Use of a continuous culture system for susceptibility testing of gram-negative bacterial isolates to piperacillin

Fifteen (15.2%) of 99 Gram-negative bacteria isolated from patients during episodes of fever and neutropenia had minimum inhibitory concentrations ranging from less than 2 to 32 mg/l but were not fully sensitive to piperacillin by disc diffusion testing. These 15 isolates were further examined using a chemostat system, in which susceptible isolates were rapidly killed by piperacillin. Only three of the 15 isolates were susceptible to piperacillin when compared to sensitive control strains using the chemostat system. The results of this study suggest that an increase in the MIC to piperacillin is a less sensitive indicator of resistance to piperacillin amongst Gram-negative isolates than a reduction in the zone of inhibition in disc diffusion testing.