Co-aggregation as a virulent factor of Streptococcus sanguis isolated from infective endocarditis.

The pathogenicity of strains of the Streptococcus sanguis group, isolated from infective endcarditis, was studied by measuring the development of subcutaneous abscesses in mice after infection with S. sanguis and Actinomyces viscosus either singly or as co-aggregated pairs. The pathogenicity of the co-aggregates was also examined in various viable combinations of the two bacterial species. More abscesses were formed by A. viscosus than the S. sanguis group including clinical isolates. Abscess formation by co-aggregates of combinations of each isolate and A. viscosus produced a higher percentage of abscess formation than those caused by infection with a pure suspension of A. viscosus or tested streptococci. Co-aggregated cells were more resistant to phagocytosis and killing by neutrophils in vivo. These results indicated that S. sanguis group streptococci isolated from infective endocarditis are able to co-aggregate and resist phagocytosis. The ability of co-aggregation of S. sanguis may serve as a survival mechanism in a host defense system and may be linked with virulence of this bacteria.