TIMED KILLING KINETIC-STUDIES OF THE INTERACTION BETWEEN CIPROFLOXACIN AND BETA-LACTAMS AGAINST GRAM-NEGATIVE BACILLI

Timed killing kinetic studies were performed with ciprofloxacin in com bination with aztreonam, ceftazidime, piperacillin/tazobactam, and tic arcillin/clavulanic acid against three isolates each of Klebsiella pne umoniae, Enterobacter cloacae, Serratia marcescens, and Pseudomonas ae ruginosa. Each antimicrobial agent in the combination was tested at it s MIC and at one-half and one-quarter of its MIC. Colony counts were d etermined at 0, 3, 5, and 7 hours. Synergy occurred most frequently at 7 hours and, when present, was most likely to occur when ciprofloxaci n and the beta-lactam were tested at concentrations equal to their res pective MICs. Synergy after 3 hours of incubation was not predictive o f a synergestic interaction at 5 or 7 hours. Antagonism was noted in s everal instances, particularly when ciprofloxacin and the beta-lactam were combined at one-quarter of their respective MICs. (C) 1996 Elsevi er Science Inc.