Hydrolysis of Cefotaxime by a beta-lactamase from Bacteroides fragilis

Phenotypic and molecular characterization of a novel beta-lactamase carried by Klebsiella pneumoniae, CTX-M-72, derived from CTX-M-3

This study reports phenotypic and molecular characterization of a novel CTX-M beta-lactamase carried by two Klebsiella pneumoniae isolates collected from two hospitals in China. Conjugation experiment, Southern hybridization, susceptibility testing, isoelectric focusing, PCR, and sequencing techniques as well as clone, expression, purification and kinetics were carried out to describe the characterization of the novel CTX-M-type enzyme. The analyses of plasmid profiling and pulsed-field gel electrophoresis of the novel enzyme were performed to investigate epidemiology. The PCR products had 967 nucleotides and a novel CTX-M enzyme with a pI of 8.5 was implicated in this resistance: CTX-M-72. Two strains exhibited a clavulanic acid-inhibited substrate profile that included extended-spectrum cephalosporins. The amino acid sequence of the CTX-M-72 beta-lactamase differed from that of the CTX-M-3 beta-lactamase by the Arg-->Gly change at position 164. The novel enzyme was susceptible to ceftazidime, the same response being observed for other CTX-M enzymes. The substrates of the beta-lactamase were also characterized. Furthermore, two resistant genes of clinical strains were closely related. The emergence of a novel CTX-M-type extended-spectrum beta-lactamase was rarely described in other areas. This study illustrated the importance of molecular surveillance in tracking CTX-M-producing strains in large teaching hospitals, suggested the horizontal transfer of plasmid-borne bla(CTX-M) genes contributed to the dissemination of CTX-M enzymes in hospital environments, and emphasized the need for epidemiological monitoring.

Phenotypic and molecular characterization of two novel CTX-M enzymes carried by Klebsiella pneumoniae

Two clinical strains (Klebsiella pneumoniae 516 and K. pneumoniae 1335) collected in September 2006 from different hospitals in Anhui Province (China) harboured two novel plasmid-mediated bla(CTX-M) genes, designated bla(CTX-M-80) and bla(CTX-M-81), respectively. Both CTX-M-80 with pI of 9.0 and CTX-M-81 with pI of 8.4 were extended-spectrum beta-lactamases (ESBLs). The results of susceptibility testing demonstrated two enzymes were highly activity against broad spectrum beta-lactams, but the level of resistance was reduced with the addition of beta-lactamase inhibitors. The bla(CTX-M-80) gene was detected on a 110-kb plasmid and the bla(CTX-M-81) gene existed on a 120-kb plasmid. The deduced amino acid sequence of CTX-M-80 differed from that of CTX-M-3 by the substitution Ala-27-->Val, and CTX-M-81 possessed the Lys-->Glu, Lys-->Gln, and Asn-->His changes at respective position 82, 98, and 132 in compassion with CTX-M-14. The enzymatic properties showed CTX-M-80 and CTX-M-81 had higher affinities for penicillin G (lower Km values) than for cephalosporins. The activities of novel enzymes against ceftazidime were undetectable or limited, as indicated by MICs data, the same response being observed for many other CTX-M enzymes. This report was evidence of the diversity of CTX-M-type ESBLs in China.

Characterization of a novel AmpC-type plasmid-mediated beta-lactamase from an Escherichia coli strain isolated in China

The aim of this work was to study the phenotypic and molecular characterization of a novel plasmid-mediated AmpC beta-lactamase from Escherichia coli E384. Conjugation experiments, isoelectric focusing, pulsed-field gel electrophoresis, plasmid profiling, and Southern blot as well as PCR, sequencing techniques, and susceptibility testing were carried out to investigate the underlying mechanism of resistance. The kinetic parameters were determined to characterize the novel enzyme. MIR-4 beta-lactamase, pI 8.2, is a novel variant with four substitutions of amino acids compared with the sequence of MIR-1. E. coli E384 displays resistance to eight beta-lactam antimicrobial agents and three fluoroquinolones. The minimal inhibitory concentrations of beta-lactam in combination with beta-lactamase inhibitors show no significant synergy. Kinetic parameters suggest that the novel enzyme effectively hydrolyzes broad-spectrum beta-lactams. The same hybridization signal was detectable only in the 54-kb plasmid band that hybridized with the bla (CTX-M)- and bla(ampC)-specific probes. This is the first description of a plasmid-mediated MIR-4 enzyme in China. This study illustrates the importance of molecular surveillance in tracking AmpC-producing strains at general hospitals and emphasizes the need for epidemiological monitoring.

A novel SHV-type beta-lactamase variant (SHV-89) in clinical isolates in China

Two clinical strains of Klebsiella pneumoniae (K. pneumoniae) and one isolate of Escherichia coli (E. coli) were collected from two large general hospitals in China. Conjugation experiment, susceptibility testing, isoelectric focusing, PCR, and sequencing techniques as well as clone, expression, purification and kinetics were carried out to describe the characterization of the novel SHV-tpye enzyme. The analysis of plasmid profiling and pulsed-field gel electrophoresis of the novel enzyme were performed to investigate epidemiology. These isolates had CTX-M-14 and SHV-89 beta-lactamases. SHV-89 beta-lactamase of pI 7.6 is a novel variant with two substitutions compared with the sequence of SHV-1: Leu35Gln and Met129Val. Its gene also had two silent mutations at positions 369 and 774, respectively. The results of substrate profiles and MIC determinations showed the activity of the novel enzyme was insufficient for the enzyme to count as an extended-spectrum beta-lactamase (ESBL). The substrates of the enzyme were also characterized. Furthermore, the three novel SHV enzyme-producing strains were epidemiologically unrelated. The emergence of a novel SHV-type beta-lactamase is rarely described in other areas. This study illustrates the importance of molecular survelliance in tracking SHV-producing strains in large teaching hospitals and emphasizes the need for epidemiological monitoring.

Phenotypic and molecular characterization of 5 novel CTX-M enzymes carried by Klebsiella pneumoniae and Escherichia coli

AIM

The aim of the present study was to study the phenotypic and molecular characterization of 5 novel CTX-M-beta-1actamases carried by 5 Klebsiella pneumoniae isolates and 3 Escherichia coli isolates collected from 4 hospitals in Hefei, China.

METHODS

The purified PCR products were ligated with pGEM-Teasy vectors, expressed, and sequenced. The complete genes of the CTX-M-beta-lactamases were ligated with the pHSG398 vector to express prokaryotic recombinant proteins. Plasmids were extracted by rapid alkaline lysis protocol, and the PCR method was performed to determine whether the prokaryotic expression was successful or not. Antimicrobial susceptibility was tested and the phenotypes of transformants were determined according to criteria recommended by the Clinical and Laboratory Standards Institute. The kinetic parameters of enzymes were confirmed. The isoelectric points (pI) were determined by isoelectric focusing assay. Pulsed-field gel electrophoresis and plasmid profiling were performed.

RESULTS

The PCR products had 1101 nucleotides and were determined as CTX-M-46, CTX-M-47, CTX-M-48, CTX-M-49, and CTX-M-50. All strains were resistant to cefotaxime, but most of them were susceptible or intermediate to ceftazidime. The phenotypes of novel enzymes were determined as extended-spectrum-beta-lactamases (ESBL). Penicillin G, cephalothin, cefuroxime, and cefotaxime were determined to good substrates, whereas ceftazidime hydrolysis was not detected. The pI of the 5 novel CTX-M-beta-lactamases were 8.0. CTX-M-derivatives could be the multiplex genesis in our area.

CONCLUSION

This is the first report of these 5 novel plasmid-mediated CTX-M ESBL produced from China in the world. Molecular typing reveals notably different origin in genes encoding different CTX-M variants of 8 strains.

Emergence of resistance during beta-lactam therapy of gram-negative infections. Bacterial mechanisms and medical responses

Some Gram-negative, non-fastidious bacilli, although classified as susceptible by conventional susceptibility testing methods, become resistant during therapy with the newer beta-lactam compounds. Emergence of resistance results primarily from the selection of resistant clones pre-existing within the susceptible bacterial populations. Most of the resistant clones produce large amounts of beta-lactamases which inhibit the beta-lactam antibiotics by hydrolysis, rather than by binding. In addition, resistant clones can limit the penetration of beta-lactam molecules through the outer membrane by a decreased expression of their porins. Less commonly, when beta-lactamase activity together with alteration of the permeability barrier does not prevent the access of the antibiotic molecules to their target, altered penicillin-binding proteins (PBPs) can produce resistance. However, the risk of resistance emerging during therapy varies with the beta-lactam drug administered. Some compounds such as cefpirome, BMY 28142, SCH 34343, or imipenem appear to be associated with a low risk. In addition, emergence of resistance can be reduced by using higher dosages of beta-lactam agents, or by combining them with other drugs such as aminoglycosides or quinolones.

Effect of clavulanic acid on the activities of ten beta-lactam agents against members of the Bacteroides fragilis group

Clavulanic acid reduced the MICs of amoxicillin, carbencillin , cefamandole, cefotaxime, ceftazidime, ceftizoxime, cephalothin, and penicillin G, but not of cefoxitin or moxalactam, against 77 isolates of the Bacteroides fragilis group, all rapidly beta-lactamase positive by the nitrocefin slide test. It had no effect on the susceptibilities of eight Bacteroides distasonis strains that were slowly beta-lactamase positive (18 h of incubation).

Comparative antibacterial activities of 7 alpha-methoxy cephalosporins and 7 beta-methoxyiminoacetamido cephalosporins against Bacteroides fragilis

The in vitro antibacterial activities of the newly developed 7 alpha-methoxy cephalosporins and 7 beta-methoxyiminoacetamido cephalosporins against 67 clinical isolates of Bacteroides fragilis and their resistance to the hydrolytic action of a beta-lactamase produced by B. fragilis were simultaneously compared. The minimal inhibitory concentrations that inhibited 90% of the 7 alpha-methoxy cephalosporins, cefoxitin, cefmetazole, moxalactam, and cefotetan, against the isolates were 4, 8, 8, and 16 micrograms/ml, respectively, and these antibiotics were entirely resistant to hydrolysis by beta-lactamases (0.10 mumol/h per mg of protein) of the isolates. By contrast, 7 beta-methoxyiminoacetamido cephalosporins represented by cefotaxime, ceftizoxime, and cefmenoxime were not effective, as indicated by the minimal inhibitory concentrations that inhibited 90%, 64, 32, and 128 micrograms/ml, respectively. Their antibacterial activities clearly corresponded to their resistance to the hydrolytic action of the beta-lactamase: namely, the correlation coefficients in regression curves of cefotaxime, ceftizoxime, and cefmenoxime, which were expressed by the antibacterial activity (x axis) and the beta-lactamase activity (y axis) were 0.098, 0.034, and 0.163, respectively.

Antimicrobial susceptibility testing (AST): a review of changing trends, quality control guidelines, test accuracy, and recommendation for the testing of beta-lactam drugs

The review that follows presents the changing trends in antimicrobial susceptibility testing observed from the author's clinical laboratory experience and the proficiency testing surveys of the College of American Pathologists (CAP). The CAP Microbiology Surveys show a clear trend toward standardized test methods of the National Committee for Clinical Laboratory Standard (NCCLS) and greater compliance with specified methods' technical steps. This has favorably influenced the laboratory performance on proficiency challenges where a 3-5% improvement has been noted over the last 5 years for the disk tests (overall acceptable rate of 95.2% in 1981). A concurrent increase in dilution test use, mainly broth microdilution methods, has resulted in greater than 25% of larger hospital laboratories reporting results as MICs (overall acceptable or good performance = 98%). Automated systems use also continues to increase, with user performance being monitored at an acceptable level. Quality control frequency may be reduced to once weekly without compromising test accuracy or patient care, but only after adequate daily or concurrent QC performance has been documented. Most methods continue to have problems in testing enterococci, methicillin-resistant staphylococci, and the class-disk concept appears to be less applicable. Recommendations are made for the testing of the newer semisynthetic penicillins and cephalosporins based on their spectrum comparability and cross-resistance studies with bacteria possessing known susceptibility or resistance mechanisms. The concept of "spectrum-class" is introduced with peer drugs within classes. The general trends and quality of antimicrobial susceptibility tests seem outstanding and point toward continued excellent intra- and interlaboratory reproducibility at the national level, primarily due to the efforts of the inspection and accreditation agencies, CAP, CDC, and other concerned professional groups.

Activity of cefoperazone and two beta-lactamase inhibitors, sulbactam and clavulanic acid, against Bacteroides spp. correlated with beta-lactamase production

A total of 102 isolates of Bacteroides spp. were studied for beta-lactamase production and susceptibility to cefoperazone alone or in combination with either of the beta-lactamase inhibitors sulbactam and clavulanic acid. The geometric mean minimal inhibitory concentration of cefoperazone alone was 31.5 micrograms/ml and when combined with 10 micrograms of sulbactam per ml or 2 micrograms of clavulanic acid per ml was reduced to 5.4 and 9.2 micrograms/ml, respectively. When bacterial suspensions were tested for beta-lactamase production with nitrocefin, 91 (89.2%) of these isolates produced the enzyme. The geometric mean minimal inhibitory concentrations of cefoperazone rose only slightly for isolates with low or intermediate enzyme activity but rose significantly for those with high activity. The addition of EDTA to cefoperazone significantly more frequently enhanced the activity of cefoperazone against beta-lactamase-negative as opposed to beta-lactamase-positive isolates. Furthermore, EDTA resulted in synergistic activity of the cefoperazone-sulbactam combination on beta-lactamase-positive isolates for which the combination had previously not shown a synergistic effect. This study demonstrates the relationship between beta-lactamase production and the resistance of Bacteroides spp. to cefoperazone and shows that inhibition of these enzymes can reverse this resistance.

Cefotaxime therapy of serious bacterial infection in adults

We evaluated the efficacy, safety, and tolerance of cefotaxime in 35 adults (25 with pleuropulmonary infections, 7 with genitourinary tract infections, and 3 with soft tissue infections). Of these 35 patients, 18 (51.4%) were seriously or critically ill. In vitro susceptibility testing revealed that 90.4% of the pathogens isolated were susceptible to cefotaxime (minimal inhibitory concentration, less than 8 micrograms/ml), 4.8% were intermediately susceptible (minimal inhibitory concentration, 8 to 32 micrograms/ml), and 4.8% were resistant (minimal inhibitory concentration, greater than 32 micrograms/ml). A total of 34 of the 35 patients (97%) were clinically and bacteriologically cured of their infections. Adverse reactions occurred in two patients, but these reactions did not require interruption of therapy.