Use of several inducer and substrate antibiotic combinations in a disk approximation assay format to screen for AmpC induction in patient isolates of Pseudomonas aeruginosa, Enterobacter spp., Citrobacter spp., and Serratia spp.

Two-hundred consecutive, single patient isolates of Enterobacter spp., Serratia spp., Citrobacter spp., and Pseudomonas aeruginosa were evaluated for AmpC production using a variety of inducer-substrate antibiotic combinations in a disk approximation format. The combinations examined included cefoxitin-piperacillin, imipenem-cefotaxime, imipenem-ceftazidime, imipenem-piperacillin-tazobactam, and imipenem-cefoxitin. All isolates were also screened for the presence of extended-spectrum R-lactamase (ESBL) activity. In total, 85.5% of isolates were shown to be inducible for the production of AmpC by one or more inducer/substrate combinations and 11% of all isolates were stably derepressed for the expression of AmpC. Of all of the combinations, imipenem/piperacillin-tazobactam provided the greatest sensitivity (97.1%). All combinations were 100% specific when a positive test was observed. Given this background among these organisms in our institution, it is reasonable to develop an antibiotic reporting strategy that favors the selection of agents for therapy of these organisms that do not serve as labile substrates of AmpC.