Virulence of Staphylococcus aureus grown in vitro or in vivo

Paired comparisons were made of various strains of Staphylococcus aureus grown in broth inside dialysis sacs anchored in the peritoneal cavities of sheep (in vivo culture) and in a variety of bacteriological media in the laboratory (in vitro culture). The organisms grown in vivo possessed enhanced virulence compared with in vitro grown organisms, when injected intradermally in sheep, when injected intraperitoneally in mice and when infused into lactating mammary glands of ewes. Growth under in vivo conditions conferred on the bacteria an increased resistance to phagocytosis by bovine neutrophils. The bacteria grown under in vivo conditions possessed an additional cell-associated component as determined by immunodiffusion tests and optic density profiles of gel filtration eluates; however, this substance was not visible in electron micrographs in the form of a capsule.