MICROBIAL INDUCTION OF DENTINAL CARIES IN HUMAN TEETH IN-VITRO

The ability of Lactobacillus casei, Streptococcus sobrinus, Actinomyce s viscosus, and Streptococcus salivarius to induce dental caries was i n vitro. A class I cavity (depth: 2 mm) was prepared in extracted huma n caries-free premolars to make dentin blocks. The blocks were inocula ted with these four bacterial strains in a monoinfective fashion and w ere incubated under anaerobic conditions. In addition to the monoinfec tion groups, mixed-infection groups of L. casei with S. sobrinus or A. viscosus were also prepared. Half of the culture medium was renewed e very 3 days, and the pH of the medium was measured. After 4 or 12 wk, these dentin blocks were prepared by Brown-Brenn staining and by conta ct microradiography for light microscopic observation and for immunohi stochemical staining. The final pH of the S. salivarius group was the highest among the experimental groups, at similar to 5.1; that of the others was similar to 4.3. Bacterial invasion into the dentinal tubule s was observed in all but the S. salivarius group. Among the monoinfec tion groups, the S. sobrinus group showed the highest invasion rate, f ollowed by the A. viscosus group and the L. casei group. The invasion rate was also high in the mixed-infection groups. Immunohistochemical staining revealed invasion only by L. casei, and not by S. sobrinus an d A. viscosus. The invasion rate by L. casei was higher in the mixed-i nfection group with either S. sobrinus or A. viscosus than in the mono infection groups. These findings suggest that lactobacillus might play an important role in the initiation and progress of dentinal caries, and that this bacterial species might exhibit a cooperative cariogenic ity when it coexists with other bacterial species that surpasses its i ndividual cariogenicity.