Anaerobic Gram-negative bacteria dominate in periodontitis locations, while Gram-positive bacteria characterize healthy sites. A well-established Gram-positive flora might therefore inhibit the colonization of Gram-negative pathogens. The purpose of the present investigation was to examine whether endogenous S. sanguis could prevent, or reduce, periodontal bone loss in rats infected with a virulent P. gingivalis strain. Sixty specific pathogen-free Wistar rats were divided into 6 groups. Doxycycline was administered in the drinking water for 2 weeks to the groups A, B, C, and D to suppress the preexisting microflora in the mouth. Rats in groups A and C were subsequently inoculated with an S. sanguis strain, isolated from one of the rats, once a day for 5 d. Infection with P. gingivalis 381 was then carried out for 5 d in groups A, B, and E. Group F was not treated with doxycycline nor infected with bacteria and served as untreated control. Six weeks after the P. gingivalis inoculation, the rats were killed. Periodontal bone levels were assessed radiographically and morphometrically, and serum antibody against P. gingivalis 381 was determined by a fluorescence immunoassay. Periodontal bone support, determined radiographically, was reduced in group B (doxycycline-treated, P. gingivalis-inoculated) compared with the other groups. In contrast, the morphometric determination showed no differences between the groups. In group B antibody levels against two different P. gingivalis 381 cell surface antigens were significantly elevated.(ABSTRACT TRUNCATED AT 250 WORDS)