Lack of virulence of bovine type III Streptococcus agalactiae strains for mice correlates with reduced in vitro production of extracellular type-specific antigen

Six strains of type III Streptococcus agalactiae isolated from milk samples from cases of bovine mastitis were examined for in vitro production of three potential extracellular virulence factors: neuraminidase, protease, and extracellular type-specific antigen. Virulence in mice, expressed as LD₅₀ values, was examined for these six strains to determine if a relationship existed between the in vitro production of any of the three extracellular products and mouse lethality. Only in vitro production of extracellular type-specific antigen showed a correlation with virulence of these organisms in the mouse model. All six bovine strains were relatively avirulent in the mouse while producing reduced levels in vitro of extracellular typespecific antigen as compared to nine human isolates. The bovine S. agalactiae strains were an average of 538-fold less virulent for the mouse than were the high type-specific antigen producers isolated from human sources.