Survey of CTX-M-3 extended-spectrum β-lactamase (ESBL) among cefotaxime-resistant Serratia marcescens at a medical center in middle Taiwan

Thirty-four clinical isolates of Serratia marcescens nonsusceptible to cefotaxime were collected from a medical center in middle Taiwan. Confirmatory tests for extended-spectrum beta-lactamases (ESBLs) by cefotaxime and ceftazidime +/- clavulanic acid using Etest ESBL Screen identified only one ESBL producer; the remaining 33 isolates revealed nondeterminable results, because of off-scale minimum inhibitory concentration (MIC) levels for cefotaxime +/- clavulanic acid. Agar microdilution method using broader MIC ranges confirmed 21 ESBL-producers and one non-determinable result, achieving a highly predicting value compared to golden standard by PCR and DNA sequencing analysis, which identified 22 (65%) isolates containing bla(CTX-M-3) genes. Only one strain carried concurrent CTX-M-3 and SHV-5 conferring high-level MICs to both cefotaxime (128 mug/mL) and ceftazidime (64 mug/mL). Other enzymatic mechanisms, such as chromosome-encoded AmpC including a novel SRT-2 enzyme, may confer resistance to cefotaxime on the remaining 12 isolates without ESBL bla genes. Thus, it is unreliable to predict the resistance mechanism by antibiogram, and current Etest ESBL Screen tests. Our study highlights expanding efforts to detect ESBLs in S. inarcescens are urgently needed in Taiwan. (C) 2004 Elsevier Inc. All rights reserved.