-β-lactamase (Ambler class B) and many OXA enzymes (OXA-23-like, OXA-24-like, OXA-48-like, OXA-58-like, class D) are considered to be responsible for the worldwide resistance epidemics. As compared with monotherapy with colistin or tigecycline, combination therapy has been shown to effectively lower case-fatality rates. However, development of new antibiotics is crucial in the present pandrug-resistant era.

Metallo-β-lactamase structure and function

β-Lactam antibiotics are the most commonly used antibacterial agents and growing resistance to these drugs is a concern. Metallo-β-lactamases are a diverse set of enzymes that catalyze the hydrolysis of a broad range of β-lactam drugs including carbapenems. This diversity is reflected in the observation that the enzyme mechanisms differ based on whether one or two zincs are bound in the active site that, in turn, is dependent on the subclass of β-lactamase. The dissemination of the genes encoding these enzymes among Gram-negative bacteria has made them an important cause of resistance. In addition, there are currently no clinically available inhibitors to block metallo-β-lactamase action. This review summarizes the numerous studies that have yielded insights into the structure, function, and mechanism of action of these enzymes.

Carbapenem resistance in Enterobacteriaceae: here is the storm!

The current worldwide emergence of resistance to the powerful antibiotic carbapenem in Enterobacteriaceae constitutes an important growing public health threat. Sporadic outbreaks or endemic situations with enterobacterial isolates not susceptible to carbapenems are now reported not only in hospital settings but also in the community. Acquired class A (KPC), class B (IMP, VIM, NDM), or class D (OXA-48, OXA-181) carbapenemases, are the most important determinants sustaining resistance to carbapenems. The corresponding genes are mostly plasmid-located and associated with various mobile genetic structures (insertion sequences, integrons, transposons), further enhancing their spread. This review summarizes the current knowledge on carbapenem resistance in Enterobacteriaceae, including activity, distribution, clinical impact, and possible novel antibiotic pathways.

A sensitive coupled HPLC/electrospray mass spectrometry assay for SPM-1 metallo-beta-lactamase inhibitors

Antibiotic-resistant bacteria continue to threaten human health through multiple mechanisms, including hydrolytic inactivation of beta-lactam antibiotics by metallo-beta-lactamases (MBLs). The SPM-1 enzyme, originally identified from a Pseudomonas aeruginosa clinical isolate, is a Class B beta-lactamase responsible for resistance in bacteria against antibiotics such as penicillins, cephalosporins, and carbapenems. Unlike Class A, C, and D beta-lactamases, which employ a serine residue in their active site, Class B enzymes possess one or two Zn atoms in the active site that play both a structural and catalytic role. A beta-lactamase inhibitor with co-administration of a beta-lactam antibiotic has proven to be an effective treatment against antibiotic-resistant bacteria whose resistance is due to serine-based beta-lactamases (e.g., amoxicillin/clavulanic acid). A similar clinical approach has not yet been developed for resistant bacteria possessing MBLs. The identification and development of specific and effective MBL inhibitors to combat this resistance could extend the utility of currently prescribed antibiotics such as cephalosporins and carbapenems. To discover MBL inhibitors, compound libraries are screened typically by enzymatic hydrolysis of a chromogenic substrate such as nitrocefin monitored by absorbance. Spectrophotometric assays, while valuable, lack the sensitivity and selectivity to screen natural product extract libraries because of the strongly absorbing nature of some extracts and the dilute concentrations of active components. An assay is described herein that monitors the SPM-1-catalyzed hydrolysis of penicillin G by high-performance (high-pressure) liquid chromatography-electrospray mass spectroscopy, which permits investigations with greater sensitivity and selectivity allowing the screening of natural product extracts for inhibitors of MBLs.

Metallo beta lactamases in Pseudomonas aeruginosa and Acinetobacter species

The multi drug resistant gram negative bacteria especially Pseudomonas aeruginosa and Acinetobacter species are on the rise. The major defense in these bacteria against beta-lactam antibiotics is production of metallo beta lactamases (MBLs) which degrade this group of antibiotics including carbapenems. Till now five main types of MBLs have been described throughout the World--IMP, VIM, SPM, GIM and SIM. A new MBL has been recently reported in P. aeruginosa from Australia--bla AIM-1. There are no standard guidelines by CLSI for detection of these enzymes in various bacteria. A number of phenotypic tests based on different beta lactam-inhibitor combinations are being evaluated and used for routine testing. Regarding the treatment options--colistin, various antibiotic combinations and a few novel antibiotics are being tried and evaluated. Prevention is based on age old practices of strict infection control and judicious use of antibiotics.

Carbapenemases: the versatile beta-lactamases

Carbapenemases are beta-lactamases with versatile hydrolytic capacities. They have the ability to hydrolyze penicillins, cephalosporins, monobactams, and carbapenems. Bacteria producing these beta-lactamases may cause serious infections in which the carbapenemase activity renders many beta-lactams ineffective. Carbapenemases are members of the molecular class A, B, and D beta-lactamases. Class A and D enzymes have a serine-based hydrolytic mechanism, while class B enzymes are metallo-beta-lactamases that contain zinc in the active site. The class A carbapenemase group includes members of the SME, IMI, NMC, GES, and KPC families. Of these, the KPC carbapenemases are the most prevalent, found mostly on plasmids in Klebsiella pneumoniae. The class D carbapenemases consist of OXA-type beta-lactamases frequently detected in Acinetobacter baumannii. The metallo-beta-lactamases belong to the IMP, VIM, SPM, GIM, and SIM families and have been detected primarily in Pseudomonas aeruginosa; however, there are increasing numbers of reports worldwide of this group of beta-lactamases in the Enterobacteriaceae. This review updates the characteristics, epidemiology, and detection of the carbapenemases found in pathogenic bacteria.

Opposing effects of NaCl on reversibility and thermal stability of halophilic β-lactamase from a moderate halophile, Chromohalobacter sp. 560

Beta-lactamase from a moderately halophilic organism is expected to show salt-dependent stability. Here we examined the temperature-dependence of stability at different salt concentrations using circular dichroism (CD) and enzyme activity. NaCl showed opposing effects on melting temperature and reversibility of the thermal melting. Increasing NaCl concentration greatly increased the melting temperature from, e.g., 41 degrees C in the absence of NaCl to 61 degrees C in 3 M NaCl. Conversely, reversibility decreased from 92% to 0% in the corresponding NaCl solutions. When beta-lactamase was heated at different temperatures and NaCl concentrations, the activity recovery followed the reversibility, not the melting temperature. Heating beta-lactamase at 63 degrees C, slightly above the onset temperature of melting in 2 M NaCl and far above the melting in 0.2 M NaCl, showed a much greater recovery of activity in 0.2 M NaCl than in 2 M NaCl, again consistent with the reversibility of melting.

Metallo-beta-lactamases: the quiet before the storm?

The ascendancy of metallo-beta-lactamases within the clinical sector, while not ubiquitous, has nonetheless been dramatic; some reports indicate that nearly 30% of imipenem-resistant Pseudomonas aeruginosa strains possess a metallo-beta-lactamase. Acquisition of a metallo-beta-lactamase gene will invariably mediate broad-spectrum beta-lactam resistance in P. aeruginosa, but the level of in vitro resistance in Acinetobacter spp. and Enterobacteriaceae is less dependable. Their clinical significance is further embellished by their ability to hydrolyze all beta-lactams and by the fact that there is currently no clinical inhibitor, nor is there likely to be for the foreseeable future. The genes encoding metallo-beta-lactamases are often procured by class 1 (sometimes class 3) integrons, which, in turn, are embedded in transposons, resulting in a highly transmissible genetic apparatus. Moreover, other gene cassettes within the integrons often confer resistance to aminoglycosides, precluding their use as an alternative treatment. Thus far, the metallo-beta-lactamases encoded on transferable genes include IMP, VIM, SPM, and GIM and have been reported from 28 countries. Their rapid dissemination is worrisome and necessitates the implementation of not just surveillance studies but also metallo-beta-lactamase inhibitor studies securing the longevity of important anti-infectives.

Highly efficient renaturation of beta-lactamase isolated from moderately halophilic bacteria

Most, if not all, beta-lactamases reported to date are irreversibly denatured at 60-70 degrees C. Here, we found that a halophilic beta-lactamase from the moderately halophilic bacterium Chromohalobacter sp. 560 was highly stable against heat inactivation: it retained approximately 75% of its activity after boiling for 5 min in the presence of 0.2 M NaCl, suggesting that the protein either incompletely denatures during the boiling process or readily renatures upon cooling to the assay temperature. Circular dichroism showed a complete unfolding at 60 degrees C and a full reversibility, indicating that the observed activity after boiling is due to efficient refolding following heat denaturation. The enzyme showed optimal activity at 50-60 degrees C, indicating that an increase in activity with temperature offsets the thermal denaturation. The gene bla was cloned, and the primary structure of the enzyme was deduced to be highly abundant in acidic amino acid residues, one of the characteristics of halophilic proteins. Despite its halophilic nature, the enzyme refolds in low salt media after heat denaturation.

Biochemical characterization of beta-lactamases Bla1 and Bla2 from Bacillus anthracis

The Sterne and Ames strains of Bacillus anthracis carry chromosomal genes bla1 and bla2, which confer beta-lactam resistance when expressed in Escherichia coli. MIC measurements and steady-state kinetic analyses indicate that Bla1 possesses penicillinase activity while Bla2 possesses penicillinase, cephalosporinase, and carbapenem-hydrolyzing activities.

Beta-lactamase genes of the penicillin-susceptible Bacillus anthracis Sterne strain

Susceptibility to penicillin and other beta-lactam-containing compounds is a common trait of Bacillus anthracis. Beta-lactam agents, particularly penicillin, have been used worldwide to treat anthrax in humans. Nonetheless, surveys of clinical and soil-derived strains reveal penicillin G resistance in 2 to 16% of isolates tested. Bacterial resistance to beta-lactam agents is often mediated by production of one or more types of beta-lactamases that hydrolyze the beta-lactam ring, inactivating the antimicrobial agent. Here, we report the presence of two beta-lactamase (bla) genes in the penicillin-susceptible Sterne strain of B. anthracis. We identified bla1 by functional cloning with Escherichia coli. bla1 is a 927-nucleotide (nt) gene predicted to encode a protein with 93.8% identity to the type I beta-lactamase gene of Bacillus cereus. A second gene, bla2, was identified by searching the unfinished B. anthracis chromosome sequence database of The Institute for Genome Research for open reading frames (ORFs) predicted to encode beta-lactamases. We found a partial ORF predicted to encode a protein with significant similarity to the carboxy-terminal end of the type II beta-lactamase of B. cereus. DNA adjacent to the 5' end of the partial ORF was cloned using inverse PCR. bla2 is a 768-nt gene predicted to encode a protein with 92% identity to the B. cereus type II enzyme. The bla1 and bla2 genes confer ampicillin resistance to E. coli and Bacillus subtilis when cloned individually in these species. The MICs of various antimicrobial agents for the E. coli clones indicate that the two beta-lactamase genes confer different susceptibility profiles to E. coli; bla1 is a penicillinase, while bla2 appears to be a cephalosporinase. The beta-galactosidase activities of B. cereus group species harboring bla promoter-lacZ transcriptional fusions indicate that bla1 is poorly transcribed in B. anthracis, B. cereus, and B. thuringiensis. The bla2 gene is strongly expressed in B. cereus and B. thuringiensis and weakly expressed in B. anthracis. Taken together, these data indicate that the bla1 and bla2 genes of the B. anthracis Sterne strain encode functional beta-lactamases of different types, but gene expression is usually not sufficient to confer resistance to beta-lactam agents.

N-arylsulfonyl hydrazones as inhibitors of IMP-1 metallo-beta-lactamase

Members of a family of N-arylsulfonyl hydrazones have been identified as novel inhibitors of IMP-1, a metallo-beta-lactamase of increasing prevalence. Structure-activity relationship studies have indicated a requirement for bulky aromatic substituents on each side of the sulfonyl hydrazone backbone for these compounds to serve as efficient inhibitors of IMP-1. Molecular modeling has provided insight into the structural basis for the anti-metallo-beta-lactamase activity exhibited by this class of compounds.

Emerging carbapenemases in Gram-negative aerobes

Carbapenemases may be defined as beta-lactamases that significantly hydrolyze at least imipenem or/and meropenem. Carbapenemases involved in acquired resistance are of Ambler molecular classes A, B, and D. Class A, clavulanic acid-inhibited carbapenemases are rare. They are either chromosomally encoded (NMC-A, Sme-1 to Sme-3, IMI-1) in Enterobacter cloacae and Serratia marcescens, or plasmid encoded, such as KPC-1 in Klebsiella pneumoniae and GES-2 in Pseudomonas aeruginosa, the latter being a point-mutant of the clavulanic acid-inhibited extended-spectrum beta-lactamase GES-1. The class B enzymes are the most clinically significant carbapenemases. They are metalloenzymes of the IMP or VIM series. They have been reported worldwide but mostly from South East Asia and Europe. Metalloenzymes, whose genes are plasmid and integron located, hydrolyze virtually all beta-lactams except aztreonam. Finally, the class D carbapenemases are increasingly reported in Acinetobacter baumannii but compromise imipenem and meropenem susceptibility only marginally. The sources of the acquired carbapenemase genes remain unknown, as does the relative importance of the spread of epidemic strains as opposed to the spread of plasmid- or integron-borne genes. Because most of these carbapenemases confer only reduced susceptibility to carbapenems in Enterobacteriaceae, they may remain underestimated as a consequence of the lack of their detection.

Genetic diversity of carbapenem-hydrolyzing metallo-beta-lactamases from Chryseobacterium (Flavobacterium) indologenes

The class B carbapenem-hydrolyzing beta-lactamase IND-1 has been characterized for Chryseobacterium indologenes strain 001. With internal primers for the bla gene for IND-1 (bla(IND-1)) and an internal bla(IND-1) probe, PCR amplifications failed, while hybridization results were positive when DNA from another C. indologenes isolate, strain CIP101026, was used as a template. Thus, a bla(IND)-related gene was cloned from this C. indologenes reference strain. Sequencing of the insert of a recombinant plasmid conferring resistance to carbapenems revealed an open reading frame with a G + C content of 39.9% and coding for a 243-amino-acid preprotein named IND-2. IND-2 shared 80% amino acid identity with IND-1 and had a similar broad-spectrum resistance profile, including resistance to carbapenems. It was classified in functional subgroup 3a of class B carbapenem-hydrolyzing beta-lactamases. IND-1 and IND-2, despite their genetic diversity, possessed similar kinetic parameters, except that ceftazidime was hydrolyzed less by IND-2. To obtain the entire bla(IND)-related gene sequences of eight other C. indologenes isolates, PCR was performed using internal and external primers, followed by inverse PCR techniques. The likely chromosome-mediated metallo-beta-lactamases of the 10 C. indologenes isolates were divided into several groups and subgroups. IND-1, IND-2, IND-2a, IND-3, and IND-4 shared 77 to 99% amino acid identity.

Functional analysis of the active site of a metallo-beta-lactamase proliferating in Japan

An R-plasmid-mediated metallo-beta-lactamase was found in Klebsiella pneumoniae DK4 isolated in Japan in 1991. The nucleotide sequence of its structural gene revealed that the beta-lactamase termed DK4 was identical to the IMP-1 metallo-beta-lactamase which was mediated by a chromosomal gene of Serratia marcescens TN9106 isolated in Japan in 1991 (E. Osano et al., Antimicrob. Agents Chemother. 38:71-78, 1994). The dose effect of DK4 beta-lactamase production on the resistance levels indicated a significant contribution of the enzyme to bacterial resistance to all the beta-lactams except monobactams. The enzymatic characteristics of the DK4 beta-lactamase and its kinetic parameters for nine beta-lactams were examined. The DK4 beta-lactamase was confirmed to contain 2 mol of zinc per mol of enzyme protein. The apoenzyme that lacked the two zincs was structurally unstable, and the activities of only 30% of the apoenzyme molecules could be restored by the addition of 1 mM zinc sulfate. The substitution of five conserved histidines (His28, His86, His88, His149, His210) and a cysteine (Cys168) for an alanine indicated that His86, His88, and His149 served as ligands to one of the zincs and that Cys168 played a role as a ligand to the second zinc. Both zinc molecules contribute to the enzymatic process. Mutant enzymes that lack only one of these retained some activity. Additionally, a conserved aspartic acid at position 90 was replaced by asparagine. This mutant enzyme showed an approximately 1,000 times lower k(cat) value for cephalothin than that of the wild-type enzyme but retained the two zincs even after dialysis against zinc-free buffer. The observed effect of pH on the activity suggested that Asp90 functions as a general base in the enzymatic process.

Molecular and biochemical heterogeneity of class B carbapenem-hydrolyzing beta-lactamases in Chryseobacterium meningosepticum

Although the carbapenem-hydrolyzing beta-lactamase (CHbetaL) BlaB-1 is known to be in Chryseobacterium meningosepticum NCTC 10585, a second CHbetaL gene, bla(GOB-1), was cloned from another C. meningosepticum clinical isolate (PINT). The G+C content of bla(GOB-1) (36%) indicated the likely chromosomal origin of this gene. Its expression in Escherichia coli DH10B yields a mature CHbetaL with a pI of 8.7 and a relative molecular mass of 28.2 kDa. In E. coli, GOB-1 conferred resistance to narrow-spectrum cephalosporins and reduced susceptibility to ureidopenicillins, broad-spectrum cephalosporins, and carbapenems. GOB-1 had a broad-spectrum hydrolysis profile including penicillins and cephalosporins (but not aztreonam). The catalytic efficiency for meropenem was higher than for imipenem. GOB-1 had low amino acid identity with the class B CHbetaLs, sharing 18% with the closest, L-1 from Stenotrophomonas maltophilia, and only 11% with BlaB-1. Most of the conserved amino acids that may be involved in the active site of CHbetaLs (His-101, Asp-103, His-162, and His-225) were identified in GOB-1. Sequence heterogeneity was found for GOB-1-like and BlaB-1-like beta-lactamases, having 90 to 100% and 86 to 100% amino acid identity, respectively, among 10 unrelated C. meningosepticum isolates. Each isolate had a GOB-1-like and a BlaB-1-like gene. The same combination of GOB-1-like and BlaB-1-like beta-lactamases was not found in two different isolates. C. meningosepticum is a bacterial species with two types of unrelated chromosome-borne class B CHbetaLs that can be expressed in E. coli and, thus, may represent a clinical threat if spread in gram-negative aerobes.

Molecular characterization of a carbapenem-hydrolyzing beta-lactamase from Chryseobacterium (Flavobacterium) indologenes

Chryseobacterium (Flavobacterium) indologenes 001 clinical strain was resistant to several beta-lactam classes including carbapenems. Shotgun cloning experiments of Sau3AI restricted genomic DNA of C. indologenes 001 into pBKCMV cloning vector followed by transformation into Escherichia coli DH10B gave one recombinant plasmid possessing a 4.2-kb DNA insert. It encoded a pI 7.2 beta-lactamase of 239 amino acids (IND-1) which is a metallo-enzyme with a broad spectrum beta-lactam hydrolysis profile. This class B carbapenem-hydrolyzing beta-lactamase shares the highest identity (43%) with BlaB from C. meningosepticum, thus showing heterogeneity of carbapenem-hydrolyzing beta-lactamases in Chryseobacterium spp.

Rapid detection and evaluation of clinical characteristics of emerging multiple-drug-resistant gram-negative rods carrying the metallo-beta-lactamase gene blaIMP

Gram-negative rods (GNR) carrying the transferable carbapenem resistance gene blaIMP, including Pseudomonas aeruginosa and Serratia marcescens, have been isolated from more than 20 hospitals in Japan. Although the emergence of such multiple-drug-resistant bacteria is of utmost clinical concern, little information in regard to the distribution of blaIMP-positive GNR in hospitals and the clinical characteristics of infected patients is available. To address this, a system for the rapid detection of the blaIMP gene with a simple DNA preparation and by enzymatic detection of PCR products was developed. A total of 933 ceftazidime-resistant strains of GNR isolated between 1991 and 1996 at Nagasaki University Hospital, Nagasaki, Japan, were screened for the blaIMP gene; 80 isolates were positive, including 53 P. aeruginosa isolates, 13 other glucose-nonfermenting bacteria, 13 S. marcescens isolates, and 1 Citrobacter freundii isolate. Most of the patients from whom blaIMP-positive organisms were isolated had malignant diseases (53. 8%). The organisms caused urinary tract infections, pneumonia, or other infections in 46.3% of the patients, while they were just colonizing the other patients evaluated. It was possible that blaIMP-positive P. aeruginosa strains contributed to the death of four patients, while the other infections caused by GNR carrying blaIMP were not lethal. DNA fingerprinting analysis by pulsed-field gel electrophoresis suggested the cross transmission of strains within the hospital. The isolates were ceftazidime resistant and were frequently resistant to other antibiotics. Although no particular means of pathogenesis of blaIMP-positive GNR is evident at present, the rapid detection of such strains is necessary to help with infection control practices for the prevention of their dissemination and the transmission of the resistance gene to other pathogenic bacteria.

The mechanism of catalysis and the inhibition of the Bacillus cereus zinc-dependent beta-lactamase

The plot of kcat/Km against pH for the Bacillus cereus 569/H beta-lactamase class B catalysed hydrolysis of benzylpenicillin and cephalosporin indicates that there are three catalytically important groups, two of pKa 5.6+/-0.2 and one of pKa 9.5+/-0.2. Below pH 5 there is an inverse second-order dependence of reactivity upon hydrogen ion concentration, indicative of the requirement of two basic residues for catalysis. These are assigned to zinc(II)-bound water and Asp-90, both with a pKa of 5.6+/-0.2. A thiol, N-(2'-mercaptoethyl)-2-phenylacetamide, is an inhibitor of the class B enzyme with a Ki of 70 microM. The pH-dependence of Ki shows similar pH inflections to those observed in the catalysed hydrolysis of substrates. The pH-independence of Ki between pH 6 and 9 indicates that the pKa of zinc(II)-bound water must be 5.6 and not the higher pKa of 9.5. The kinetic solvent isotope effect on kcat/Km is 1.3+/-0.5 and that on kcat is 1.5. There is no effect on reactivity by either added zinc(II) or methanol. The possible mechanisms of action for the class B beta-lactamase are discussed, and it is concluded that zinc(II) acts as a Lewis acid to stabilize the dianionic form of the tetrahedral intermediate and to provide a hydroxide-ion bound nucleophile, whereas the carboxylate anion of Asp-90 acts as a general base to form the dianion and also, presumably, as a general acid catalyst facilitating C-N bond fission.

Molecular heterogeneity of the L-1 metallo-beta-lactamase family from Stenotrophomonas maltophilia

We have determined the nucleotide sequence of the blaS gene encoding the carbapenem-hydrolyzing L-1 beta-lactamase from Stenotrophomonas maltophilia GN12873. Analysis of the DNA and deduced amino acid sequences identified a product of 290 amino acids. Comparisons of the L-1 amino acid sequence with those of other zinc beta-lactamases showed 88.6% identity with the L-1 enzyme from S. maltophilia IID1275 and less than 20% identity with other class B metalloenzymes.

The renal membrane dipeptidase (dehydropeptidase I) inhibitor, cilastatin, inhibits the bacterial metallo-beta-lactamase enzyme CphA

The Aeromonas hydrophila AE036 chromosome contains a cphA gene encoding a metallo-beta-lactamase which is highly active against carbapenem antibiotics such as imipenem. Here we show that the cphA gene product shares inhibitory similarities with a mammalian zinc peptidase, membrane dipeptidase (MDP; dehydropeptidase I). Both enzymes are able to hydrolyze imipenem and are inhibited by cilastatin. The active site similarities of these enzymes are not reflected in any significant primary sequence similarity.

Kinetic analysis of extension of substrate specificity with Xanthomonas maltophilia, Aeromonas hydrophila, and Bacillus cereus metallo-beta-lactamases

Twenty beta-lactam molecules, including penicillins, cephalosporins, penems, carbapenems, and monobactams, were investigated as potential substrates for Xanthomonas maltophilia ULA-511, Aeromonas hydrophila AE036, and Bacillus cereus 5/B/6 metallo-beta-lactamases. A detailed analysis of the kinetic parameters examined confirmed these enzymes to be broad-spectrum beta-lactamases with different ranges of catalytic efficiency. Cefoxitin and moxalactam, substrates for the beta-lactamases from X. maltophilia ULA-511 and B. cereus 5/B/6, behaved as inactivators of the A. hydrophila AE036 metallo-beta-lactamase, which appeared to be unique among the enzymes tested in this study. In addition, we report a new, faster, and reliable purification procedure for the B. cereus 5/B/6 metallo-beta-lactamase, cloned in Escherichia coli HB101.

Contribution of enzymatic properties, cell permeability, and enzyme expression to microbiological activities of beta-lactams in three Bacteroides fragilis isolates that harbor a metallo-beta-lactamase gene

The metallo-beta-lactamase gene, ccrA, has been cloned from three clinical isolates of Bacteroides fragilis, TAL3636, QMCN3, and QMCN4. Although all three isolates harbored a gene encoding a potent beta-lactamase, the MICs of benzylpenicillin, piperacillin, cefotaxime, ceftazidime, imipenem, and biapenem for the three isolates varied from 4- to > 128-fold. QMCN4 was the most susceptible of the three isolates, followed by QMCN3. TAL3636 was resistant to all of the beta-lactams. Previous DNA sequence analysis of the three ccrA genes revealed that the enzymes differed at 5 amino acid residues (B. A. Rasmussen, Y. Gluzman, and F. P. Tally, Mol. Microbiol. 5:1211-1219, 1991). Biochemical characterization of the three enzymes revealed only small differences in kcat and Km values for the majority of beta-lactams tested. Thus, the 5 amino acid substitutions affected the hydrolyzing activity of the enzymes only modestly. Crypticity differences between the three isolates showed that QMCN4 was the least permeable of the isolates to cephaloridine, followed by TAL3636, and that QMCN3 was highly permeable to cephaloridine. Therefore, neither catalytic activity nor permeability was a major contributor to the dramatic differences in the MICs. Instead, microbiological susceptibility was closely related to the level of metallo-beta-lactamase present in each isolate. Both biochemical and physical studies indicated that TAL3636 produced 5- to 10-fold and 50- to 100-fold more metallo-beta-lactamase than QMCN3 and QMCN4, respectively. Therefore, the level of CcrA enzyme production is the dominant contributing factor to high-level resistance among strains harboring a ccrA gene.

Analysis of a carbapenem-hydrolyzing class A beta-lactamase from Enterobacter cloacae and of its LysR-type regulatory protein

Carbapenems such as imipenem are extended-spectrum beta-lactam antibiotics, which are not hydrolyzed by the beta-lactamases commonly found in Enterobacteriaceae. Here we report a gene encoding a carbapenemase, which was cloned from the chromosome of a clinical isolate of Enterobacter cloacae, strain NOR-1, into pACYC184 plasmid in Escherichia coli. Unlike all the sequenced carbapenemases, which are class B metallo-beta-lactamases, the mature protein (NmcA) is a class A serine beta-lactamase. NmcA shares the highest amino acid identity (50%) with the extended-spectrum class A beta-lactamase MEN-1 from E. coli. In the opposite orientation from the nmcA promoter, an overlapping and divergent promoter was detected, along with an open reading frame, which encoded a 33.5-kDa protein (NmcR). The NmcR amino acid sequence displays homology with LysR-type transcriptional regulatory proteins, including the conserved residues near its N terminus within a helix-turn-helix motif. Deletion of nmcR resulted in decreased carbapenem resistance and a loss of beta-lactamase inducibility, demonstrating a positive role of NmcR in NmcA expression.

Cloning and sequence analysis of the gene for a carbapenem-hydrolyzing class A beta-lactamase, Sme-1, from Serratia marcescens S6

Serratia marcescens S6 produces a pI 9.7 carbapenem-hydrolyzing beta-lactamase that is probably encoded by the chromosome (Y. Yang, P. Wu, and D. M. Livermore, Antimicrob. Agents Chemother. 34:755-758, 1990). A total of 11.3 kb of genomic DNA from this strain was cloned into plasmid pACYC184 in Escherichia coli. After further subclonings, the carbapenem-hydrolyzing beta-lactamase gene (blaSme-1) was sequenced (EMBL accession number Z28968). The gene corresponded to an 882-bp open reading frame which encoded a 294-amino-acid polypeptide. This open reading frame was preceded by a -10 and a -35 region consistent with a putative promoter sequence of members of the family Enterobacteriaceae. This promoter was active in E. coli and S. marcescens, as demonstrated by primer extension analysis. N-terminal sequencing showed that the Sme-1 enzyme had a 27-amino-acid leader peptide and enabled calculation of the molecular mass of the mature protein (29.3 kDa). Sequence alignment revealed that Sme-1 is a class A serine beta-lactamase and not a class B metalloenzyme. The earlier view that the enzyme was zinc dependent was discounted. Among class A beta-lactamases, Sme-1 had the greatest amino acid identity (70%) with the pI 6.9 carbapenem-hydrolyzing beta-lactamase, NMC-A, from Enterobacter cloacae NOR-1. Comparison of these two protein sequences suggested a role for specific residues in carbapenem hydrolysis. The relatedness of Sme-1 to other class A beta-lactamases such as the TEM and SHV types was remote. This work details the sequence of the second carbapenem-hydrolyzing class A beta-lactamase from an enterobacterial species and the first in the genus Serratia.

Molecular characterization of an enterobacterial metallo beta-lactamase found in a clinical isolate of Serratia marcescens that shows imipenem resistance

A clinical isolate of Serratia marcescens (TN9106) produced a metallo beta-lactamase (IMP-1) which conferred resistance to imipenem and broad-spectrum beta-lactams. The blaIMP gene providing imipenem resistance was cloned and expressed in Escherichia coli HB101. The IMP-1 was purified from E. coli HB101 that harbors pSMBNU24 carrying blaIMP, and its apparent molecular mass was calculated to be about 30 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Kinetic studies of IMP-1 against various beta-lactams revealed that this enzyme hydrolyzes not only various broad-spectrum beta-lactams but also carbapenems. However, aztreonam was relatively stable against IMP-1. Although clavulanate or cloxacillin failed to inhibit IMP-1, Hg2+, Fe2+, or Cu2+ blocked the enzyme's activity. Moreover, the presence of EDTA in the reaction buffer resulted in a decrease in the enzyme's activity. Carbapenem resistance was not transferred from S. marcescens TN9106 to E. coli CSH2 by conjugation. A hybridization study confirmed that blaIMP was encoded on the chromosome of S. marcescens TN9106. By nucleotide sequencing analysis, blaIMP was found to encode a protein of 246 amino acid residues and was shown to have considerable homology to the metallo beta-lactamase genes of Bacillus cereus, Bacteroides fragilis, and Aeromonas hydrophila. The G+C content of blaIMP was 39.4%. Four consensus amino acid residues, His-95, His-97, Cys-176, and His-215, which form putative zinc ligands, were conserved in the deduced amino acid sequence of IMP-1. By determination of the amino acid sequence at the N terminus of purified mature IMP-1, 18 amino acid residues were found to be processed from the N terminus of the premature enzyme as a signal peptide. These results clearly show that IMP-1 is an enterobacterial metallo beta-lactamase, of which the primary structure has been completely determined, that confers resistance to carbapenems and other broad-spectrum beta-lactams.

Biochemical properties of a carbapenem-hydrolyzing beta-lactamase from Enterobacter cloacae and cloning of the gene into Escherichia coli

A clinical isolate of Enterobacter cloacae, strain NOR-1, exhibited resistance to imipenem and remained susceptible to extended-spectrum cephalosporins. Clavulanic acid partially restored the susceptibility of the strain to imipenem. Two beta-lactamases with isoelectric points (pI) of 6.9 and > 9.2 were detected in strain E. cloacae NOR-1; the higher pI corresponded to AmpC cephalosporinase. Plasmid DNA was not detected in E. cloacae NOR-1 and imipenem resistance could not be transferred into Escherichia coli JM109. The carbapenem-hydrolyzing beta-lactamase gene was cloned into plasmid pACYC184. One recombinant plasmid, pPTN1, harbored a 5.3-kb Sau3A fragment from E. cloacae NOR-1 expressing the carbapenem-hydrolyzing beta-lactamase. This enzyme (pI 6.9) hydrolyzed ampicillin, cephalothin, and imipenem more rapidly than it did meropenem and aztreonam, but it hydrolyzed extended-spectrum cephalosporins only weakly and did not hydrolyze cefoxitin. Hydrolytic activity was partially inhibited by clavulanic acid, sulbactam, and tazobactam, was nonsusceptible to chelating agents such as EDTA and 1,10-o-phenanthroline, and was independent of the presence of ZnCl2. Its relative molecular mass was 30,000 Da. Induction experiments concluded that the carbapenem-hydrolyzing beta-lactamase biosynthesis was inducible by cefoxitin and imipenem. Subcloning experiments with HindIII partial digests of pPTN1 resulted in a recombinant plasmid, designated pPTN2, which contained a 1.3-kb insert from pPTN1 and which conferred resistance to beta-lactam antibiotics. Hybridization studies performed with a 1.2-kb HindIII fragment from pPtN2 failed to determine any homology with ampC of E. cloacae, with other known beta-lactamase genes commonly found in members of the family Enterobacteriaceae (bla(TEM-1)) and bla(SHV-3) derivatives), and with previously described carbapenemase genes such as those from Xanthomonas maltophilia, Bacillus cereus, Bacteroides fragilis (cfiA), and Aeromonas hydrophila (cphA). This work describing the biochemical properties of a novel chromosome-encoded beta-lactamase from E. cloacae indicates that this enzyme differs from all the previously described carbapenemases. This is the first reported cloning of a carbapenem-hydrolyzing gene from a member of the family Enterobacteriaceae.

An overview of the kinetic parameters of class B beta-lactamases

The catalytic properties of three class B beta-lactamases (from Pseudomonas maltophilia, Aeromonas hydrophila and Bacillus cereus) were studied and compared with those of the Bacteroides fragilis enzyme. The A. hydrophila beta-lactamase exhibited a unique specificity profile and could be considered as a rather specific 'carbapenemase'. No relationships were found between sequence similarities and catalytic properties. The problem of the repartition of class B beta-lactamases into sub-classes is discussed. Improved purification methods were devised for the P. maltophilia and A. hydrophila beta-lactamases including, for the latter enzyme, a very efficient affinity chromatography step on a Zn(2+)-chelate column.

Protein secretion in Bacillus species

Bacilli secrete numerous proteins into the environment. Many of the secretory proteins, their export signals, and their processing steps during secretion have been characterized in detail. In contrast, the molecular mechanisms of protein secretion have been relatively poorly characterized. However, several components of the protein secretion machinery have been identified and cloned recently, which is likely to lead to rapid expansion of the knowledge of the protein secretion mechanism in Bacillus species. Comparison of the presently known export components of Bacillus species with those of Escherichia coli suggests that the mechanism of protein translocation across the cytoplasmic membrane is conserved among gram-negative and gram-positive bacteria differences are found in steps preceding and following the translocation process. Many of the secretory proteins of bacilli are produced industrially, but several problems have been encountered in the production of Bacillus heterologous secretory proteins. In the final section we discuss these problems and point out some possibilities to overcome them.