In vitro activity of beta-lactam antibiotics against CTX-M-producing Escherichia coli.

Beta-lactam antibiotics have been discussed as options for the treatment of infections caused by multiresistant extended-spectrum beta-lactamase (ESBL)-producing bacteria if the minimum inhibitory concentration (MIC) is low. The objective of this study was to investigate the in vitro activity of different beta-lactam antibiotics against CTX-M-producing Escherichia coli. A total of 198 isolates of E. coli with the ESBL phenotype were studied. Polymerase chain reaction (PCR) amplification of CTX-M genes and amplicon sequencing were performed. The MICs for amoxicillin-clavulanic acid, aztreonam, cefepime, cefotaxime, ceftazidime, ceftibuten, ertapenem, imipenem, mecillinam, meropenem, piperacillin-tazobactam, and temocillin were determined with the Etest. Susceptibility was defined according to the breakpoints of the European Committee on Antimicrobial Susceptibility Testing (EUCAST). MIC(50) and MIC(90) values were calculated. Isolates from CTX-M group 9 showed higher susceptibility to the beta-lactam antibiotics tested than isolates belonging to CTX-M group 1. More than 90% of the isolates belonging to CTX-M group 9 were susceptible to amoxicillin-clavulanic acid, ceftazidime, ceftibuten, piperacillin-tazobactam, and temocillin. The susceptibility was high to mecillinam, being 91%, regardless of the CTX-M group. All isolates were susceptible to imipenem and meropenem, and 99% to ertapenem. This study shows significant differences in susceptibility to different beta-lactam antibiotics among the CTX-M-producing E. coli isolates and a significant difference for many antibiotics tested between the CTX-M-producing groups 1 and 9. The good in vitro activity of other beta-lactam antibiotics compared to carbapenems indicate that clinical studies are warranted in order to examine the potential role of these beta-lactam antibiotics in the treatment of infections caused by multiresistant ESBL-producing E. coli.

Multiplex PCR amplification assay for the detection of blaSHV, blaTEM and blaCTX-M genes in Enterobacteriaceae.

Extended-spectrum beta-lactamases (ESBLs) are often mediated by (bla-)SHV, (bla)TEM and (bla)CTX-M genes in Enterobacteriaceae and other Gram-negative bacteria. Numerous molecular typing methods, including PCR-based assays, have been developed for their identification. To reduce the number of PCR amplifications needed we have developed a multiplex PCR assay which detects and discriminates between (bla-)SHV, (bla)TEM and (bla)CTX-M PCR amplicons of 747, 445 and 593 bp, respectively. This multiplex PCR assay allowed the identification of (bla-)SHV, (bla)TEM and (bla)CTX-M genes in a series of clinical isolates of Enterobacteriaceae with previously characterised ESBL phenotype. The presence of (bla)SHV, (bla)TEM and (bla)CTX-M genes was confirmed by partial DNA sequence analysis. Apparently, the universal well-established CTX-M primer pair used here to reveal plasmid-encoded (bla)CTX-M genes would also amplify the chromosomally located K-1 enzyme gene in all Klebsiella oxytoca strains included in the study.