Interactions between polymorphonuclear leucocytes, Bacteroides sp, and Escherichia coli: their role in the pathogenesis of mixed infection.

Five Bacteroides fragilis strains and five Bacteroides vulgatus strains were compared with regard to their ability to consume complement and to fix C3, their killing by polymorphonuclear leucocytes, and their ability to inhibit the bactericidal effect of serum and polymorphs on Escherichia coli strains. Complement consumption was positively related to C3 fixation, but no relation was observed between these variables and the killing of the anaerobes. Greatest inhibition of the killing of E coli by serum and polymorphs was achieved with the bacteroides strains that fixed most complement. The greater virulence of B fragilis in mixed infections with E coli was not reflected either by a greater ability to inhibit the killing of E coli or a greater resistance of the anaerobes themselves to the bactericidal effect of serum and polymorphs.