Demonstration of capsule in a strain of Staphylococcus hyicus by an electron microscope.

A compact type variant designated as strain ST67V was isolated by a high-temperature subculture method from strain ST67P of Staphylococcus hyicus, which was a diffuse type in serum-soft agar. The parent strain was relatively virulent in mice and resisted ingestion by mouse peritoneal cells. The variant strain, however, was avirulent in mice and no antiphagocytic activity was observed in the peritoneal cavity. In ultra-thin sections of the organisms treated with anti-ST67P rabbit anti-serum conjugated with ferritin, the outermost layer of the cell wall of the parent strain was covered with a well-defined capsule while no capsule was shown in the variant strain.