Effects of a mixed infection with Porphyromonas gingivalis and Treponema denticola on abscess formation and immune responses in mice.

Porphyromonas gingivalis and Treponema denticola have been found together in lesions of human periodontitis. We examined the ability of a mixed infection by both bacteria to synergistically form abscesses and disturb immune responses in mice. Absorbance of an invasive P. gingivalis 16-1 strain grown in tryptic soy broth and T. denticola ATCC 33520 strain grown in TYGVS medium were adjusted. BALB/c mice were injected with 200 microliters of the cell suspension at a site on the lateral dorsal area. The sizes of the subsequent subcutaneous abscesses were measured with a caliper gauge, and the area was expressed in square mm. Mixed infections by P. gingivalis and T.denticola produced larger abscesses than those formed after mono-infections by either P. gingivalis or T.denticola. The abscesses caused by mixed infection reached their maxima on the 6th day and maintained that size for the subsequent 5 days. The delayed type hypersensitivities against extracted antigens of P.gingivalis in mixed infection mice were significantly lower than those in the mono-infected mice. However, the IgG response to sonicated antigen of P.gingivalis did not differ between the two groups. The sizes of the abscesses caused by mixed infections in mice immunized with whole cells of P.gingivalis 16-1 were compared to those caused in sham-immunized mice. The average size of the abscess caused by mixed infection in immunized mice did not differ from that in sham-immunized mice, but many of the abscesses in immunized mice ruptured on the 4th or 5th day, followed by recovery in two weeks. These results suggest that mixed infection with P.gingivalis and T.denticola attenuates protective immune responses.