Strain M, classified as a Staphylococcus aureus, behaves like the other rare encapsulated staphylococcal strains. It was clumping-factor negative, grew in diffuse-type colonies in serum-soft agar, and produced rapidly fatal disease in mice. Strain M was highly resistant to phagocytosis by human or mouse leukocytes and required both specific antibody and heat-labile serum factor(s) for efficient ingestion by human polymorphonuclear leukocytes. Electron micrographs confirmed the presence of a large capsule. Agglutination studies, active or passive mouse protection experiments, and opsonic studies revealed that strain M represents a new, immunologically distinct strain of encapsulated staphylococcus. Strain M differs from other known encapsulated staphylococci in several other respects: its cellular and colonial morphology is atypical; its LD50 in the mouse peritoneal model is 100 times less than that of other mouse lethal strains; it is poorly opsonized by normal human serum; and, finally, it possesses an unusually large capsule as seen in electron micrographs. MATERIALS AND METHODS Cultures. Strain M. This encapsulated strain was isolated from a human infection in 1962 and has been described by Scott (20). The organism (NCTC 10649) was obtained from the National Collection of Type J. Bacteriol, in press). Smith diffuse. This variant has been previously described and has been used extensively as the prototype of an encapsulated mouse-virulent staphylococcus in this laboratory (9). T mutant. This is a mutant of S. aureus H possessing an additional surface polysaccharide antigen (24). The organism was kindly supplied by J. T. Park, Tufts University, Boston, Mass. Wiley. This strain (Wound) was kindly supplied by B. B. Wiley. When grown under certain conditions this strain has been reported to undergo a specific capsular reaction (23). Giorgio. This strain of S. aureus has been previously described (14) and is a prototypic non-ecapsulated S. aureus. All cultures were maintained on brain heart infusion agar (Difco) slants at 4 C. Unless otherwise noted, 18-h subcultures in brain heart infusion broth were used in all experiments.

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Annals New York Academy of Sciences\ \ \ t -CsCl O D 260-CPM 0 D 260-CPM m t 0.0 3 0.6