Pathogenic synergy between Escherichia coli and Bacteroides fragilis or B. vulgatus in experimental infections: a non-specific phenomenon.

The virulence of Bacteroides fragilis and B. vulgatus for mice was compared in a skin-infection model. These strains were also tested for pathogenic synergy in mixed infections with Escherichia coli. Strains of B. fragilis were generally more virulent than strains of B. vulgatus and, with one exception, the effect of Bacteroides strains in mixed infections merely reflected their inherent virulence.