Confirmation of extended-spectrum β-lactamase-producing Serratia marcescens: Preliminary report from Taiwan

Although Serratia marcescens is a common cause of nosocomial infection in Taiwan, strains producing extended-spectrum β-lactamases (ESBLs) are rare. We detected four clinical isolates of S. marcescens from Taiwan that exhibited resistance to cefotaxime (MICs, > 256 μg/ml) and cefepime (MICs, ≥ 32 μg/ml), but were susceptible to imipenem and meropenem. Transconjugants revealed similar MIC profiles when compared to the parental strains. Isoelectric focusing revealed one major transferable β-lactamase (pI 8.4), which was further identified as CTX-M-3 by polymerase chain reaction and gene sequencing. An AmpC-like enzyme (pI 8.8) was not transferable. All four isolates had significant MIC reductions of ≥3 log₂ dilutions for cefepime in the presence of clavulanic acid, compatible with the presence of an ESBL (CTX-M-3). Clavulanate did not significantly reduce the cefotaxime MIC for one isolate that may co-produce high-level AmpC β-lactamase (pI 8.8). Since high-level AmpC expression has minimal effect on the activity of cefepime, isolates co-producing AmpC β-lactamase may be recognized as additional ESBL producers by using cefepime as an ESBL screening agent.