Success through diversity - how Staphylococcus epidermidis establishes as a nosocomial pathogen.

Staphylococcus epidermidis normally is a commensal inhabitant of the healthy human skin and mucosa, but also a common nosocomial pathogen in immunocompromised patients. Living at the edge between commensalism and pathogenicity, S. epidermidis has developed interesting strategies to conquer the hospital environment as a novel ecological niche and to transform into a notorious pathogen. Recent progress in genome analysis and molecular epidemiology gave interesting insights into the enormous flexibility by which these bacteria generate continuously novel phenotypic and genotypic variants. Recent multilocus sequence typing studies identified S. epidermidis as a highly diverse species that evolves mainly by recombination and acquires readily mobile genetic elements. With respect to healthcare-associated isolates, a limited number of epidemic clonal lineages were found to have emerged and established in hospital settings worldwide. These isolates are characterised by the carriage of various SCCmec gene cassettes, conferring methicillin resistance, and by a striking ability to form biofilms on medical devices. Moreover, nosocomial S. epidermidis strains typically harbour multiple copies of the insertion sequence element IS256 in their genomes. Nosocomial S. epidermidis strains vary virulence- and resistance-associated gene expression in the course of an infection to a remarkably high degree. Heterogenous gene expression in S. epidermidis is achieved, on the one hand, by complex regulatory pathways. On the other hand, it is associated with genetic mechanisms that were found to be mediated by the action of the IS256 element which obviously represents an important driving force for the flexibility of the S. epidermidis genome. The data accumulated so far suggest that recombination along with the frequent acquisition of mobile genetic elements are crucial factors for the success of S. epidermidis as a nosocomial pathogen.