Effect of different beta-lactams in combination with beta-lactamase inhibitors in the presence or absence of tobramycin against some enterobacteriaceae producing extended-spectrum beta-lactamases.

Extended-spectrum beta-lactamase (ESBL) production among members of the family Enterobacteriaceae generally involves resistance to oxyimino-cephalosporins and monobactams, while implying different susceptibility profiles with other antimicrobial agents. We have investigated the activity of some beta-lactam antibiotics, alone or in double and triple combinations with beta-lactamase inhibitors, in the presence or absence of tobramycin (TOB), against some Enterobacteriaceae producing ESBL by means of time-kill curves. Antimicrobials employed were ceftazidime (CAZ), cefotaxime (CTX), TOB, ampicillin (AMP), ampicillin-sulbactam (ASL), amoxicillin (AML), amoxicillin-clavulanic acid (AMC), piperacillin (PIP) and piperacillin-tazobactam (TZP), at 1/4 minimum inhibitory concentration for susceptible strains and at achievable serum concentrations for resistant strains. Only the combinations CTX-ASL, CAZ-ASL and PIP-ASL were synergistic, both at 6 and 24 h, on some strains of Klebsiella species.