Screening method for beta-lactamase substrate profiles

Cefuroxime resistance in Escherichia coli. Resistance mechanisms and prevalence

In order to characterize cefuroxime resistance in Escherichia coli 22 clinical isolates were investigated for susceptibility to different beta-lactam antibiotics and ciprofloxacin. The production of beta-lactamases, the pattern of the major outer membrane proteins (OMPs), and the plasmid profiles were determined for these isolates. Ten of the isolates were resistant to ceftazidime, two to cefotaxime, and none was resistant to imipenem or ciprofloxacin. The dominating resistance mechanism was hyperproduction of the chromosomally encoded beta-lactamase to some extent accompanied by alterations of the OMP's. Two isolates with low ampicillin MIOs seemed solely to have alteration of the OMPs. None of the isolates produced plasmid-mediated extended-spectrum beta-lactamases. In addition, the prevalence of cefuroxime resistance was investigated. The prevalence as attained in 8704 clinical isolates of E. coli collected from Copenhagen County during a 5-year period (1990-1994) was 4.4%, but there was considerable variation among specimens from different sites of the body. Isolates from blood were much less resistant (2.5%) than isolates from the respiratory tract (9.7%).

Resistance characteristics of blood culture isolates of Enterobacter cloacae with special reference to beta-lactamases and relation to preceding antimicrobial therapy

Resistance characteristics of 53 blood culture isolates of E. cloacae were examined and correlated with antimicrobial treatment preceding bacteraemia. Resistance patterns of 22 antimicrobial agents, presence of resistant mutants, and inducibility of beta-lactamase were investigated; furthermore, population analysis and investigation of beta-lactamase production of selected isolates were performed. Thirty-two isolates (60%) were resistant to cephalothin and/or cefoxitin and/or ampicillin, and 14 isolates (26%) had further resistance characteristics, 7 of the 14 being resistant to non-beta-lactam antibiotics. All ampicillin-susceptible and 76% of cefotaxime-susceptible isolates had resistant mutants in the zone of inhibition when high inoculum was used. All isolates investigated had inducible chromosomal beta-lactamases, and, in addition, two isolates had an enzyme corresponding with TEM-1. Correlation of resistance patterns and antimicrobial treatment preceding bacteraemia showed that treatment with a third-generation cephalosporin was associated with beta-lactam multiresistance. In conclusion, susceptibility testing of beta-lactam antibiotics of Enterobacter must be interpreted with caution and monotherapy with an extended-spectrum cephalosporin should be avoided unless presence of resistant mutants and inducibility of beta-lactamase can be excluded.