Classification of beta-lactamases: groups 2c, 2d, 2e, 3, and 4

An analysis of contributing factors of head and neck space infections of odontogenic origin: A long-term retrospective clinical study (including COVID-19 pandemic period)

BACKGROUND

The purpose of this study is to investigate predisposing factors for the head and neck infections (HNIs), regarding to the demographic data, anatomical spaces, microbiology and antibiotic sensitivity for affected patients.

MATERIAL AND METHODS

A 13-year of retrospective study evaluating 470 patients with HNIs, treated as inpatient management in the Department of Oral and Maxillofacial Surgery of KyungHee University school of Dentistry, Seoul, Korea, from January 2009 to February 2022. Statistical analysis of demographic, time-related, anatomic, microbiologic, and treatment variables were investigated for each patient.

RESULTS

The frequency of HNIs was significantly higher in 50's in males, followed by 70's in females. High Severity score (SS) were significantly associated with increased LOH (Length of hospital stay) and LOM (Length of medication), while LOH showed more intensive relationship compared with LOM. The most frequently involved space in abscess was submandibular space, though incidence and severity of HNIs shows declining tendency throughout 13-year research. Streptococcus viridans was the most predominant species isolated from pus culture growth, and a combination of ampicillin and sulbactam was the 1st choice of antibiotics intravenously. According to the comparison analysis between recommended antibiotics from resistance testing result and clinically administered antibiotics, final coincidence rate was estimated about 55%.

CONCLUSIONS

Due to HNIs being multifactorial, predicting progression and management of HNIs is still a challenge for oral and maxillofacial surgeons. The present study showed several predisposing factors of SHNIs and their correlations, which could contribute to earlier diagnosis and more effective treatment planning for clinicians, thereby leading to the improvement of prognosis for patients, ultimately.

The functional convergence of antibiotic resistance in β-lactamases is not conferred by a simple convergent substitution of amino acid

Bacterial resistance to antibiotics is a serious medical and public health concern worldwide. Such resistance is conferred by a variety of mechanisms, but the extensive variability in levels of resistance across bacteria is a common finding. Understanding the underlying evolutionary processes governing this functional variation in antibiotic resistance is important as it may allow the development of appropriate strategies to improve treatment options for bacterial infections. The main objective of this study was to examine the functional evolution of β-lactamases, a common mechanism of enzymatic resistance that inactivates a widely used class of antibiotics. We first obtained β-lactamase protein sequences and minimal inhibitory concentration (MIC), a measure of antibiotic function, from previously published literature. We then used a molecular phylogenetic framework to examine the evolution of β-lactamase functional activity. We found that the functional activity of antibiotic resistance mediated by β-lactamase has evolved in a convergent manner within molecular classes, but is not associated with any single amino acid substitution. This suggests that the dynamics of convergent evolution in this system can vary between the functional and molecular (sequence) levels. Such disassociation may hamper bioinformatic approaches to antibiotic resistance determination and underscore the need for (less efficient but more effective) activity assays as an essential step in evaluating resistance in a given case.

Past and Present Perspectives on β-Lactamases

β-Lactamases, the major resistance determinant for β-lactam antibiotics in Gram-negative bacteria, are ancient enzymes whose origins can be traced back millions of years ago. These well-studied enzymes, currently numbering almost 2,800 unique proteins, initially emerged from environmental sources, most likely to protect a producing bacterium from attack by naturally occurring β-lactams. Their ancestors were presumably penicillin-binding proteins that share sequence homology with β-lactamases possessing an active-site serine. Metallo-β-lactamases also exist, with one or two catalytically functional zinc ions. Although penicillinases in Gram-positive bacteria were reported shortly after penicillin was introduced clinically, transmissible β-lactamases that could hydrolyze recently approved cephalosporins, monobactams, and carbapenems later became important in Gram-negative pathogens. Nomenclature is based on one of two major systems. Originally, functional classifications were used, based on substrate and inhibitor profiles. A later scheme classifies β-lactamases according to amino acid sequences, resulting in class A, B, C, and D enzymes. A more recent nomenclature combines the molecular and biochemical classifications into 17 functional groups that describe most β-lactamases. Some of the most problematic enzymes in the clinical community include extended-spectrum β-lactamases (ESBLs) and the serine and metallo-carbapenemases, all of which are at least partially addressed with new β-lactamase inhibitor combinations. New enzyme variants continue to be described, partly because of the ease of obtaining sequence data from whole-genome sequencing studies. Often, these new enzymes are devoid of any phenotypic descriptions, making it more difficult for clinicians and antibiotic researchers to address new challenges that may be posed by unusual β-lactamases.

Dissemination of Pseudomonas aeruginosa producing bla IMP-1 and bla VIM-1 in Qazvin and Alborz educational hospitals, Iran

BACKGROUND AND OBJECTIVES

Pseudomonas aeruginosa is a frequent opportunistic pathogen in health care associated infections that is highly resistant to the majority of β-lactams. The aims of this study were to access the antimicrobial susceptibility pattern of P. aeruginosa isolated from educational hospitals of Qazvin and Alborz provinces, to determine the prevalence of metallo-β-lactamase (MBL) among carbapenem non-susceptible isolates by combined disk (CD) method, and to detect the bla IMP, bla VIM, bla SIM, bla GIM, bla SPM and bla NDM-1-MBL genes.

MATERIALS AND METHODS

In this cross-sectional study, 300 P. aeruginosa isolates were collected from different clinical specimens in two provinces of Qazvin and Alborz hospitals, Iran. After identification of isolates by standard laboratory methods, antimicrobial susceptibility was done against 17 antibiotics according to clinical and laboratory standards institute (CLSI) guideline. CD method was carried out for detection of MBLs and the presence of bla IMP, bla VIM, bla SIM, bla GIM, bla NDM-1 and bla SPM-genes was further assessed by PCR and sequencing methods.

RESULTS

In this study, 107 (35.66%) isolates were non-susceptible to imipenem and/or meropenem among those 56 (52.3%) isolates were metallo-β-lactamase producer. Twenty-four of 56 (42.85%) MBL-positive isolates were confirmed to be positive for MBL-encoding genes in which 14 (25%) and 10 (17.85%) isolates carried bla IMP-1 and bla VIM-1 genes either alone or in combination. Three (5.35%) isolates carried bla IMP and bla VIM genes, simultaneously.

CONCLUSION

Considering the moderate prevalence and clinical importance of MBL-producing isolates, rapid identification and use of appropriate infection control (IC) measures are necessary to prevent further spread of infections by these resistant organisms.

Incidence and susceptibility of aerobic Gram-negative bacilli from 20 Canadian intensive care units: 1989-1993

OBJECTIVE

To assess the prevalence of antibiotic resistance in Canadian intensive care units.

DESIGN

The antimicrobial profiles of 1939 Gram-negative bacilli isolated in 20 Canadian intensive care units were analyzed using a custom designed MicroScan panel.

SETTING

The majority of the hospitals were tertiary care institutions, but some community hospitals were included.

PATIENTS

Adult intensive care unit patients were the sources of isolates.

MAIN RESULTS

Pseudomonas aeruginosa was the most frequently isolated microorganism overall, with Escherichia coli the most common initial isolate. Comparison of initial and repeat isolates showed that P aeruginosa readily acquired resistance to all antibiotic classes except the aminoglycosides. Enterobacter aerogenes developed resistance to ciprofloxacin and Enterobacter cloacae demonstrated resistance to all beta-lactam antibiotics except for imipenem on repeat isolation. Other Enterobacteriaceae remained susceptible. Historical comparison with data derived four years previously from 15 of the centres showed increased resistance of P aeruginosa and Acinetobacter species to ciprofloxacin while other susceptibility patterns remained stable.

CONCLUSIONS

The prevalence of Gram-negative resistance in Canadian hospitals is less than that reported in surveys done in some other countries, and was relatively stable over four years.

Three factors that modulate the activity of class D β-lactamases and interfere with the post-translational carboxylation of Lys70

The activity of class D β-lactamases is dependent on Lys70 carboxylation in the active site. Structural, kinetic and affinity studies show that this post-translational modification can be affected by the presence of a poor substrate such as moxalactam but also by the V117T substitution. Val117 is a strictly conserved hydrophobic residue located in the active site. In addition, inhibition of class D β-lactamases by chloride ions is due to a competition between the side chain carboxylate of the modified Lys70 and chloride ions. Determination of the individual kinetic constants shows that the deacylation of the acyl-enzyme is the rate-limiting step for the wild-type OXA-10 β-lactamase.

In vitro activity of ampicillin, cefoperazone, their combinations with sulbactam and other antimicrobials: survey of Russian isolates

Methodological differences makes it difficult to compare results of antimicrobial susceptibility testing obtained in Russia and in other regions. During the period from October 1993 to May 1994 susceptibility of 1296 isolates of bacteria was investigated according to NCCLS standards by the broth microdilution method. The order of activity of antibiotics against Gram-negative bacteria based on percent of susceptible strains from common hospitals were: amikacin (98%) > ciprofloxacin (93%) = imipenem (93%) > cefoperazone/sulbactam (92%) > ceftazidime (89%) > ceftriaxone (81%) > cefotaxime (80%) > cefoperazone (77%) > gentamicin (71%) > ampicillin/sulbactam (51%) > cefazoline (45%) > ampicillin (25%). The order of activity against strains from teaching hospitals was similar but the percent of susceptible strains was 10-20% less for the majority of antibiotics. The susceptibility level of Gram-negative isolates from Moscow teaching hospitals is lower than from Northern America and Europe. Ampicillln/sulbactam and cefoperazone/sulbactam, as well as other cephalosporins, demonstrated high activity against methicillin-susceptible staphylococci and penicillin-susceptible pneumococci. B-lactams/beta-lactamase inhibitor combinations were active against 100% strains of anaerobic bacteria.

Antibiotic susceptibility and resistance of the odontogenic microbiological spectrum and its clinical impact on severe deep space head and neck infections

OBJECTIVE

The purpose of this retrospective study was to identify the major pathogens responsible for deep space head and neck infections and their current resistance to routinely used antibiotics in a university hospital setting.

STUDY DESIGN

A total of 206 patients suffering from odontogenic deep space infections were treated at our department by means of surgical intervention and intravenous administration of antibiotics.

RESULTS

The predominant bacteria were viridans group streptococci (VGS), staphylococci, Prevotella, Peptostreptococcus, and Bacteroides. In the aerobic spectrum, resistance against clindamycin was found in 18%, against macrolides in 14%, and against penicillin G in 7%. The anaerobes were resistant to clindamycin in 11%, to metronidazole in 6%, and to penicillin G in 8%.

CONCLUSION

The high resistance rate for clindamycin and macrolides was especially striking and may necessitate an adaptation of our antibiotic regime in the future.

Critical role of tryptophan 154 for the activity and stability of class D beta-lactamases

The catalytic efficiency of the class D beta-lactamase OXA-10 depends critically on an unusual carboxylated lysine as the general base residue for both the enzyme acylation and deacylation steps of catalysis. Evidence is presented that the interaction between the indole group of Trp154 and the carboxylated lysine is essential for the stability of the posttranslationally modified Lys70. Substitution of Trp154 by Gly, Ala, or Phe yielded noncarboxylated enzymes which displayed poor catalytic efficiencies and reduced stability when compared to the wild-type OXA-10. The W154H mutant was partially carboxylated. In addition, the maximum values of k(cat) and k(cat)/K(M) were shifted toward pH 7, indicating that the carboxylation state of Lys70 is dependent on the protonation level of the histidine. A comparison of the three-dimensional structures of the different proteins also indicated that the Trp154 mutations did not modify the overall structures of OXA-10 but induced an increased flexibility of the Omega-loop in the active site. Finally, the deacylation-impaired W154A mutant was used to determine the structure of the acyl-enzyme complex with benzylpenicillin. These results indicate a role of the Lys70 carboxylation during the deacylation step and emphasize the importance of Trp154 for the ideal positioning of active site residues leading to an optimum activity.

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.

[Bacterial spectra and antibiotics in odontogenic infections. Renaissance of the penicillins?]

BACKGROUND

The role played by odontogenic infection in dental, oral, and maxillofacial surgery is not to be underestimated even at the present time. An extensive, standardized, prospective study was performed with the intention of verifying the bacterial spectrum of odontogenic infections to evaluate antibiotic sensitivity.

MATERIAL AND METHODS

Bacterial spectra and resistograms of 65 patients with an odontogenic infection were analyzed in a prospective study under standardized conditions for specimen collection and transport.

RESULTS

A total of 226 bacterial strains were analyzed. The ratio between anaerobes and aerobes was approximately 2:1. The most frequent aerobes were members of the genera Streptococcus (46 isolates), Staphylococcus (10 isolates), and Neisseria (9 isolates), respectively. The anaerobic gram-positive spectrum was dominated by members of the genera Eubacterium (19 isolates), Peptostreptococcus (16 isolates), and Actinomyces (12 isolates). The most frequently isolated gram-negative anaerobes were Prevotella (46 isolates), and Fusobacterium (21 isolates). The overall resistance to antibiotics was very low: only 7.3% of all bacteria were resistant to penicillin G/V, and 8.8% showed resistance to ampicillin. The resistance rates to other beta-lactam antibiotics were 4.4% to piperacillin and 0.6% to imipenem, respectively. Penicillin G presented the highest antimicrobial activity among aerobes: only 4.5% of anaerobic strains were resistant of penicillin G. The other resistance rates of anaerobic bacteria to antibiotics were as follows: ampicillin 24%, doxycycline 34%, erythromycin 18%, and clindamycin 9.3%. Penicillin G was also highly antimicrobially active to anaerobes. The resistance rates were: penicillin G 8.1%, ampicillin 2.6%, doxycycline 9.2%, erythromycin 10.2%, and clindamycin 1.4%, respectively.

Enzymes acting on peptides containing D-amino acid

Mainly microorganisms but only a few higher organisms are presently known to express enzymes that hydrolyze peptides containing D-amino acids. These enzymes can be involved in proceedings at the bacterial cell wall, in either assembly or modification, and thus cause resistance to glycopeptide antibiotics, or mediate resistance against beta-lactam antibiotics. In other cases the in vivo function is still unknown. New enzymes screened from nature, such as D-aminopeptidase, D-amino acid amidase, alkaline D-peptidase or D-aminoacylase, offer potential application in the production of D-amino acids, the synthesis of D-amino acid oligomers by promoting the reversed reaction under appropriate conditions, or in the field of semi-synthetic antibiotics.

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.

Probing the dynamics of a mobile loop above the active site of L1, a metallo-beta-lactamase from Stenotrophomonas maltophilia, via site-directed mutagenesis and stopped-flow fluorescence spectroscopy

A structural feature shared by the metallo-beta-lactamases is a flexible loop of amino acids that extends over their active sites and that has been proposed to move during the catalytic cycle of the enzymes, clamping down on substrate. To probe the movement of this loop (residues 152-164), a site-directed mutant of metallo-beta-lactamase L1 was engineered that contained a Trp residue on the loop to serve as a fluorescent probe. It was necessary first, however, to evaluate the contribution of each native Trp residue to the fluorescence changes observed during the catalytic cycle of wild-type L1. Five site-directed mutants of L1 (W39F, W53F, W204F, W206F, and W269F) were prepared and characterized using metal analyses, CD spectroscopy, steady-state kinetics, stopped-flow fluorescence, and fluorescence titrations. All mutants retained the wild-type tertiary structure and bound Zn(II) at levels comparable with wild type and exhibited only slight (<10-fold) decreases in k(cat) values as compared with wild-type L1 for all substrates tested. Fluorescence studies revealed a single mutant, W39F, to be void of the fluorescence changes observed with wild-type L1 during substrate binding and catalysis. Using W39F as a template, a Trp residue was added to the flexile loop over the active site of L1, to generate the double mutant, W39F/D160W. This double mutant retained all the structural and kinetic characteristics of wild-type L1. Stopped-flow fluorescence and rapid-scanning UV-visible studies revealed the motion of the loop (k(obs) = 27 +/- 2 s(-1)) to be similar to the formation rate of a reaction intermediate (k(obs) = 25 +/- 2 s(-1)).

In vivo folding of recombinant metallo-beta-lactamase L1 requires the presence of Zn(II)

Metallo-beta-lactamase L1, secreted by pathogenic Stenotrophomonas maltophilia, is a dinuclear Zn(II)-containing enzyme that hydrolyzes almost all known penicillins, cephalosporins, and carbapenems. The presence of Zn(II) ions in both metal binding sites is essential for full enzymatic activity; however, the mechanism of physiological metal incorporation is unknown. To probe metal incorporation, L1 was over-expressed in minimal media with (mmL1+Zn) and without (mmL1-Zn) Zn(II) added to the media, and the resulting proteins were purified and characterized. The mmL1+Zn sample was bound by a Q-Sepharose column, exhibited steady-state kinetic properties, bound Zn(II), existed as a tetramer, and yielded fluorescence emission and CD spectra similar to L1 overexpressed in rich media. On the other hand, the mmL1-Zn sample did not bind to a Q-Sepharose column, and gel filtration studies demonstrated that this protein was monomeric. The mmL1-Zn sample exhibited a lower kcat value, bound less Zn(II), and yielded fluorescence emission and CD spectra consistent with this enzyme being folded improperly. Taken together, these data demonstrate that the proper folding of L1 requires the presence of Zn(II) and suggest that in vitro, thermodynamic metal binding studies do not accurately reflect physiological metal incorporation into L1.

Role of a solvent-exposed tryptophan in the recognition and binding of antibiotic substrates for a metallo-beta-lactamase

Numerous X-ray crystal structures of the metallo-beta-lactamase from Bacteroides fragilis and related organisms show a beta-hairpin loop immediately adjacent to the active-site zinc atom(s). Both crystallographic and NMR information show that the end of this beta-hairpin loop, which contains a solvent exposed tryptophan residue, Trp49, is highly flexible in the absence of substrates or other ligands, giving rise in some of the X-ray structures to a lack of observable electron density in this region. We report an investigation of the role of this mobile, solvent-exposed tryptophan using site-directed mutagenesis, steady state kinetics measurements and characterization by NMR. Trp49 appears to have a role both in substrate binding and in promotion of catalysis. Substitution of this residue with a number of different side chains indicates that the binding interaction depends on the bulky hydrophobic and aromatic nature of the indole ring, which can provide relatively non-specific interactions with a variety of antibiotic substrates. In this way, the tryptophan at this position provides a large degree of the breadth of substrate specificity for the metallo-beta-lactamase. Previous studies established that the antibiotic binding site was sufficiently plastic that the derivatization of existing antibiotics is unlikely to result in the successful treatment of bacterial infections incorporating this resistance element. Rather, a more productive approach may be to design therapeutics directed towards this solvent-exposed tryptophan residue.

Probing substrate binding to metallo-beta-lactamase L1 from Stenotrophomonas maltophilia by using site-directed mutagenesis

BACKGROUND

The metallo-beta-lactamases are Zn(II)-containing enzymes that hydrolyze the beta-lactam bond in penicillins, cephalosporins, and carbapenems and are involved in bacterial antibiotic resistance. There are at least 20 distinct organisms that produce a metallo-beta-lactamase, and these enzymes have been extensively studied using X-ray crystallographic, computational, kinetic, and inhibition studies; however, much is still unknown about how substrates bind and the catalytic mechanism. In an effort to probe substrate binding to metallo-beta-lactamase L1 from Stenotrophomonas maltophilia, nine site-directed mutants of L1 were prepared and characterized using metal analyses, CD spectroscopy, and pre-steady state and steady state kinetics.

RESULTS

Site-directed mutations were generated of amino acids previously predicted to be important in substrate binding. Steady-state kinetic studies using the mutant enzymes and 9 different substrates demonstrated varying Km and kcat values for the different enzymes and substrates and that no direct correlation between Km and the effect of the mutation on substrate binding could be drawn. Stopped-flow fluorescence studies using nitrocefin as the substrate showed that only the S224D and Y228A mutants exhibited weaker nitrocefin binding.

CONCLUSIONS

The data presented herein indicate that Ser224, Ile164, Phe158, Tyr228, and Asn233 are not essential for tight binding of substrate to metallo-beta-lactamase L1. The results in this work also show that Km values are not reliable for showing substrate binding, and there is no correlation between substrate binding and the amount of reaction intermediate formed during the reaction. This work represents the first experimental testing of one of the computational models of the metallo-beta-lactamases.

Clinical resistance encountered in the respiratory surveillance program (RESP) study: a review of the implications for the treatment of community-acquired respiratory tract infections

The Respiratory Surveillance Program (RESP) is a large-scale surveillance study of potential bacterial pathogens from respiratory tract infections that was performed over a 10-month period (July to April) during the 1999-2000 respiratory infection season. It is also the first study of its kind to derive its information entirely from community-based medical practices. This study, therefore, provides insight into the identity, frequency, and susceptibility of the possible pathogens isolated from patients encountered by primary care physicians. Reduction of antibiotic susceptibility in various bacterial pathogens may be of academic interest. However, it is only the emergence of clinical resistance (strains exhibiting minimum inhibitory concentrations above the resistance breakpoint) to commonly used antibacterial agents in the most prevalent species that has significant impact on empiric therapy choices. A review of data from RESP indicated that the most prevalent species were Moraxella catarrhalis, Haemophilus influenzae, Staphylococcus aureus, and Streptococcus pneumoniae. As expected, the prevalence of these bacterial isolates varied by disease state. The prevalence of clinical resistance to various antibiotics ranged, within these 4 species, between 0% and 92%. Resistance to the greatest number of drugs was expressed by S pneumoniae, followed by S aureus, H influenzae, and M catarrhalis. The prevalence of antibiotic resistance found among these community-isolated pathogens was surprisingly similar to that reported in hospital-based studies, suggesting that resistance is as important an issue in the community as it is in hospitals. With few exceptions, the prevalence of resistance was fairly uniform across disease states. The antibiotics most likely to encounter clinically resistant isolates during the treatment of community-acquired respiratory tract infections were penicillins, macrolides, and trimethoprim/sulfamethoxazole. The antibiotics least likely to encounter resistance were quinolones, followed by ceftriaxone and amoxicillin/clavulanate.

In vitro activity of R-95867, the active metabolite of a new oral carbapenem, CS-834, against anaerobic bacteria

The in vitro activity of R-95867, the active metabolite of a new oral carbapenem, CS-834, was compared with those of DU-6859a, cefditoren, ampicillin/sulbactam and clindamycin against a variety of anaerobic bacteria. R-95867 inhibited 90% of anaerobic strains at </=2 mg/L. In general, R-95867 was 2- to 4-fold less active than DU-6859a but more active than other agents tested against strains of peptostreptococci, clostridia, the Bacteroides fragilis group, Porphyromonas spp. and fusobacteria. R-95867 was stable to hydrolysis by beta-lactamase type 2e derived from B. fragilis, Prevotella bivia and Prevotella intermedia, but unstable to hydrolysis by carbapenemase from B. fragilis.

Evolution and spread of SHV extended-spectrum beta-lactamases in gram-negative bacteria

Resistance to beta-lactam antibiotics has been a problem for as long as these drugs have been used in clinical practice. In clinically significant bacteria the most important mechanism of resistance is the production of one or more beta-lactamases, enzymes that hydrolyse the beta-lactam bond characteristic of this family of antibiotics. Prominent among the beta-lactamases produced by the Enterobacteriaceae is the SHV family. The first reported SHV beta-lactamase had a narrow spectrum of activity. By the accumulation of point mutations at sites that affect the active site of the enzyme, a family of derivatives of SHV-1 has evolved. Derivatives of SHV-1 either have an extended spectrum of activity, capable of inactivating third-generation cephalosporins, or are resistant to beta-lactamase inhibitors. This review describes the evolution and spread of the SHV family of beta-lactamases, introducing the structure-function analysis made possible by DNA sequence analysis. It also reviews the methods used to characterize members of this family of beta-lactamases, indicating some of the difficulties involved.

Beta-lactam degradation catalysed by Cd2+ ion in methanol

Kinetic schemes are established for degradation catalysed by Cd2+ ions in methanolic medium for penicillin G, penicillin V and cephalothin, a cephalosporin. Methanolysis of penicillin V and cephalothin occurs with the formation of a single substrate-metal ion intermediate complex, SM, while degradation of penicillin G occurs with the initial formation of two complexes with different stoichiometry, SM and S2M. In each case. degradation is of first order with respect to SM with rate constant values equal to 0.079 min(-1), 0.120 min(-1) and 0.166 min(-1) at 20, 25 and 30 degrees C, respectively, for penicillin G; 0.061 min(-1) at 20 degrees C for penicillin V; and 2.0 x 10(-3) min(-1) at 20 degrees C for cephalothin. Activation energy for the decomposition process of the SM intermediate for penicillin G was calculated to be about 5.5 x 10(4) J/mol. Equilibrium constant values between SM compound and S2M at 20 degrees C (77.1 l/mol), 25 degrees C (45.3 l/mol) and at 30 degrees C (25.7 l/mol) were also calculated as well as the normal enthalpy of this equilibrium. With respect to the reaction products there is evidence that Cd2+ becomes part of their structure, forming complexes between Cd2+ and the product resulting from antibiotic methanolysis (L). Some characteristics of these complexes are discussed.

Purification, characterization, and kinetic studies of a soluble Bacteroides fragilis metallo-beta-lactamase that provides multiple antibiotic resistance

Resistance to multiple beta-lactam antibiotics traced to the expression of Zn(II) requiring metallo-beta-lactamases has emerged in clinical isolates of several bacterial strains including Bacteroides fragilis, a pathogen commonly found in suppurative/surgical infections. A soluble B. fragilis metallo-beta-lactamase has been purified to homogeneity from the cell growth medium after expression as a secretory protein in Escherichia coli. The enzyme requires two tightly bound Zn(II) ions for full activity, and the Zn(II) ions can be removed by EDTA from the enzyme. The apoenzyme is reactivated by stoichiometric amounts of Zn(II) and Co(II) ions. The Co(II)-substituted enzyme exhibits a UV-visible spectrum characterized by strong Co(II) d-d transitions at 510, 548, 615, and 635 nm and an EPR spectrum with g values of 5. 52, 4.25, and 2.01: features that serve as useful spectroscopic handles for the mechanistic studies of the enzyme. Although steady-state and transient-state kinetic studies of the soluble Zn(II) enzyme with nitrocefin as substrate found no ionizable groups with pKa values between 5.25 and 10.0 involved in catalysis, a kinetically significant proton transfer step in turnover was implicated by studies in deuterium oxide. These studies also detected the accumulation of an enzyme-bound intermediate and provide the basis for a minimal kinetic scheme describing metallo-beta-lactamase-catalyzed nitrocefin hydrolysis.

Activities of three quinolones, alone and in combination with extended-spectrum cephalosporins or gentamicin, against Stenotrophomonas maltophilia

The present study examined the activities of trovafloxacin, levofloxacin, and ciprofloxacin, alone and in combination with cefoperazone, ceftazidime, cefpirome, and gentamicin, against 100 strains of Stenotrophomonas maltophilia by the MIC determination method and by synergy testing of the combinations by the time-kill and checkerboard titration methods for 20 strains. The respective MICs at which 50% and 90% of isolates were inhibited for the drugs used alone were as follows: trovafloxacin, 0.5 and 2.0 microg/ml; levofloxacin, 2.0 and 4.0 microg/ml; ciprofloxacin, 4.0 and 16.0 microg/ml; cefoperazone, >128.0 and >128.0 microg/ml; ceftazidime, 32.0 and >128.0 microg/ml; cefpirome, >128.0 and >128.0 microg/ml; and gentamicin, 128.0 and >128.0 microg/ml. Synergistic fractional inhibitory concentration indices (</=0.5) were found for >/=50% of strains for trovafloxacin-cefoperazone, trovafloxacin-ceftazidime, levofloxacin-cefoperazone, levofloxacin-ceftazidime, ciprofloxacin-cefoperazone, and ciprofloxacin-ceftazidime, with other combinations affecting fewer strains. For 20 strains tested by the checkerboard titration and time-kill methods, synergy (>/=100-fold drop in count compared to the count achieved with the more active compound) was more pronounced after 12 h due to regrowth after 24 h. At 12 h, trovafloxacin at 0.004 to 0.5 microg/ml showed synergy with cefoperazone for 90% of strains, with ceftazidime for 95% of strains with cefpirome for 95% of strains, and with gentamicin for 65% of strains. Levofloxacin at 0.03 to 0.5 microg/ml and ciprofloxacin at 0.5 to 2.0 microg/ml showed synergy with cefoperazone for 80% of strains, with ceftazidime for 90 and 85% of strains, respectively, with cefpirome for 85 and 75% of strains, respectively, and with gentamicin for 65 and 75% of strains, respectively. Time-kill assays were more discriminatory than checkerboard titration assays in demonstrating synergy for all combinations.

Catalytic properties of class A beta-lactamases: efficiency and diversity

beta-Lactamases are the main cause of bacterial resistance to penicillins, cephalosporins and related beta-lactam compounds. These enzymes inactivate the antibiotics by hydrolysing the amide bond of the beta-lactam ring. Class A beta-lactamases are the most widespread enzymes and are responsible for numerous failures in the treatment of infectious diseases. The introduction of new beta-lactam compounds, which are meant to be 'beta-lactamase-stable' or beta-lactamase inhibitors, is thus continuously challenged either by point mutations in the ubiquitous TEM and SHV plasmid-borne beta-lactamase genes or by the acquisition of new genes coding for beta-lactamases with different catalytic properties. On the basis of the X-ray crystallography structures of several class A beta-lactamases, including that of the clinically relevant TEM-1 enzyme, it has become possible to analyse how particular structural changes in the enzyme structures might modify their catalytic properties. However, despite the many available kinetic, structural and mutagenesis data, the factors explaining the diversity of the specificity profiles of class A beta-lactamases and their amazing catalytic efficiency have not been thoroughly elucidated. The detailed understanding of these phenomena constitutes the cornerstone for the design of future generations of antibiotics.

A review of the in vitro activity of meropenem and comparative antimicrobial agents tested against 30,254 aerobic and anaerobic pathogens isolated world wide

The in vitro activity of meropenem (formerly SM-7738), a new carbapenem, was compared with that of imipenem and five other broad-spectrum antimicrobials (ceftazidime, cefotaxime, piperacillin, piperacillin/tazobactam, and ciprofloxacin) against 30,254 clinically significant pathogens isolated in nine countries worldwide. Overall, the carbapenems, meropenem and imipenem, were the most active drugs. Meropenem was four- to 64-fold more active than imipenem against Gram-negative bacteria, including the Enterobacteriaceae, Pseudomonas aeruginosa, Burkholderia cepacia, Haemophilus influenzae, and Neisseria meningitidis. Meropenem was also quite active against ceftazidime-resistant strains of Enterobacteriaceae, inhibiting 87.5 to 100% at < or = 4 micrograms/ml. In contrast, imipenem was four- to eight-fold more active than meropenem against Gram-positive species, including methicillin-susceptible strains of Staphylococcus aureus and Staphylococcus epidermidis, Streptococcus pneumoniae, and Enterococcus faecalis. Among the anaerobes, strains resistant to meropenem or imipenem were encountered very rarely. These extensive data provide additional in vitro support for the clinical use of meropenem as a broad spectrum antimicrobial agent active against key pathogenic species of bacteria.

High-yield expression, purification, and characterization of active, soluble Bacteroides fragilis metallo-beta-lactamase, CcrA

The gene from Bacteroides fragilis encoding a metallo-beta-lactamase, ccrA, was expressed in Escherichia coli BL21(DE3) containing the wild-type disulfide bond-catalyzing system dsb as an active, soluble enzyme in quantities exceeding 100 mg/liter using both rich and minimal media. Both the nonfusion and a glutathione S-transferase fusion enzyme lacking the periplasmic signal sequence were purified to homogeneity. Characteristics of the purified nonfusion enzyme are shown to be similar to those of the renatured enzyme previously reported. Thermal denaturation studies using circular dichroism and fluorescence spectroscopy show that CcrA undergoes a transition at approximately 50 degrees C which corresponds to the transition temperature of catalytic activity. The secondary structure of the protein and the catalytic apparatus are thus intimately linked.

Characterization of the metal-binding sites of the beta-lactamase from Bacteroides fragilis

In an effort to better understand the structure and function of the metallo-beta-lactamase from Bacteroides fragilis, spectroscopic and metal-binding studies were performed on the native, metal-substituted, and mutant forms of the enzyme. Atomic absorption studies demonstrate that the native B. fragilis enzyme tightly binds 2 mol of Zn(II) and, along with mutagenesis studies, that the presence of both metal ions is required for full catalytic activity. EPR spectroscopy was used to confirm that the Co(II)-substituted beta-lactamase binds 2 mol of Co(II) per mole of enzyme, that the two Co(II)'s are highspin and probably uncoupled, with apparent g values of 6.5, 4.2, and 2.0, and that the coordination number of the Co(II) is 5 or 6. This number of ligands for the Co(II)-substituted enzyme is confirmed by UV-Vis spectra, which demonstrate the presence of very weak d-d transitions between 550 and 650 nm (epsilon approximately 30 M-1.cm-1) and an intense feature at 320 nm (epsilon approximately 1570 M-1.cm-1). The latter is assigned to a cysteine sulfur to Co(II) ligand-to-metal charge transfer band, and this assignment is confirmed by the disappearance of this band in the UV-Vis spectrum of a Co(II)-substituted C168S mutant. H NMR studies on the Co(II)-substituted enzyme suggest the presence of three histidine ligands bound to Co(II). Taken together, these studies support the sequence comparison study of Rasmussen et al., in which there is a catalytic metal-binding site with three histidines and one cysteine (C168). The remaining ligands are postulated to be water molecules involved in catalysis. Mutagenesis studies, in combination with activity assays and metal-binding studies, have been used to identify Asp61, Asp90, Asp152, and Asp183 as possible ligands to the second metal-binding site, with Asp90 and Asp152 having a pronounced effect on kcat. These results are discussed in light of the recent crystal structure of the metallo-beta-lactamase from B. cereus.

Characterization of IMI-1 beta-lactamase, a class A carbapenem-hydrolyzing enzyme from Enterobacter cloacae

In 1984, a year prior to the U.S. approval of imipenem for clinical use, a wound isolate and a bile isolate of Enterobacter cloacae were obtained from two patients in a California hospital. These isolates were resistant to imipenem, penicillins, and inhibitor combinations; early cephalosporins such as cephalothin, cefamandole, and cefoxitin; and cefoperazone. However, they were susceptible (MICs, < 4 micrograms/ml) to cefotaxime, ceftriaxone, ceftazidime, and moxalactam. Both strains produced an apparent TEM-1 beta-lactamase; an inducible NmcA-type imipenem-hydrolyzing beta-lactamase, IMI-1, with a pl of 7.05; and an inducible beta-lactamase with a pI of 8.1, typical of an E. cloacae AmpC beta-lactamase. Purified IMI-1 hydrolyzed imipenem and benzylpenicillin at modest rates, but more slowly than cephaloridine. The enzyme was inhibited by clavulanic acid and tazobactam. EDTA did not inhibit the cephaloridine-hydrolyzing activity. The beta-lactamase gene encoding IMI-1, imiA1, was cloned from E. cloacae 1413B. Sequence analysis identified the imiA1 gene as encoding a class A serine beta-lactamase. Both the imiA1 DNA and encoded amino acid sequences shared greater than 95% identity with the NmcA gene and its encoded protein. DNA sequence analysis also identified a gene upstream of imiA1 that shares > 95% identity with nmcR and that may encode a regulatory protein. In conclusion, IMI-1, a carbapenem-hydrolyzing beta-lactamase inhibited by clavulanic acid, was identified as a group 2f, class A, carbapenem-hydrolyzing cephalosporinase.

The emergent needs for basic research, education, and surveillance of antimicrobial resistance. Problems facing the report from the American Society for Microbiology Task Force on Antibiotic Resistance

The American Society for Microbiology (ASM) convened a task force to study the current prevelance of antibiotic resistance and the problems associated with it. The task force produced a series of recommendations centered around three key elements: (a) education of the physician and the public; (b) encouragement of more basic research directed to the development of new antimicrobials and vaccines; and (c) the setting-up of a national surveillance system to both confirm and monitor the extent of the problem. Since the publication of this report in 1995, progress has been slow. No "consortium-style" funding has been set aside and any initiatives, such as the first steps in an education program by the ASM, are still in their planning phases. The spirit of cooperation and trust needed to deal with this problem appears to be lacking.

The beta-lactamases of Moraxella (Branhamella) catarrhalis isolated from Danish children

Two distinct beta-lactamases have been isolated from Moraxella catarrhalis: the stronger acting BRO-1 enzyme and the weaker acting BRO-2. Several reports have noted an effect of penicillin and ampicillin on infections caused by M. catarrhalis in spite of the presence of beta-lactamase production. The purpose of this work was to charaterize the beta-lactamases of M. catarrhalis isolated from Danish children regarding type and susceptibility, and to relate these findings to the eradication of beta-lactamases-producing strains by use of antibiotic treatment with penicillin or ampicillin. MICs for penicillin V, ampicillin, cefuroxime and amoxicillin/clavulanic acid (2:1) were determined in 70 strains of M. catarrhalis: 46 strains from children with lower respiratory tract infection (LRTI) and 24 strains from respiratory healthy children, beta-lactamase production was found in 59 strains. The BRO-1 enzyme was identified by isoelectric focusing in 55 strains (93.2%) and BRO-2 in 3 strains (5.1%); in 1 strain no isoelectric bands were produced. All strains were susceptible to cefuroxime and amoxicillin/clavulanic acid, and non-beta-lactamase-producing strains were susceptible to penicillin and ampicillin. For the beta-lactamase-producing strains, MIC50 of penicillin was 8.0 micrograms/ml, while MIC50 of ampicillin was 1.0 microgram/ml and MIC90 of ampicillin was 2.0 micrograms/ml. M. catarrhalis was more often eradicated from the children who received antibiotic treatment with penicillin or ampicillin than from those who did not receive any treatment, indicating an in vivo effect of penicillin and ampicillin in spite of the beta-lactamase production.

Comparative activity of eight antimicrobial agents against clinical bacterial isolates from the United States, measured by two methods

In a surveillance study conducted during 1992-1993 at 83 medical institutions of different types and sizes (e.g., laboratories, community hospitals, teaching hospitals) and from different geographical areas of the United States, clinical bacterial isolates were tested for their susceptibility to eight comparative antimicrobial agents (cefepime, ceftazidime, cefotaxime, ceftriaxone, ciprofloxacin, gentamicin, imipenem, and piperacillin). A total of 12,574 isolates were tested by either the Etest method (AB Biodisk) or a microdilution method (MicroScan) in the participating laboratories; 11.8% of these isolates were subsequently retested for quality assurance purposes by both methods in a central laboratory. The results obtained in the central laboratory were essentially the same as the results obtained in the participating laboratories. This article presents data for gram-negative and gram-positive isolates other than Streptococcus pneumoniae, the results of which have been previously published. Antimicrobial susceptibility results obtained with the two different minimum inhibitory concentration (MIC) methods--MicroScan and Etest--showed that most isolates of Enterobacteriaceae were susceptible to cefepime, exceeding the activity of ceftazidime, ceftriaxone, and cefotaxime, principally because of the greater activity of cefepime against the species that produce Bush group 1 beta-lactamases (predominantly Enterobacter cloacae, Enterobacter aerogenes, and Citrobacter freundii). In addition, the activity of cefepime against Pseudomonas aeruginosa isolates was essentially equivalent to that of ceftazidime and greater than that of third-generation cephalosporins. Most methicillin-susceptible Staphylococcus aureus were susceptible to all the cephalosporins, whereas methicillin-resistant S. aureus and enterococci were resistant. Overall, the most active antimicrobials in this study were imipenem, ciprofloxacin, and cefepime, but the activity of all the antimicrobials varied with different species. Categorically, the results from the microdilution and Etest methods were equivalent.

Beta-lactam resistance in anaerobic bacteria: a review

The majority of the human microflora consists of anaerobic bacteria. Normally these bacteria have low pathogenicity, but under certain conditions, such as destruction of tissues and poor circulation or impaired host defense, they may cause serious infections. Bacteroides species are the most frequently isolated microorganisms from suppurative anaerobic infections and they have the broadest spectrum of resistance to the commonly used antimicrobial agents. Resistance to antimicrobial agents is an increasing problem, especially to beta-lactam compounds. Multiresistant clinical isolates, resistant to beta-lactam antibiotics as well as other antimicrobial agents used in the treatment and prophylaxis of anaerobic infections are now occurring. Resistance to beta-lactam antibiotics is usually mediated by beta-lactamase production. A few isolates of Bacteroides fragilis are producing metallo-beta-lactamases which are capable of hydrolyzing beta-lactamase stable compounds such as cefoxitin and imipenem. The enzyme activity in metallo-beta-lactamases is not affected by the clinically used beta-lactamase inhibitors clavulanic acid, sulbactam and tazobactam. Other resistance mechanisms are alterations in the penicillin-binding proteins (PBPs) or a decreased permeability through the outer membrane. Beta-lactam resistance and beta-lactamase production have also been detected in some species of clostridia, fusobacteria, Prevotella, Porphyromonas and in some other anaerobic bacteria.

Ascorbate as an induction inhibitor of beta-lactamase in a strain of Enterobacter cloacae

The effect of ascorbate and anaerobiosis of beta-lactamase content (constitutive and inducible) in relation to the susceptibility of a standard strain of Enterobacter cloacae to ampicillin was studied. Enterobacter cloacae ATCC 13047 showed increasing susceptibility to ampicillin when incubated anaerobically in the presence of increasing concentrations of ascorbic acid. The inducible beta-lactamase activity in the cell-free extracts of Ent. cloacae decreased when the bacterium was grown aerobically in the presence of ascorbic acid. Under anaerobic growth conditions, however, ascorbic acid abrogated the induction of the enzyme completely. On the other hand, the constitutive enzymatic activity was markedly decreased as the bacterium was grown anaerobically. Thus under these growth conditions ascorbate-anaerobiosis, the total beta-lactamase level in the presence of ampicillin as inducer fell below the basal constitutive activity observed in the absence of ampicillin.

Lack of additive effect between mechanisms of resistance to carbapenems and other beta-lactam agents in Pseudomonas aeruginosa

Eighty-nine clinical isolates resistant (n = 61) or susceptible (n = 28) to imipenem and exhibiting the main patterns of susceptibility to other beta-lactam agents (wild type pattern, penicillinase pattern, constitutive cephalosporinase pattern) were studied in order to investigate (i) the mechanism of resistance involved and (ii) whether resistance to carbapenems affects the level of resistance to other beta-lactam agents and, conversely, if resistance to other beta-lactam agents affects the level of resistance to carbapenems. For this purpose, the presence of OprD protein in the cell wall was detected by Western blot and beta-lactamase activity by spectrophotometric assay and isoelectric focusing. OprD expression was not detectable in the imipenem-resistant (MIC > or = 16 micrograms/ml) strains. It was decreased in half the strains for which MICs of imipenem were 2 to 8 micrograms/ml and was close to a normal level in the most susceptible strains (MIC < or = 1 microgram/ml), thus demonstrating a direct correlation between the level of susceptibility to imipenem and the level of OprD expression. No imipenemase activity was detected in imipenem-resistant strains. Synergy between imipenem or meropenem and BRL 42715 was observed for all of the strains, demonstrating the role of cephalosporinase in carbapenem resistance. Within each pattern of susceptibility, the mean MICs of beta-lactam agents other than carbapenems were similar, whether the strains were susceptible or resistant to imipenem. Conversely, the mean MICs of imipenem or meropenem for either the imipenem-resistant or the imipenem-susceptible strains were similar, regardless of the susceptibility of these strains to the other beta-lactam agents. Thus, when several mechanisms of resistance to beta-lactam agents are present in the same strain of Pseudomonas aeruginosa, there is no additive effect between these mechanisms.

[beta-Lactamases of Branhamella catarrhalis and their phenotypic implications]

Of the 50 strains of beta-lactamase-producing Branhamella catarrhalis isolated at Saint Joseph's Hospital (Paris) that were studied, 94% produced BRO-1 type beta-lactamase and 6% produced the BRO-2 type. We examined the transfer of BRO-1 and BRO-2 genes and found that, among 7 donor strains producing BRO-1, all were able to transfer the gene for BRO-1 production by conjugation. Of the 4 donor strains producing BRO-2, 2 were able to transfer the gene for BRO-2 production by conjugation. Three BRO-1 beta-lactamase-producing transformants were obtained from total DNA extracted from 3 strains producing BRO-1. Plasmid bands were demonstrated in strains of B. catarrhalis, but no change in plasmid profiles was seen in beta-lactamase-positive recombinants, supporting previous studies that suggested the beta-lactamases are chromosomal. In vitro activity of oral beta-lactams was tested for 67 strains of B. catarrhalis (56 beta-lactamase-producing strains). Cefixime, cefpodoxime and the combination ampicillin-clavulanic acid were very active against the beta-lactamase-producing strains. BRO-1 beta-lactamase appears to affect the activity of cefaclor, cefuroxime and loracarbef. BRO-2 beta-lactamases have no effect on the activity of these cephalosporins. Cefixime and cefpodoxime seemed the least affected by beta-lactamase production.

beta-Lactamases in laboratory and clinical resistance

beta-Lactamases are the commonest single cause of bacterial resistance to beta-lactam antibiotics. Numerous chromosomal and plasmid-mediated types are known and may be classified by their sequences or phenotypic properties. The ability of a beta-lactamase to cause resistance varies with its activity, quantity, and cellular location and, for gram-negative organisms, the permeability of the producer strain. beta-Lactamases sometimes cause obvious resistance to substrate drugs in routine tests; often, however, these enzymes reduce susceptibility without causing resistance at current, pharmacologically chosen breakpoints. This review considers the ability of the prevalent beta-lactamases to cause resistance to widely used beta-lactams, whether resistance is accurately reflected in routine tests, and the extent to which the antibiogram for an organism can be used to predict the type of beta-lactamase that it produces.

In vitro activities of antimicrobial combinations against Stenotrophomonas (Xanthomonas) maltophilia

Stenotrophomonas (Xanthomonas) maltophilia is inherently resistant to multiple antimicrobial agents. In order to investigate the in vitro potential of combinations of antimicrobial agents, we obtained 230 epidemiologically unrelated clinical isolates from seven hospitals across Canada and from Northwestern Memorial Hospital in Chicago. Ticarcillin-clavulanate combined with ciprofloxacin or trimethoprim-sulfamethoxazole were assayed for synergy against 31 ticarcillin-resistant strains of S. maltophilia by using microtiter checkerboard panels and against 20 strains by using time-kill methodology. The combination of ciprofloxacin with ceftazidime was also evaluated by time-kill studies. Ticarcillin-clavulanate plus trimethoprim-sulfamethoxazole demonstrated synergy by checkerboard panels, with fractional inhibitory concentration indices ranging from 0.033 to 0.49, and by time-kill studies for all 20 strains tested. Synergy between ticarcillin-clavulanate plus ciprofloxacin was found by the checkerboard method for 24 of 31 strains (77%), with fractional inhibitory concentration indices ranging from 0.188 to 0.75. A correlation between synergy by the checkerboard method and the reference time-kill study method was not observed for ticarcillin-clavulanate plus ciprofloxacin, with results for 3 of 10 strains being nonconcordant. Synergy with both ticarcillin-clavulanate plus ciprofloxacin and ceftazidime plus ciprofloxacin by the time-kill method was found to correlate with ciprofloxacin MICs of <32 micrograms/ml and zone diameters of >15 mm on Mueller-Hinton agar. Evaluation of these combinations in vivo may be warranted.

Cloning and sequence of the gene encoding a cefotaxime-hydrolyzing class A beta-lactamase isolated from Escherichia coli

Escherichia coli TUH12191, which is resistant to piperacillin, cefazolin, cefotiam, ceftizoxime, cefuzonam, and aztreonam but is susceptible to cefoxitin, latamoxef, flomoxef, and imipenem, was isolated from the urine of a patient treated with beta-lactam antibiotics. The beta-lactamase (Toho-1) purified from the bacteria had a pI of 7.8, had a molecular weight of about 29,000, and hydrolyzed beta-lactam antibiotics such as penicillin G, ampicillin, oxacillin, carbenicillin, piperacillin, cephalothin, cefoxitin, cefotaxime, ceftazidime, and aztreonam. Toho-1 was markedly inhibited by beta-lactamase inhibitors such as clavulanic acid and tazobactam. Resistance to beta-lactams, streptomycin, spectinomycin, sulfamethoxazole, and trimethoprim was transferred by conjugational transfer from E. coli TUH12191 to E. coli ML4903, and the transferred plasmid was about 58 kbp, belonging to incompatibility group M. The cefotaxime resistance gene for Toho-1 was subcloned from the 58-kbp plasmid by transformation of E. coli MV1184. The sequence of the gene for Toho-1 was determined, and the open reading frame of the gene consisted of 873 or 876 bases (initial sequence, ATGATG). The nucleotide sequence of the gene (DDBJ accession number D37830) was found to be about 73% homologous to the sequence of the gene encoding a class A beta-lactamase produced by Klebsiella oxytoca E23004. According to the amino acid sequence deduced from the DNA sequence, the precursor consisted of 290 or 291 amino acid residues, which contained amino acid motifs common to class A beta-lactamases (70SXXK, 130SDN, and 234KTG). Toho-1 was about 83% homologous to the beta-lactamase mediated by the chromosome of K. oxytoca D488 and the beta-lactamase mediated by the plasmid of E. coli MEN-1. Therefore, the newly isolated beta-lactamase Toho-1 produced by E. coli TUH12191 is similar to beta-lactamases produced by K. oxytoca D488, K. oxytoca E23004, and E. coli MEN-1 rather than to mutants of TEM or SHV enzymes. Toho-1 has shown the highest degree of similarity to K. oxytoca class A beta-lactamase. Detailed comparison of Toho-1 with other beta-lactamases implied that replacement of Asn-276 by Arg with the concomitant substitution of Thr for Arg-244 is an important mutation in the extension of the substrate specificity.

A beta-lactamase from Serratia marcescens hydrolyzing the 2-carboxypenam T-5575

A beta-lactamase was purified from Serratia marcescens GN16694; it hydrolyzed T-5575 and oxime-type cephalosporins, i.e., cefuroxime and ceftazidime. Its isoelectric point and molecular weight were 8.6 and 42,000, respectively. This enzyme was not inhibited by EDTA and clavulanic acid. This enzyme is an unusual beta-lactamase and has been classified as a group 1 cephalosporinase.

Effect of pH on activities of novel beta-lactamases and beta-lactamase inhibitors against these beta-lactamases

The effects of acidic conditions on activities of seven beta-lactamases--TEM-1 (class A), KOXY (class A), IMP-1 (class B), AmpC (class C), MOX-1 (class C), OXA-5 (class D), and PSE-2 (class D)--and their inhibitors were measured. The enzymatic activities of KOXY, IMP-1, and MOX-1 at pH 5.8 were slightly lower than those at pH 7.5. However, the activities of PSE-2 and OXA-5 were greatly reduced at pH 5.8. All of the beta-lactamase inhibitors tested had poorer inhibitory activities at pH 5.8 than at pH 7.5 except clavulanic acid for TEM-1.

Purification and characterization of a new beta-lactamase from Bacteroides uniformis

A beta-lactam-resistant Bacteroides uniformis strain was isolated from a clinical specimen. The strain produced large amounts of beta-lactamase and was resistant to penicillins and cephalosporins. The specific activity of the unpurified beta-lactamase was 4.8 U/mg of protein with nitrocefin as the substrate. The enzyme was purified 188-fold by Q-Sepharose, Sephacryl S-300, and Mono Q column passages. Kinetic parameters of the enzyme were determined by a micromethod performed in microtiter plates. beta-Lactamase was inhibited by cefoxitin and imipenem and hydrolyzed cephalosporins more rapidly than penicillins. The molecular weight was determined by sodium dodecyl sulfate-gradient gel electrophoresis to be 32,500, and the isoelectric point was 4.5.