In vitro growth, acidogenicity and cariogenicity of predominant human root caries flora

Streptococcus mutans (Sm), Lactobacillus acidophilus (La) and Actinomyces israelii (Ai) have been associated with root surface caries, which is an increasing problem in elderly Chinese. The aim of this study therefore, was to evaluate in vitro, the growth, acidogenicity and cariogenicity of these organisms, both in mono- and co-cultures using an in vitro model. Forty-eight root specimens were prepared using intact extracted human molars. Fresh, wild-type bacteria obtained from root caries lesions were assembled into seven experimental groups as either mono- or co-cultures and incubated with the root specimens. Appropriate controls were included. Growth curve of each experimental group was monitored for 24h, aerobically, at 37 degrees C using a microplate reader. The pH of the medium was recorded after 24-h incubation using a pH meter. Mean depths of artificial root lesions produced in each cultural group were measured using polarized light microscopy in specimens cut into thin sections (100+/-20 microm). Compared with mono-cultures, synergistic growth was observed in co-cultures of 'La+Sm', 'Ai+La' and 'Ai+La+Sm'. Mean lesion depth produced in La group was significantly shallower than other mono- or co-culture groups (p<0.01). The pH values of all culture media were similar after 24-h incubation. The current data elucidate the complex interactions of three predominant bacterial species considered prime agents of human root surface caries.