[Detection of beta-lactamases resistant to inhibitors (IRT) by the disk diffusion method]

85 E. coli strains producing various beta-lactamase were studied: TEM low level: n = 11, TEM high level: n = 4, oxacillinase (OXA): n = 5, céphalosporinase (CASE) +/- TEM: n = 5, inhibitor resistant TEM (IRT) n = 50, IRT+TEM: n = 3, IRT+

n = 7. Their susceptibility to amoxicillin (AMX), amoxicillin+clavulanate (AMC), ticarcillin (TIC), ticarcillin+clavulanate (TCC), piperacillin (PIP), piperacillin+tazobactam (TAZ), mecillinam (MEC), cephalotin (CF), cefoxitin (FOX) et ceftazidime (CAZ) were evaluated by a disk diffusion method, in order to determine the resistance pattern which allows a reliable detection of IRT-producing strains. The phenotype AMX R, TIC R, AMCI/R, TCCI/R et CFS was observed in 46 out of 53 IRT +/- TEM strains, 4/5 of OXA strains and 1/11 of TEM low level strains. A better sensitivity could be obtained by a modification of breakpoints for AMC, TCC and CF, however OXA-producing strains remain indistinguishable from IRT-strains.