Presence of a capsule in Erysipelothrix rhusiopathiae and its relationship to virulence for mice.

Three avirulent insertional mutants of Erysipelothrix rhusiopathiae were obtained by the technique of transposon mutagenesis with the self-conjugative transposon Tn916. The interactions between murine polymorphonuclear leukocytes and parent and mutant strains were studied in vitro. In the presence of normal serum, the virulent parent strain was resistant to phagocytosis, whereas the avirulent mutant strains were efficiently phagocytosed. In the presence of immune serum, the parent and the mutant strains were both efficiently phagocytosed. Electron microscopic examination of the parent strain demonstrated the presence of a structure resembling a capsule which was absent on the mutant strains, suggesting that a capsule may be involved in virulence. This was confirmed in studies in which an avirulent mutant strain reverted to virulence following acquisition of a capsule when the transposon was lost by spontaneous excision. These results strongly suggest that virulence of E. rhusiopathiae is associated, at least in part, with resistance to phagocytosis by polymorphonuclear leukocytes and that this antiphagocytic ability of the bacterium results from its possession of a capsule.