A temporal disc co-diffusion method detects piperacillin-tazobactam resistance in Serratia marcescens isolates reportedly susceptible by conventional methods

Abstract

Laboratory interpretation of susceptibility data concerning the AmpC-producing Enterobacteriaceae is confusing to clinicians. Typical organisms included in this category are Enterobacter cloacae, Enterobacter aerogenes, Citrobacter freundii, Morganella morganii, and Serratia marcescens. These organisms frequently appear susceptible to extended spectrum cephalosporins by standard in vitro testing, although this is highly contested by treatment failures in cases of invasive infections. The mechanisms of resistance were postulated to be ampC induction or de-repression during therapy. As an alternative to carbapenems, piperacillin-tazobactam has been commonly used for its apparent activities against AmpC producers. In this laboratory study, when cefoxitin was used as an ampC-inducing agent in vitro, piperacillin-tazobactam showed activity in only a limited number of AmpC producers. Most notably, a temporal disc co-diffusion method detected inducible piperacillin- tazobactam resistance in all 20 strains of S. marcescens tested. With more standardized measurement, this method improves our previous Kirby-Bauer disc approximation method for the detection of ampC-mediated inducible resistance.