Three sequential samples from the distal surface of the upper first premolars of ten 13- to 14-year-old children were collected over a period of 12 weeks. Viable bacterial counts were made on each sample and representative strains of the genera Streptococcus, Actinomyces, Veillonella, Neisseria, Fusobacterium, Rothia, Lactobacillus, Bacterionema, Eubacteria and Arachnia were identified on the basis of morphology, physiological tests, cell wall and acid end product analysis and serology. The qualitative composition of the 58 samples obtained from the 20 sampled areas examined was similar, although some surfaces were consistently free of specific organisms during the test period, this was particularly evident with the lactobacilli. The mean percentages of the predominant genera were Actinomyces 34.92, Streptococcus 22.9, Veillonella 13.07, and ‘bacteroides’ 7.97. Streptococcus mutans mean percentage was 2.17, Strep. sanguis 5.9, Strep. salivarius 0.67, Strep. milleri 0.51, A. israelii 16.5 and A. viscosus together with A. naeslundii 19.05. Consideration of the results indicates that each of the sites represents a distinct ecological niche that should be looked at in isolation, and that the following parameters may be important in relating the bacterial population of a site to the formation of a carious lesion: (a) qualitative composition; (b) persistence of specific bacteria; (c) concentration of specific bacteria, and (d) the stability of the numerical concentration of specific bacteria.

1. During the two-year period, caries developed at 20% of the target premolar sites. The attack rate for these surfaces was similar in the plaque panel and the other subjects in the study. 2. The microbial composition of plaque samples from caries-free sites and from carious sites before and after radiographic detection of lesions was broadly similar. 3. Numerical domination of particular sites by S mutans before detection of caries can occur, but has only been observed so far in 2 of 15 sites. 4. Pooled date from sites which have developed lesions indicate a rise in the isolation frequency and mean numbers of S. mutans after detection of caries. This trend was particularly obvious in the one subject who developed bilateral lesions by the second examination and in three of four sites where caries was detected at the fourth examination. Similar observations have been made with lactobacilli. 5. In two of 15 instances no isolations of S mutans were made from sites which developed caries. 6. To date, no single species appears to be uniquely associated with the onset of dental caries.

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