Four clumping factor-negative strains of Staphylococcus aureus were found to closely resemble the diffuse colonial variant of the Smith strain. All produced fatal intraperitoneal infections in mice, all grew in diffuse, streaming colonies in plasma or serum soft agar, and all behaved like encapsulated microorganisms in in vitro opsonic systems.

These staphylococci were resistant to phagocytosis in the peritoneal cavities of normal mice. When mice were immunized with heat-killed vaccines prepared from the Smith diffuse variant these strains were rapidly ingested by peritoneal leukocytes and the animals survived. This observation suggests that these strains share the same or a similar phagocytosis-retarding antigen.

While most pathogenic staphylococci isolated from human material do not behave like these unusual mouse-virulent strains, indirect evidence is cited to support the suggestion that other staphylococci may acquire similar phagocytosis-resisting characteristics during in vivo multiplication. Studies to support or refute this thesis are in progress.

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