Institutional spread of clonally related Serratia marcescens isolates with a novel AmpC cephalosporinase (S4): a 4-year experience in Taiwan.

Resistance of Serratia marcescens, a nosocomial pathogen of Enterobacteriaceae, to the extended-spectrum beta-lactams is usually mediated by an overproduced AmpC cephalosporinase. We aimed to characterize the molecular epidemiology and AmpC of S. marcescens isolates recovered from 1 medical center in southern Taiwan. AmpC-encoding genes for SRT families were investigated by polymerase chain reaction and DNA sequencing. From August 1999 through July 2003, 69 nonrepetitive bloodstream isolates were enrolled. Excluding 11 isolates, which also produced an extended-spectrum beta-lactamase, 58 isolates carried an AmpC-encoding gene, including a novel S4 gene with 98% identity to SRT-1 gene (n = 50), SRT-2 gene (n = 3), SST-1 gene (n = 1), and others (n = 4). Isolates with S4 exhibited a phenotype of resistance to cefotaxime (CTX) but not ceftazidime. Genotype analysis by pulsed-field gel electrophoresis revealed that 45 (90%) of the isolates carrying S4 gene belonged to 2 major epidemic clones, including types A (n = 28) and B (n = 17). In conclusion, the AmpC-like S4 beta-lactamase may confer CTX resistance of the S. marcescens population. Strains carrying the S4 gene with prolonged dissemination were closely related.