Clinical significance of infections caused by plasmid-mediated AmpC β-lactamases and extended-spectrum β-lactamase-producing Escherichia coli

Abstract

Escherichia coli frequently causes both nosocomial and community-acquired infections, and antimicrobial resistance in E. coli is a growing problem worldwide. Resistance to broad-spectrum antibiotics in E. coli is often mediated by the production of extended-spectrum b-lactamases (ESBLs) [1]. In recent years, some E. coli strains have been isolated that harbor plasmid-mediated AmpC genes imported from other Enterobacteriaceae carrying chromosomal AmpC genes, such as Citrobacter freundii, Enterobacter cloacae, and Aeromonas species [2]. Recent studies on plasmid-mediated AmpC b-lactamase (PAB)producing E. coli have shown only the prevalence of PAB [3–6]. Furthermore, most previous studies regarding PABproducing E. coli have involved all clinical isolates, including colonized pathogens [1, 4–6]. Thus, we investigated the clinical features and clinical outcomes of patients with PAB-producing E. coli bacteremia. Also, we conducted this study in order to delineate the differences in the clinical manifestations and outcome of E. coli bacteremia between ESBL and non-ESBL groups, and to evaluate the impact of ESBL-producing organisms on outcome in E. coli bacteremia. We reviewed the medical records of individuals diagnosed with E. coli bacteremia from July 2006 to August 2007 at Samsung Medical Center, Seoul, Republic of Korea, a 1,950-bed tertiary-care university hospital. Patients were included in the study if their blood culture results were positive for E. coli. Only the first bacteremic episode for each patient was included in this study. E. coli bacteremia was defined as a growth of E. coli in a blood culture specimen. Nosocomial infection was defined as an infection that occurred [72 h after admission to the hospital. Community-onset infections occurring B72 h after admission were further classified as healthcare-associated if any of the established criteria were present [7]. We considered antimicrobial therapy to be inappropriate if the drugs used did not have in vitro activity against the isolates or if the patient did not receive antimicrobial therapy. The bacterial isolates were obtained from the Asian Bacterial Bank (ABB) of the Asia Pacific Foundation for Infectious Disease (APFID). In vitro antimicrobial susceptibility testing of the isolates was performed in Mueller–Hinton broth (Becton–Dickinson, Sparks, MD) by the broth microdilution method in accordance with the guidelines established by the Clinical and Laboratory Standards Institute (CLSI). Isolates with resistance to cefoxitin were considered to be positive for AmpC b-lactamase. Cefoxitin-positive isolates were further tested using the previously described polymerase chain reaction (PCR) primers and conditions [6]. ESBL production was confirmed in screening-positive isolates via a double-disk synergy test using BD BBL Sensi-Disc (Becton–Dickinson). Doubledisk synergy test-positive isolates were further tested by PCR and sequence analyses to determine the gene This study was presented, in part, at the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), San Francisco, CA, USA, September 2009 (abstract 2413).