Colonization of the teeth of rats by human and rodent oral strains of the bacterium Actinomyces viscosus.

The mouths of rats were infected with human and rodent strains of A. viscosus and streptomycin-rifamycin-resistant (SR) mutants of these strains. The establishment, adherence to the teeth and cell dose required for infection were determined. Human and rodent strains established equally well on the teeth and did not differ in their initial adherence to teeth. The cell dose required for infection was much lower for rodent than for human strains (less than 10(5) compared to 10(7) - 10(8) c.f.u.). The SR mutants established as well as their parents except for one SR strain, that did not establish at all. Another SR mutant appeared to have lost its SR resistance after passage through the animal. The results stress the need of in-vivo testing of antibiotic resistant mutants to be used in ecological studies.