RESUMO
BACKGROUND: Antimicrobial resistance to third-generation cephalosporins in gram-negative bacteria, especially Enterobacter, Citrobacter, and Serratia spp., is increasing. The resistance mechanism of these organisms are hyperproduction of AmpC beta-lactamase and plasmid-mediated extended-spectrum beta-lactamase (ESBL). This study was to determine the occurrence of AmpC hyperproduction and ESBLs in E. cloacae, C. freundii, and S. marcescens over a 3-month period in 2002. METHODS: We tested total of 619 consecutive, nonduplicate isolates (229 E. cloacae, 183 C. freundii, 207 S. marcescens) from 12 university hospitals and a commercial laboratory in Korea. Antimicrobial susceptibilities were tested using the disk diffusion method. AmpC hyperproduction was defined as nonsusceptible to cefotaxime or ceftazidime for E. cloacae and C. freundii and as nonsusceptible to cefotaxime for S. marcescens. ESBL production was determined by the double disk synergy test. RESULTS: Among the E. cloacae, C. freundii and S. marcescens derepressed strains were 20.5%, 30.1%, and 31.4% and ESBL producers were 23.6%, 10.9%, and 15.5%, respectively. The AmpC derepressed strains and ESBL producers revealed lower susceptibility rates for ciprofloxacin, piperacillin, piperacillin-tazobactam and aminoglycosides. CONCLUSIONS: These data reveal that the occurrence of AmpC derepressants and ESBL producers among E. cloacae, C. freundii and S. marcescens is relatively high. Continued nationwide surveillance is necessary to provide information on the spread of these important mechanisms of resistance to beta-lactams.