Affiliations: Microbiology Laboratory, Department of Laboratory
Medicine, Seattle, Washington, USA | Division of Infectious Diseases, Department of
Pediatrics, Seattle, Washington, USA | Office of Research Administration, Clinical Research
Center, Children's Hospital and Regional Medical Center, University of
Washington School of Medicine, Seattle, Washington, USA
Note: [] Correspondence: Xuan Qin, PhD., Microbiology Laboratory,
Department of Laboratories and Pathology, A6901, Children's Hospital and
Regional Medical Center, Seattle, Washington 98105, USA. Tel.: +1 206 987 2586;
Fax: +1 206 987 3840; E-mail: [email protected]
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.
