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In the present investigation, the proportions of Streptococcus mutans, lactobacilli, Streptococcus sanguis, veillonellae, and an unidentified actinomyces-like organism in dental plaque on occlusal fissures of first mandibular molars were monitored at 6-month intervals over a 3-year period in 368 children who were initially in grades 1 or 2. Teeth destined to become decayed exhibited a significant increase in the proportions of S. mutans from 6 to 24 months before the diagnosis of dental decay. Lactobacilli were sporadically detected but when present were associated with dental decay. Children whose teeth exhibited the greatest number of decayed surfaces had, at all time periods, significantly higher proportions of S. mutans than did children who were caries free. Many teeth had high proportions of S. mutans at their entry into the study. About 10% of the monitored teeth erupted during the period of observation, and in these teeth both S. mutans and lactobacilli could be significantly associated with decay. In these newly erupted teeth S. mutans outnumbered lactobacilli by ca. 20 to 1. S. sanguis, veillonellae, and the unidentified actinomyces-like organism could not be associated with the development of decay. These findings strongly implicate S. mutans and possibly lactobacilli as dental pathogens and suggest that if decay is to be controlled by strategies based upon a S. mutans infection, then the various tactics used probably will have to be performed on primary teeth, as these teeth are the most likely sources of infection for the permanent teeth.

The natural history of Streptococcus mutans was studied in occlusal fissures of 238 right and 235 left lower mandibular first molars in 279 children, initially aged 6–7, over a 2-year period. S. mutans infection was endemic in this group. Results showed that the levels of S. mutans were fairly stable – an initially high level of infection tended to stay high, and vice versa. High levels of infection were more associated with subsequent development of caries. None of the 21 teeth with nondetectable levels of S. mutans developed caries, though a number of teeth with high levels of S. mutans remained sound. S. sanguis appeared to have no independent influence on whether caries developed or not.

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