Enhancement of Bacteroides intermedius growth by Fusobacterium necrophorum.

Previous work from this laboratory has demonstrated the persistence of Bacteroides intermedius in the livers of mice receiving an intraperitoneal inoculum of B. intermedius and Fusobacterium necrophorum. This study was undertaken to determine whether F. necrophorum enhanced the in vitro growth of B. intermedius. Tryptose phosphate broth did not support the growth of B. intermedius alone, but the bacterium did survive in a tryptose phosphate broth culture of F. necrophorum. B. intermedius cultured in F. necrophorum-conditioned tryptose phosphate broth grew impressively, reaching maximal absorbance at 24 h after inoculation. The growth of B. intermedius in F. necrophorum-conditioned tryptose phosphate broth was proportional to the amount of conditioned medium present. The B. intermedius growth-stimulating factor was detectable in conditioned medium 8 h after inoculation with F. necrophorum and could be detected throughout the 96-h incubation period. Growth-factor-active fractions eluted from a Sephadex G-100 column did not absorb at 280 nm and were retained on the column until 4 column volumes were eluted. The growth factor was nondialyzable and stable to boiling, lyophilization, extraction with hot aqueous phenol, and trypsin digestion. The factor was inactivated by exposure to pH 2.0 in the pepsin digestion protocol. Significant amounts of hexose, methyl pentose, and 2-keto-3-deoxyoctonate were detected in pooled growth-factor-active fractions eluted from the Sephadex column. This pool was also active in the Limulus lysate endotoxin assay. These results suggest that the B. intermedius growth-stimulating factor produced by F. necrophorum is a lipopolysaccharide.