Synergism between Bacteroides fragilis and Staphylococcus aureus in experimental tibial osteomyelitis

We explored the potential role of microbial synergy in an experimental rat osteomyelitis model. Osteomyelitis was assessed by gross pathologic conditions and quantitative cultivation of rat tibiae for the implanted organisms 21 days after challenge. When Staphylococcus aureus was used alone, the 50% infective dose (ID₅₀) and the 100% infective dose (ID₁₀₀) were 400 and 25,000 colonyforming units (CFU), respectively. When Bacteroides fragilis was inoculated alone, the ID₅₀ was 150,000 organisms, and the ID₁₀₀ was not confidently determined. Subinfectious numbers of B. fragilis added to the staphylococcal inocula yielded an ID₁₀₀ as low as 20 staphylococci. Mixed inocula with 20 or 200 staphylococci and increasing numbers of B. fragilis yielded a dose-dependent increase in the number of staphylococci isolated from osteomyelitic tibiae. Multiple linear regression analysis confirmed the two inocula and their interaction to be significantly predictive of the 21-day quantitative assessment of staphylococci (r = 0.80). Synergy was most striking at low bacterial inocula. When even large numbers of S. aureus were added to B. fragilis, the B. fragilis inoculum required to initiate B. fragilis osteomyelitis was essentially unchanged. We conclude that small numbers of B. fragilis allow remarkably low numbers of S. aureus (20 or 200 CFU) to establish osteomyelitis in the rat.