[Studies on caries of Ishibashi rat--II. Streptococcus mutans population in oral cavity].

Previously, it was demonstrated that extensive carious lesions occurred in the Ishibashi rats (ISR) while they were bred with conventional diets. In this study, specimens from the oral cavity of the ISR were examined for the presence of Streptococcus mutans, and its occurrence and population were compared with those of the Wistar rats. The results were as follows: 1. S. mutans was first found in the mouth of the ISR when the first molars erupt. 2. The number of S. mutans (CFU on the gelatin agar) and its mean percentage of the total streptococci (CFU on the gelatin agar/CFU on the MS agar) in the mouth of the ISR increased rapidly during the few days after weaning, and this increment was comparable to that of the Wistar rats fed caries-producing diets, 6PMV, after weaning. However, the population of the S. mutans remained low in the mouth of the Wistar rats fed conventional diets. 3. In the aged ISR, the number of S. mutans and its mean percentage of total streptococci were also much higher than in the aged Wistar rats. These data suggested that the ISR harboured increased the number of S. mutans strains in the oral cavity when compared with the Wistar rats.