[Detection of extended broad-spectrum beta-lactamases with the ATB MIC system]

The Enterobacteriaceae producing extended broad-spectrum beta-lactamases (ESB) must be detected even when their MIC to oxyiminocephalosporins are low. We tested 164 isolates (133 ESB, 31 non ESB) on two experimental ATB MIC plates containing cefotaxime or ceftazidime (0.06-128 mg/l) alone or associated with sulbactam (4 mg/l). Fifty four per cent of the ESB isolates are sensitive to cefotaxime, and 30% to ceftazidime. Respectively 95% and 94% of these bacteria have a synergism index (MIC CIII alone/MIC CIIII + sulbactam) greater than or equal to 4 with cefotaxime and ceftazidime. The isolates that do not produce ESB have all a rate less than 4. The ATB system can detect the extended broad spectrum beta-lactamases when low concentrations of cephalosporins are used, alone and in association with an inhibitor of beta-lactamases.