Genome evolution and plasticity of serratia marcescens, an important multidrug-resistant nosocomial pathogen

Serratiamarcescens is an important nosocomial pathogen that can cause an array of infections,most notably of the urinary tract and bloodstream. Naturally, it is found inmany environmental niches, and is capable of infecting plants and animals. The emergence and spread of multidrug-resistant strains producing extended-spectrum or metallo beta-lactamases now pose a threat to public health worldwide. Here we report the complete genome sequences of two carefully selected S. marcescens strains, a multidrug-resistant clinical isolate (strainSM39)andaninsect isolate (strain Db11).Our comparative analyses reveal the coregenomeof S. marcescensand define the potential metabolic capacity, virulence, and multidrug resistance of this species. We show a remarkable intraspecies genetic diversity, both at the sequence level and with regardsgenomeflexibility, whichmay reflect the diversity of niches inhabited by members of this species.Abroader analysis with other Serratia species identifies a set of approximately 3,000 genes that characterize the genus.Within this apparent genetic diversity, we identifiedmany genes implicated in the high virulence potential and antibiotic resistance of SM39, including the metallo beta-lactamase and multiple other drug resistance determinants carried on plasmid pSMC1. We further show that pSMC1 ismost closely related to plasmids circulating in Pseudomonas species. Our data will provide a valuable basis for future studies on S. marcescens and new insights into the genetic mechanisms that underlie the emergence of pathogens highly resistant to multiple antimicrobial agents.