Antiphagocytic effect of the capsule of Staphylococcus simulans.

An encapsulated strain of Staphylococcus simulans was observed to be more resistant to phagocytosis by human granulocytes than was a nonencapsulated strain. Phagocytosis of the encapsulated strain was enhanced by antisera to S. simulans, but opsonic activity of antisera was removed by absorption with S. simulans capsular material. The encapsulated strain of S. simulans was also more invasive than the nonencapsulated S. simulans in vivo. More encapsulated than nonencapsulated S. simulans were found in heart blood when equal numbers of organisms were injected intraperitoneally into mice. Invasion of the bloodstreams of mice by encapsulated S. simulans was prevented by passive immunization (rabbit antiserum). Thus, the capsule of S. simulans inhibited phagocytosis in vitro and contributed to virulence in vivo.