THE FINAL PH OF BACTERIA COMPRISING THE PREDOMINANT FLORA ON SOUND AND CARIOUS HUMAN ROOT AND ENAMEL SURFACES

Acidogenesis at low pH appears to be an important bacterial cariogenic trait. However, most information in this regard pertains to only a fe w of the acidogenic dental plaque bacteria. Therefore, the 'final' pH in sugar broth was determined for a wide variety of oral bacteria. The ir source was: (1) carious material from advanced root lesions (ARL), (2) plaque from sound root surfaces of root-caries-free subjects (SRS) , (3) plaque from ''white spot'' coronal lesions and sound coronal sur faces of caries-active subjects, and (4) plaque from sound coronal sur faces of caries-free subjects. Strains from groups 1 and 2 (ARL, 389 s trains; SRS, 358 strains) were previously identified (van Houte et al. , 1994) to the genus/species level and belonged to the predominant cul tivable flora (PCF). Strains from groups 3 and 4 also belonged to the PCF but were not identified. All strains were placed in one of 4 final pH categories: < 4.2, 4.2 - 4.4, 4.4 - 4.6, and greater than or equal to 4.6. The main findings were: (1) ARL samples contained many strain s with a final pH < 4.2 (mean percentage of 25.7). They included all s trains of Lactobacillus and mutans streptococci (MS), most Bifidobacte rium strains and non-mutans streptococci (non-MS), and about 20% of th e Actinomyces strains. By contrast, SRS samples contained far fewer st rains with a final pH < 4.2 (mean percentage of 8.4) which were nearly all non-MS. (2) Organisms with a final pH < 4.4 constituted mean perc entages of 41.5 and 32.1 for the ARL and SRS samples, respectively. (3 ) The final pH distribution of strains in samples from coronal surface s showed a tendency relative to caries activity (group 3 vs. group 4) similar to that for groups 1 and 2. Our findings further support the c oncept that increased cariogenic conditions are associated with increa sed proportions of organisms capable of acidogenesis at a low pH and t hat this shift involves organisms other than the MS and lactobacilli.