In Vitro Susceptibility of piperacillin/tazobactam Against extended-spectrum beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae

Minimal inhibitory concentrations (MIC) of piperacillin/tazobactam were determined on 20 clinical isolates of extended-spectrum beta-lactamase (ESBL)-producing E. coli and 30 isolates of ESBL-producing Klebsiella pneumoniae. MIC50 and MIC90 for ESBL-producing E. coli were 8/4 mu g/ml and 256/4 mu g/mL, respectively. MIC50 and MIC90 for ESBL-producing K. pneumoniae were 8/4 mu g/mL and >512/4 mu g/mL, respectively. The susceptibilities of ESBL-producing E. coli and K. pneumoniae to piperacillin/tazobactam were 80% and 60%, respectively. Of 20 ESBL-producing E. coli strains, 11 (55%) were TEM-and CTX-M-positive, and SHV-negative. Of 30 ESBL-producing K. pneumoniae strains, ten (33%) were PCR positive for SHV and negative for TEM and CTX-M.