Effect of Streptococcus faecalis on the growth of Bacteroides species and anaerobic cocci in mixed infection.

The relationship between Streptococcus faecalis and anaerobic bacteria was studied in a subcutaneous abscess model in mice. Included in the study were one strain of S. faecalis, three strains each of the Bacteroides fragilis group and B. melaninogenicus group, and five strains of anaerobic cocci. The relationship between the organisms was studied by determining the individual change in colony-forming units (CFUs) of the two bacterial components that induces abscesses in mice, compared to the CFUs of each of the isolated when inoculated alone. Of the eleven combinations, the CFUs of S. faecalis increased in seven and decreased in two. The CFUs of anaerobic bacteria were never increased and were decreased in two. A mutual decrease in growth occurred only in the combinations of S. faecalis and B. melaninogenicus group. This study demonstrates the potential for pathogenicity of S. faecalis in mixed infections with the B. fragilis group and anaerobic cocci.