Pseudocompact-type growth and conversion of growth types of strains of Staphylococcus aureus in vitro and in vivo.

Some strains of Staphylococcus aureus grew as compact colonies in Brain Heart Infusion-serum-soft agar but as diffuse colonies in a modified Staphylococcus 110-serum-soft agar. These strains were designated "pseudocompact." Strains showing compact-type colonial morphology in both media were designated "compact," whereas strains showing diffuse-type growth in both media were designated "diffuse." It was observed that the most recently isolated strains of S. aureus were of the pseudocompact type, whereas most stock culture strains tested were of the compact type. Using cultures recently isolated from clinical material, it was shown that pseudocompact strains convert to compact-type growth after prolonged incubation. Interconversion of compact, diffuse, and pseudocompact growth forms could be induced in vitro by appropriate cultural conditions, and conversion of growth type was also observed in vivo. Femoral abscesses produced in mice by four different compact-type strains showed conversion to diffuse or pseudocompact-type growth during the course of the infection.