The bactericidal action of human serum on Bacteroides species.

Sera obtained from 60 healthy normal subjects were tested for their inhibitory action against four clinical isolates of the Bacteroides fragilis group. All the sera inhibited clinical strains of B. vulgatus and B. ovatus, 92% of 60 sera inhibited B. thetaiotaomicron and 62% inhibited B. fragilis. Bacterial inhibition required components of uninactivated serum and was related to the serum concentrations and species of the bacteria. The viable counts of all the strains, except B. fragilis, were significantly reduced in pooled sera from patients with anaerobic systemic infections and from non-infected patients. The bactericidal activity of the sera from infected patients was more pronounced than that of the 'normal' sera, on all strains except B. fragilis. These observations may explain in part the predominance of B. fragilis in serious clinical infections involving anaerobes.