Phenotypic variation of Staphylococcus epidermidis slime production in vitro and in vivo

Clinical studies performed by us and others have found an association between slime production and strains of coagulase-negative staphylococci that infect indwelling medical devices. By serial low-speed centrifugation of broth cultures we have isolated a stable, weakly adherent strain (RP62A-NA) from a strongly adherent, slime-producing, pathogenic strain of Staphylococcus epidermidis sensu stricto (RP62A, ATCC 35984). We obtained a second strain from RP62A-NA (RP62A-NAR) by serial subculture of glass-adherent cells of RP62A-NA. All three strains had the same pattern of biochemical reactions, antimicrobial susceptibilities, and plasmid analysis. Transmission electron micrograph sections stained with the mucopolysaccharide-specific stain alcian blue demonstrated that the adherent strains RP62A and RP62A-NAR were covered with an extracellular coat of polysaccharide-rich material. In contrast, the nonadherent RP62A-NA strain lacked this external coat. All three strains were used in a mouse model of foreign body infection and a rat model of catheter-induced infective endocarditis. The adherence characteristics of isolates of RP62A and RP62A-NA recovered from experimental animals were relatively stable, although we noted a slight but a significant increase in the adherence of RP62A-NA isolates recovered from the foreign body model. The adherence characteristics of RP62A-NAR isolates recovered from infected animals were variable; in general these isolates were less adherent than the laboratory strain of RP62A-NAR. In both models the 50% infective dose (calculated by the Reed and Muench method) was three times greater for the RP62A-NA strain than for the RP62A strain. The phenotypic expression of slime production is subject to both in vitro and in vivo variation and could play a role in the pathogenesis of foreign body infection.