Phenotypic Selection of Small-Colony Variant Forms of Staphylococcus epidermidis in the Rat Model of Endocarditis

Four pathogenic strains of Staphylococcus epidermidis (sensu strictu) ? EC, RP62A, LW, and HBSN?exhibited a mixed population of colony phenotypes when plated onto a highsalt, low-glucose agar ("Memphis agar"). When compared with challenge inocula, significant shifts in colony populations to the small-colony variant forms occurred for all four strains for isolates recovered from infected vegetations of rats with catheter-induced endocarditis. Various colony phenotypes of the HBSN strain were compared in virulence studies by using the rat model of endocarditis. The infectivity rate of the small-colony variant forms was significantly less than that of either the normal parent form (P < .05) or the mixed phenotypic inoculum (P < .05). These data indicate that the small-colony variant forms may be selected for, once endocardial infections are established, and that the small-colony variant forms alone are much less able to initiate and/or sustain intracardiac infections in experimental endocarditis.