Effect of substitution of Asn for Arg-276 in the cefotaxime-hydrolyzing class A beta-lactamase CTX-M-4

Broad-Spectrum Inhibitors against Class A, B, and C Type β-Lactamases to Block the Hydrolysis against Antibiotics: Kinetics and Structural Characterization

The emergence of antibiotic resistance has led to a global crisis for the physician to handle infection control issues. All antibiotics, including colistin, have lost efficiency against emerging drug-resistant bacterial strains due to the production of metallo-β-lactamases (MBLs) and serine-β-lactamases (SBLs). Therefore, it is of the utmost importance to design inhibitors against these enzymes to block the hydrolytic action against antibiotics being used. Although various novel β-lactamase inhibitors are being authorized or are under clinical studies, the coverage of their activity spectrum does not include MDR organisms expressing multiple classes of β-lactamases at a single time. This study reports three novel broad-spectrum inhibitors effective against both SBLs and MBLs. Virtual screening, molecular docking, molecular dynamics simulations, and an in silico pharmacokinetic study were performed to identify the lead molecules with broad-spectrum ability to inhibit the hydrolysis of β-lactam. The selected compounds were further assessed by in vitro cell assays (MIC, 50% inhibitory concentration [IC₅₀], kinetics, and fluorescence against class A, B, and C type β-lactamases) to confirm their efficacies. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay was performed to check the toxicity of screened lead molecules. All three selected inhibitors were found to reduce MIC and showed good affinity against all the SBLs and MBLs produced by class A, B, and C type β-lactamases. These nontoxic novel non-β-lactam broad-spectrum inhibitors bind to the active site residues of selected β-lactamases, which are crucial for β-lactam antibiotic hydrolysis. These inhibitors may be proposed as a future drug candidate in combination with antibiotics as a single formulation to control infection caused by resistant strains. Hence, this study plays a significant role in the cure of infections caused by antibiotic-resistant bacteria.

IMPORTANCE Several inhibitors for usage in conjunction with antibiotics have been developed. However, to date, there is no commercially available broad-spectrum β-lactamase inhibitor that targets both MBLs and SBLs. Here, we showed three novel broad-spectrum inhibitors with promising results through computational techniques and in vitro studies. These inhibitors are effective against both SBLs and MBLs and hence could be used as future drug candidates to treat infections caused by multidrug-resistant bacteria producing both types of enzymes (SBLs and MBLs).

Molecular basis for the catalytic specificity of the CTX-M extended-spectrum β-lactamases

Extended-spectrum β-lactamases (ESBLs) pose a threat to public health because of their ability to confer resistance to extended-spectrum cephalosporins such as cefotaxime. The CTX-M β-lactamases are the most widespread ESBL enzymes among antibiotic resistant bacteria. Many of the active site residues are conserved between the CTX-M family and non-ESBL β-lactamases such as TEM-1, but the residues Ser237 and Arg276 are specific to the CTX-M family, suggesting that they may help to define the increased specificity for cefotaxime hydrolysis. To test this hypothesis, site-directed mutagenesis of these positions was performed in the CTX-M-14 β-lactamase. Substitutions of Ser237 and Arg276 with their TEM-1 counterparts, Ala237 and Asn276, had a modest effect on cefotaxime hydrolysis, as did removal of the Arg276 side chain in an R276A mutant. The S237A:R276N and S237A:R276A double mutants, however, exhibited 29- and 14-fold losses in catalytic efficiency for cefotaxime hydrolysis, respectively, while the catalytic efficiency for benzylpenicillin hydrolysis was unchanged. Therefore, together, the Ser237 and Arg276 residues are important contributors to the cefotaximase substrate profile of the enzyme. High-resolution crystal structures of the CTX-M-14 S70G, S70G:S237A, and S70G:S237A:R276A variants alone and in complex with cefotaxime show that residues Ser237 and Arg276 in the wild-type enzyme promote the expansion of the active site to accommodate cefotaxime and favor a conformation of cefotaxime that allows optimal contacts between the enzyme and substrate. The conservation of these residues, linked to their effects on structure and catalysis, imply that their coevolution is an important specificity determinant in the CTX-M family.

Distant and new mutations in CTX-M-1 beta-lactamase affect cefotaxime hydrolysis

The CTX-M β-lactamases are an increasingly prevalent group of extended-spectrum β-lactamases (ESBL). Point mutations in CTX-M β-lactamases are considered critical for enhanced hydrolysis of cefotaxime. In order to clarify the structural determinants of the activity against cefotaxime in CTX-M β-lactamases, screening for random mutations was carried out to search for decreased activity against cefotaxime, with the CTX-M-1 gene as a model. Thirteen single mutants with a considerable reduction in cefotaxime MICs were selected for biochemical and stability studies. The 13 mutated genes of the CTX-M-1 β-lactamase were expressed, and the proteins were purified for kinetic studies against cephalothin and cefotaxime (as the main antibiotics). Some of the positions, such as Val103Asp, Asn104Asp, Asn106Lys, and Pro107Ser, are located in the (103)VNYN(106) loop, which had been described as important in cefotaxime hydrolysis, although this has not been experimentally confirmed. There are four mutations located close to catalytic residues-Thr71Ile, Met135Ile, Arg164His, and Asn244Asp-that may affect the positioning of these residues. We show here that some distant mutations, such as Ala219Val, are critical for cefotaxime hydrolysis and highlight the role of this loop at the top of the active site. Other distant substitutions, such as Val80Ala, Arg191, Ala247Ser, and Val260Leu, are in hydrophobic cores and may affect the dynamics and flexibility of the enzyme. We describe here, in conclusion, new residues involved in cefotaxime hydrolysis in CTX-M β-lactamases, five of which are in positions distant from the catalytic center.

Roles of residues Cys69, Asn104, Phe160, Gly232, Ser237, and Asp240 in extended-spectrum beta-lactamase Toho-1

Toho-1, which is also designated CTX-M-44, is an extended-spectrum class A β-lactamase that has high activity toward cefotaxime. In this study, we investigated the roles of residues suggested to be critical for the substrate specificity expansion of Toho-1 in previous structural analyses. Six amino acid residues were replaced one by one with amino acids that are often observed in the corresponding position of non-extended-spectrum β-lactamases. The mutants produced in Escherichia coli strains were analyzed both for their kinetic properties and their effect on drug susceptibilities. The results indicate that the substitutions of Asn104 and Ser237 have certain effects on expansion of substrate specificity, while those of Cys69 and Phe160 have less effect, and that of Asp240 has no effect on the hydrolysis of any substrates tested. Gly232, which had been assumed to increase the flexibility of the substrate binding site, was revealed not to be critical for the expansion of substrate specificity of this enzyme, although this substitution resulted in deleterious effects on expression and stability of the enzyme.

Molecular and biochemical characterization of the natural chromosome-encoded class A beta-lactamase from Pseudomonas luteola

Pseudomonas luteola (formerly classified as CDC group Ve-1 and named Chryseomonas luteola) is an unusual pathogen implicated in rare but serious infections in humans. A novel beta-lactamase gene, bla(LUT-1), was cloned from the whole-cell DNA of the P. luteola clinical isolate LAM, which had a weak narrow-spectrum beta-lactam-resistant phenotype, and expressed in Escherichia coli. This gene encoded LUT-1, a 296-amino-acid Ambler class A beta-lactamase with a pI of 6 and a theoretical molecular mass of 28.9 kDa. The catalytic efficiency of this enzyme was higher for cephalothin, cefuroxime, and cefotaxime than for penicillins. It was found to be 49% to 59% identical to other Ambler class A beta-lactamases from Burkholderia sp. (PenA to PenL), Ralstonia eutropha (REUT), Citrobacter sedlakii (SED-1), Serratia fonticola (FONA and SFC-1), Klebsiella sp. (KPC and OXY), and CTX-M extended-spectrum beta-lactamases. No gene homologous to the regulatory ampR genes of class A beta-lactamases was found in the vicinity of the bla(LUT-1) gene. The entire bla(LUT-1) coding region was amplified by PCR and sequenced in five other genetically unrelated P. luteola strains (including the P. luteola type strain). A new variant of bla(LUT-1) was found for each strain. These genes (named bla(LUT-2) to bla(LUT-6)) had nucleotide sequences 98.1 to 99.5% identical to that of bla(LUT-1) and differing from this gene by two to four nonsynonymous single nucleotide polymorphisms. The bla(LUT) gene was located on a 700- to 800-kb chromosomal I-CeuI fragment, the precise size of this fragment depending on the P. luteola strain.

Analysis of the plasticity of location of the Arg244 positive charge within the active site of the TEM-1 beta-lactamase

A large number of beta-lactamases have emerged that are capable of conferring bacterial resistance to beta-lactam antibiotics. Comparison of the structural and functional features of this family has refined understanding of the catalytic properties of these enzymes. An arginine residue present at position 244 in TEM-1 beta-lactamase interacts with the carboxyl group common to penicillin and cephalosporin antibiotics and thereby stabilizes both the substrate and transition state complexes. A comparison of class A beta-lactamase sequences reveals that arginine at position 244 is not conserved, although a positive charge at this structural location is conserved and is provided by an arginine at positions 220 or 276 for those enzymes lacking arginine at position 244. The plasticity of the location of positive charge in the beta-lactamase active site was experimentally investigated by relocating the arginine at position 244 in TEM-1 beta-lactamase to positions 220, 272, and 276 by site-directed mutagenesis. Kinetic analysis of the engineered beta-lactamases revealed that removal of arginine 244 by alanine mutation reduced catalytic efficiency against all substrates tested and restoration of an arginine at positions 272 or 276 partially suppresses the catalytic defect of the Arg244Ala substitution. These results suggest an evolutionary mechanism for the observed divergence of the position of positive charge in the active site of class A beta-lactamases.

Nosocomial outbreak of CTX-M-15-producing E. coli in Norway

Seven E. coli isolates expressing resistance to 3rd generation cephalosporins were recovered from blood (n=2), kidney and lung tissue (n=1), and urinary tract (n=4) samples from seven patients hospitalised or recently discharged from the Divisions of Geriatrics and Pulmonary Medicine, Central Hospital of Rogaland, between July and September 2004. All isolates expressed a typical ESBL-cefotaximase profile (cefotaxime MIC>ceftazidime MIC) with clavulanic acid synergy. A bla(CTX-M-15) genotype was confirmed in six strains that were coresistant to gentamicin, nitrofurantoin, trimethoprim-sulfamethoxazole and ciprofloxacin. A bla(CTX-M-3) genotype was detected in the last strain. XbaI-PFGE patterns of the six bla(CTX-M-15) isolates revealed a clonal relationship. Bla(CTX-M-15) strains were also positive for the ISEcp1-like insertion sequences that have been shown to be involved in the mobilization of bla(CTX-M.) Further analyses revealed two bla(CTX-M-15)-positive E. coli urinary isolates clonally related to the outbreak strain from two different patients at the same divisions in January and February 2004. These patients were later re-hospitalised and one had E. coli with an ESBL-cefotaximase profile in sputum and nasopharyngeal specimen during the outbreak period. Clinical evaluation suggests that the CTX-M-producing E. coli strains contributed to death in three patients due to delayed efficient antimicrobial therapy. The outbreak emphasises the epidemic potential of multiple-antibiotic-resistant CTX-M-15-producing E. coli also in a country with low antibiotic usage and low prevalence of antimicrobial resistance.

Crystal structure of the Mycobacterium fortuitum class A beta-lactamase: structural basis for broad substrate specificity

beta-Lactamases are the main cause of bacterial resistance to penicillins and cephalosporins. Class A beta-lactamases, the largest group of beta-lactamases, have been found in many bacterial strains, including mycobacteria, for which no beta-lactamase structure has been previously reported. The crystal structure of the class A beta-lactamase from Mycobacterium fortuitum (MFO) has been solved at 2.13-A resolution. The enzyme is a chromosomally encoded broad-spectrum beta-lactamase with low specific activity on cefotaxime. Specific features of the active site of the class A beta-lactamase from M. fortuitum are consistent with its specificity profile. Arg278 and Ser237 favor cephalosporinase activity and could explain its broad substrate activity. The MFO active site presents similarities with the CTX-M type extended-spectrum beta-lactamases but lacks a specific feature of these enzymes, the VNYN motif (residues 103 to 106), which confers on CTX-M-type extended-spectrum beta-lactamases a more efficient cefotaximase activity.

Structure, function, and inhibition along the reaction coordinate of CTX-M beta-lactamases

CTX-M enzymes are an emerging group of extended spectrum beta-lactamases (ESBLs) that hydrolyze not only the penicillins but also the first-, second-, and third-generation cephalosporins. Although they have become the most frequently observed ESBLs in certain areas, there are few effective inhibitors and relatively little is known about their detailed mechanism. Here we describe the X-ray crystal structures of CTX-M enzymes in complex with different transition-state analogues and beta-lactam inhibitors, representing the enzyme as it progresses from its acylation transition state to its acyl enzyme complex to the deacylation transition state. As the enzyme moves along this reaction coordinate, two key catalytic residues, Lys73 and Glu166, change conformations, tracking the state of the reaction. Unexpectedly, the acyl enzyme complex with the beta-lactam inhibitor cefoxitin still has the catalytic water bound; this water had been predicted to be displaced by the unusual 7alpha-methoxy of the inhibitor. Instead, the 7alpha-group appears to inhibit by preventing the formation of the deacylation transition state through steric hindrance. From an inhibitor design standpoint, we note that the best of the reversible inhibitors, a ceftazidime-like boronic acid compound, binds to CTX-M-16 with a K(i) value of 4 nM. When used together in cell culture, this inhibitor reversed cefotaxime resistance in CTX-M-producing bacteria. The structure of its complex with CTX-M enzyme and the structural view of the reaction coordinate described here provide templates for inhibitor design and intervention to combat this family of antibiotic resistance enzymes.

Role of a mutation at position 167 of CTX-M-19 in ceftazidime hydrolysis

CTX-M-19 is a recently identified ceftazidime-hydrolyzing extended-spectrum beta-lactamase, which differs from the majority of CTX-M-type beta-lactamases that preferentially hydrolyze cefotaxime but not ceftazidime. To elucidate the mechanism of ceftazidime hydrolysis by CTX-M-19, the beta-lactam MICs of a CTX-M-19 producer, and the kinetic parameters of the enzyme were confirmed. We reconfirmed here that CTX-M-19 is also stable at a high enzyme concentration in the presence of bovine serum albumin (20 micro g/ml). Under this condition, we obtained more accurate kinetic parameters and determined that cefotaxime (k(cat)/K(m), 1.47 x 10(6) s(-1) M(-1)), cefoxitin (k(cat)/K(m), 62.2 s(-1) M(-1)), and aztreonam (k(cat)/K(m), 1.34 x 10(3) s(-1) M(-1)) are good substrates and that imipenem (k(+2)/K, 1.20 x 10(2) s(-1) M(-1)) is a poor substrate. However, CTX-M-18 and CTX-M-19 exhibited too high a K(m) value (2.7 to 5.6 mM) against ceftazidime to obtain their catalytic activity (k(cat)). Comparison of the MICs with the catalytic efficiency (k(cat)/K(m)) of these enzymes showed that some beta-lactams, including cefotaxime, ceftazidime, and aztreonam showed a similar correlation. Using the previously reported crystal structure of the Toho-1 beta-lactamase, which belongs to the CTX-M-type beta-lactamase group, we have suggested characteristic interactions between the enzymes and the beta-lactams ceftazidime, cefotaxime, and aztreonam by molecular modeling. Aminothiazole-bearing beta-lactams require a displacement of the aminothiazole moiety due to a severe steric interaction with the hydroxyl group of Ser167 in CTX-M-19, and the displacement affects the interaction between Ser130 and the acidic group such as carboxylate and sulfonate of beta-lactams.

ColE1-like plasmid pIP843 of Klebsiella pneumoniae encoding extended-spectrum beta-lactamase CTX-M-17

The resistance of Klebsiella pneumoniae BM4493, isolated in Ho Chi Minh City, Vietnam, to cefotaxime and aztreonam was due to production of a novel beta-lactamase, CTX-M-17. The bla(CTX-M-17) gene was borne by 7,086-bp plasmid pIP843, which was entirely sequenced and which was found to belong to the ColE1 family. The 876-bp bla(CTX-M-17) gene differed from bla(CTX-M-14) by 2 nucleotides, which led to the single amino acid substitution Glu289-->Lys. bla(CTX-M-17) was flanked upstream by an ISEcp1-like element and downstream by an insertion sequence (IS) IS903 variant designated IS903-C. The transcriptional start site of bla(CTX-M-17) was located 109 nucleotides upstream from the initiation codon in the ISEcp1-like element, which also provided the promoter sequences. Plasmid pIP843, which was non-self-transferable and nonmobilizable, contained five open reading frames transcribed in the same orientation. Regions homologous to sequences coding for putative RNA II and RNA I transcripts, a rom gene, which is involved in initiation of replication, and a cer-like gene, which is responsible for the stability of ColE1-like plasmids, were identified. Consensus sequences for putative replication (oriV) and transfer (oriT) origins were present. Results of primer extension experiments indicated that ISEcp1 provides the promoter for expression of bla(CTX-M-17) and may contribute to dissemination of this gene.

Extended-spectrum beta-lactamases in the 21st century: characterization, epidemiology, and detection of this important resistance threat

Beta-lactamases continue to be the leading cause of resistance to beta-lactam antibiotics among gram-negative bacteria. In recent years there has been an increased incidence and prevalence of extended-spectrum beta-lactamases (ESBLs), enzymes that hydrolyze and cause resistance to oxyimino-cephalosporins and aztreonam. The majority of ESBLs are derived from the widespread broad-spectrum beta-lactamases TEM-1 and SHV-1. There are also new families of ESBLs, including the CTX-M and OXA-type enzymes as well as novel, unrelated beta-lactamases. Several different methods for the detection of ESBLs in clinical isolates have been suggested. While each of the tests has merit, none of the tests is able to detect all of the ESBLs encountered. ESBLs have become widespread throughout the world and are now found in a significant percentage of Escherichia coli and Klebsiella pneumoniae strains in certain countries. They have also been found in other Enterobacteriaceae strains and Pseudomonas aeruginosa. Strains expressing these beta-lactamases will present a host of therapeutic challenges as we head into the 21st century.

Novel cefotaximase (CTX-M-16) with increased catalytic efficiency due to substitution Asp-240-->Gly

Three clinical strains (Escherichia coli Rio-6, E. coli Rio-7, and Enterobacter cloacae Rio-9) collected in 1996 and 1999 from hospitals in Rio de Janeiro (Brazil) were resistant to broad-spectrum cephalosporins and gave a positive double-disk synergy test. Two bla(CTX-M) genes encoding beta-lactamases of pl 7.9 and 8.2 were implicated in this resistance: the bla(CTX-M-9) gene observed in E. coli Rio-7 and E. cloacae Rio-9 and a novel CTX-M-encoding gene, designated bla(CTX-M-16), observed in E. coli strain Rio-6. The deduced amino acid sequence of CTX-M-16 differed from CTX-M-9 only by the substitution Asp-240-->Gly. The CTX-M-16-producing E. coli transformant exhibited the same level of resistance to cefotaxime (MIC, 16 microg/ml) but had a higher MIC of ceftazidime (MIC, 8 versus 1 microg/ml) than the CTX-M-9-producing transformant. Enzymatic studies revealed that CTX-M-16 had a 13-fold higher affinity for aztreonam and a 7.5-fold higher k(cat) for ceftazidime than CTX-M-9, thereby showing that the residue in position 240 can modulate the enzymatic properties of CTX-M enzymes. The two bla(CTX-M-9) genes and the bla(CTX-M-16) gene were located on different plasmids, suggesting the presence of mobile elements associated with CTX-M-encoding genes. CTX-M-2 and CTX-M-8 enzymes were found in Brazil in 1996, and two other CTX-M beta-lactamases, CTX-M-9 and CTX-M-16, were subsequently observed. These reports are evidence of the diversity of CTX-M-type extended-spectrum beta-lactamases in Brazil.

A novel class A extended-spectrum beta-lactamase (BES-1) in Serratia marcescens isolated in Brazil

Serratia marcescens Rio-5, one of 18 extended-spectrum beta-lactamase (ESBL)-producing strains isolated in several hospitals in Rio de Janeiro (Brazil) in 1996 and 1997, exhibited a high level of resistance to aztreonam (MIC, 512 microgram/ml) and a distinctly higher level of resistance to cefotaxime (MIC, 64 microgram/ml) than to ceftazidime (MIC, 8 microgram/ml). The strain produced a plasmid-encoded ESBL with a pI of 7.5 whose bla gene was not related to those of other plasmid-mediated Ambler class A ESBLs. Cloning and sequencing revealed a bla gene encoding a novel class A beta-lactamase in functional group 2be, designated BES-1 (Brazil extended-spectrum beta-lactamase). This enzyme had 51% identity with chromosomal class A penicillinase of Yersinia enterocolitica Y56, which was the most closely related enzyme and 47 to 48% identity with CTX-M-type beta-lactamases, which were the most closely related ESBLs. In common with CTX-M enzymes, BES-1 exhibited high cefotaxime-hydrolyzing activity (k(cat), 425 s(-1)). However, BES-1 differed from CTX-M enzymes by its significant ceftazidime-hydrolyzing activity (k(cat), 25 s(-1)), high affinity for aztreonam (K(i), 1 microM), and lower susceptibility to tazobactam (50% inhibitory concentration [IC(50)], 0.820 microM) than to clavulanate (IC(50), 0.045 microM). Likewise, certain characteristic structural features of CTX-M enzymes, such as Phe-160, Ser-237, and Arg-276, were observed for BES-1, which, in addition, harbored different residues (Ala-104, Ser-171, Arg-220, Gly-240) and six additional residues at the end of the sequence. BES-1, therefore, may be an interesting model for further investigations of the structure-function relationships of class A ESBLs.

A novel CTX-M beta-lactamase (CTX-M-8) in cefotaxime-resistant Enterobacteriaceae isolated in Brazil

To estimate the diversity of extended-spectrum beta-lactamases in Brazil, 18 strains from different species of the family Enterobacteriaceae exhibiting a positive double-disk synergy test were collected by a clinical laboratory from several hospitals in Rio de Janeiro, Brazil, in 1996 and 1997. Four strains (Proteus mirabilis, Enterobacter cloacae, Enterobacter aerogenes, and Citrobacter amalonaticus) hybridized with a 550-bp CTX-M probe. The P. mirabilis strain produced a CTX-M-2 enzyme. The E. cloacae, E. aerogenes, and C. amalonaticus isolates harbored a bla gene which was identified by cloning and sequencing as a bla(CTX-M) gene. E. coli HB101 transconjugants and the E. coli DH5alpha transformant harboring a recombinant plasmid produced a CTX-M beta-lactamase with an isoelectric point of 7.6 conferring a resistance phenotype characterized by a higher level of resistance to cefotaxime than to ceftazidime, as observed with the other CTX-M enzymes. The deduced protein sequence showed a novel Ambler class A CTX-M enzyme, named CTX-M-8, which had 83 to 88% identity with the previously described CTX-M enzymes. The phylogenic study of the CTX-M family including CTX-M-8 revealed four CTX-M types, CTX-M-8 being the first member of a new phylum of CTX-M enzymes. The evolutionary distances between the four types of CTX-M were large, suggesting that the four clusters branched off early from a distant unknown enzyme and that intermediate enzymes probably existed.

TLA-1: a new plasmid-mediated extended-spectrum beta-lactamase from Escherichia coli

Escherichia coli R170, isolated from the urine of an infected patient, was resistant to expanded-spectrum cephalosporins, aztreonam, ciprofloxacin, and ofloxacin but was susceptible to amikacin, cefotetan, and imipenem. This particular strain contained three different plasmids that encoded two beta-lactamases with pIs of 7.0 and 9.0. Resistance to cefotaxime, ceftazidime, aztreonam, trimethoprim, and sulfamethoxazole was transferred by conjugation from E. coli R170 to E. coli J53-2. The transferred plasmid, RZA92, which encoded a single beta-lactamase, was 150 kb in length. The cefotaxime resistance gene that encodes the TLA-1 beta-lactamase (pI 9.0) was cloned from the transconjugant by transformation to E. coli DH5alpha. Sequencing of the bla(TLA-1) gene revealed an open reading frame of 906 bp, which corresponded to 301 amino acid residues, including motifs common to class A beta-lactamases: (70)SXXK, (130)SDN, and (234)KTG. The amino acid sequence of TLA-1 shared 50% identity with the CME-1 chromosomal class A beta-lactamase from Chryseobacterium (Flavobacterium) meningosepticum; 48.8% identity with the VEB-1 class A beta-lactamase from E. coli; 40 to 42% identity with CblA of Bacteroides uniformis, PER-1 of Pseudomonas aeruginosa, and PER-2 of Salmonella typhimurium; and 39% identity with CepA of Bacteroides fragilis. The partially purified TLA-1 beta-lactamase had a molecular mass of 31.4 kDa and a pI of 9.0 and preferentially hydrolyzed cephaloridine, cefotaxime, cephalothin, benzylpenicillin, and ceftazidime. The enzyme was markedly inhibited by sulbactam, tazobactam, and clavulanic acid. TLA-1 is a new extended-spectrum beta-lactamase of Ambler class A.