J. Van Houte

Role of Micro-organisms in Caries Etiology

The microbial etiology of dental caries is discussed in terms of the dynamic relationship among the dental plaque microbiota, dietary carbohydrate, saliva, and the pH-lowering and cariogenic potential of dental plaque. The evidence supports a concept of caries as a dietary carbohydrate-modified bacterial infectious disease. Its key feature is a dietary carbohydrate-induced enrichment of the plaque microbiota with organisms such as the mutans streptococci and lactobacilli which causes an increase of plaques pH-lowering and cariogenic potential. The shift in the plaque proportions of these organisms appears to be related to their relatively high acid tolerance. A large body of evidence also supports a major effect of saliva on caries development. Integration of salivary effects with the concept of caries as a dietary carbohydrate-modified bacterial infectious disease suggests a broader concept whichincludes a major role of saliva in the regulation of the exposure of tooth surfaces to carbohydrate and of plaque acidity and, hence, the microbial composition and the pH-lowering and cariogenic potential of dental plaque. It is proposed that caries occurs preferentially in dentition sites characterized by a relatively high exposure to carbohydrate and diminished salivary effects. Some implications of this concept are discussed.The microbial etiology of dental caries is discussed in terms of the dynamic relationship among the dental plaque microbiota, dietary carbohydrate, saliva, and the pH-lowering and cariogenic potential of dental plaque. The evidence supports a concept of caries as a dietary carbohydrate-modified bacterial infectious disease. Its key feature is a dietary carbohydrate-induced enrichment of the plaque microbiota with organisms such as the mutans streptococci and lactobacilli which causes an increase of plaques pH-lowering and cariogenic potential. The shift in the plaque proportions of these organisms appears to be related to their relatively high acid tolerance.A large body of evidence also supports a major effect of saliva on caries development. Integration of salivary effects with the concept of caries as a dietary carbohydrate-modified bacterial infectious disease suggests a broader concept whichincludes a major role of saliva in the regulation of the exposure of tooth surfaces to carbohydrate and of pla...

The Association of Mutans Streptococci and Non-Mutans Streptococci Capable of Acidogenesis at a Low pH with Dental Caries on Enamel and Root Surfaces

Coronal dental plaque from each of 12 caries-positive subjects (Group I) was pooled from white spot tooth surface areas and, separately, from sound surface areas; sound surface areas in each of 18 caries-free subjects (Group II) were sampled similarly. Two samples, one consisting of material from a root-surface lesion and another of plaque from a sound root-surface area, were obtained from each of another 10 subjects (Group III). The samples from Groups I and II were evaluated for: (1) pH-lowering potential in vitro with dispersed plaque suspensions, excess glucose supply, and a 60-minute test; (2) the levels of mutans streptococci (MS) and lactobacilli; and (3) the distribution of the predominant non-mutans streptococci (non-MS) according to their final pH in glucose broth; only microbial analysis was done for the Group III samples. The levels of the MS were generally positively associated with caries. A weaker positive association was found for the levels of those non-MS capable of acidogenesis at low pH (final pH < 4.4). The latter generally far outnumbered the MS in all types of samples. The levels of lactobacilli were nearly always very low. The pH-lowering potential (final pH and pH drop rate) was higher for plaque from white spot areas than for plaque from sound surface areas (Group I). The samples from caries-free subjects (Group II), however, exhibited a pH-lowering potential which was not significantly different from that of both types of samples from the caries-active subjects (Group I). Collectively, the findings suggest that the role of non-MS capable of acidogenesis at low pH in caries development merits further study.

Food Starches and Dental Caries

Sucrose and starches are the predominant dietary carbohydrates in modern societies. While the causal relationship between sucrose and dental caries development is indisputable, the relationship between food starch and dental caries continues to be debated and is the topic of this review. The current view of dental caries etiology suggests that in-depth evaluation of the starch-caries relationship requires the consideration of several critical cariogenic determinants: (1) the intensity (i.e., the amount and frequency) of exposure of tooth surfaces to both sugars and starches, (2) the bioavailability of the starches, (3) the nature of the microbial flora of dental plaque, (4) the pH-lowering capacity of dental plaque, and (5) the flow rate of saliva. Studies of caries in animals, human plaque pH response, and enamel/dentin demineralization leave no doubt that processed food starches in modern human diets possess a significant cariogenic potential. However, the available studies with humans do not provide unequivocal data on their actual cariogenicity. In this regard, we found it helpful to distinguish between two types of situations. The first, exemplified by our forebears, people in developing countries, and special subject groups in more modern countries, is characterized by starch consumption in combination with a low sugar intake, an eating frequency which is essentially limited to two or three meals per day, and a low-to-negligible caries activity. The second, exemplified by people in the more modern societies, e.g., urban populations, is characterized by starch consumption in combination with significantly increased sugar consumption, an eating frequency of three or more times per day, and a significantly elevated caries activity. It is in the first situation that food starches do not appear to be particularly caries-inducive. However, their contribution to caries development in the second situation is uncertain and requires further clarification. Although food starches do not appear to be particularly caries inducive in the first situation, the possibility cannot be excluded that they contribute significantly to caries activity in modern human populations. The commonly used term dietary starch content is misleading, since it represents a large array of single manufactured and processed foods of widely varying composition and potential cariogenicity. Hence, increased focus on the cariogenicity of single starchy foods is warranted. Other aspects of starchy foods consumption, deserving greater attention, include the bioavailability of starches in processed foods, their retentive properties, also in relation to sugars present (starches as co-cariogens), their consumption frequency, the effect of hyposalivation on their cariogenicity, and their impact on root caries. The starch-caries issue is a very complex problem, and much remains uncertain. More focused studies are needed. At present, it appears premature to consider or promote food starches in modern diets as safe for teeth.

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 few of the acidogenic dental plaque bacteria. Therefore, the final pH in sugar broth was determined for a wide variety of oral bacteria. Their 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 surfaces of caries-active subjects, and (4) plaque from sound coronal surfaces of caries-free subjects. Strains from groups 1 and 2 (ARL, 389 strains; SRS, 358 strains) were previously identified (van Houte et al., 1994) to the genus/species level and belonged to the predominant cultivable 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 ≥ 4.6. The main findings were: (1) ARL samples contained many strains with a final pH < 4.2 (mean percentage of 25.7). They included all strains of Lactobacillus and mutans streptococci (MS), most Bifidobacterium strains and non-mutans streptococci (non-MS), and about 20% of the Actinomyces strains. By contrast, SRS samples contained far fewer strains 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 percentages of 41.5 and 32.1 for the ARL and SRS samples, respectively. (3) The final pH distribution of strains in samples from coronal surfaces 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 concept that increased cariogenic conditions are associated with increased proportions of organisms capable of acidogenesis at a low pH and that this shift involves organisms other than the MS and lactobacilli.

Oral Flora of Children with "Nursing Bottle Caries"

Plaque from carious and sound tooth surfaces, as well as saliva from children with nursing bottle caries, contained unusually high levels of Streptococcus mutans; the levels of Streptococcus sanguis in plaque on anterior teeth and Streptococcus salivarius in saliva were unusually low. Lactobacilli were detected in nearly all plaque samples. The findings are discussed in relation to the role of S. mutans and lactobacilli in the initiation of human dental caries and diet-plaque interactions.

The Predominant Cultivable Flora of Sound and Carious Human Root Surfaces

Little detailed knowledge exists about the composition of the human root flora. Therefore, the predominant cultivable flora (PCF) was determined for samples of: (1) plaque from sound root surfaces (SRS) in eight subjects without root caries, (2) plaque from incipient root lesions (IRL) in eight subjects with root caries, and (3) carious material from advanced root lesions (ARL) in nine other subjects with root caries. Generally, one root surface was sampled per subject, and organisms-358, 512, and 389 for SRS, IRL, and ARL, respectively-were identified by standard methods. It was found that: (1) streptococci, actinomyces, and veillonellae constituted 84.2, 57.8, and 65.7% of the PCF of SRS, IRL, and ARL samples, respectively; (2) a wide variety of other Gram-positive cocci and Gram-positive and -negative rods was also present; (3) the PCF of many samples was often dominated by few organisms, the identity of which differed from sample to sample; (4) a negative and no association with root caries existed for the PCF levels of the non-mutans streptococci (non-MS) and the actinomyces; and (5) mutans streptococci (MS) and lactobacilli (L) were not always present among the PCF of IRL and ARL, respectively, and non-MS were isolated from the PCF of most lesions and actinomyces from all lesions. Our findings illustrate the complexity of the root surface flora and suggest that root caries development involves organisms other than MS and L.

Relationship among Mutans Streptococci, "Low-pH" Bacteria, and lodophilic Polysaccharide-producing Bacteria in Dental Plaque and Early Enamel Caries in Humans

Multiple interactions occur among major determinants of dental caries. We have studied the bacterial flora and pH-lowering capacity of the same dental plaques in relation to caries. The findings on the plaque flora are reported here. The buccal surfaces of upper teeth in each subject were selected for study. A low-caries group had no white spot caries (ws) in the selected dentition area; a higher-caries group averaged 4.1 ws in this area. The latter group was divided into subjects with 2, 3, or 4 ws and subjects with 5, 6, or 7 ws. Enumerated organisms in plaque samples (sound and ws sites) from all subjects were: (1) mutans streptococci (MS) on mitis-salivarius-bacitracin and mitis-salivarius agar; (2) non-mutans streptococci (non-MS) on mitis-salivarius agar; (3) organisms that were categorized according to their minimum pH in sugar broth, i.e., the predominant undifferentiated total flora on blood agar or the predominant non-MS flora on mitis-salivarius agar; and (4) iodophilic polysaccharide-storing organisms on trypticaseyeast extract-salts agar. Plaques covering ws lesions contained generally only low proportions (< 0.1%) of MS. The plaque proportions of all the above 4 bacterial groups were increased in the higher-caries group but were similar for s and ws sites in this group. Over half of the total plaque flora in subjects with 5, 6, or 7 ws consisted of low-pH-type organisms (minimum pH < 4.4). Many of these were neither MS nor low-pH non-MS. The numerical emergence of MS in plaque appeared to be preceded often by other types of low-pH bacteria, including the non-MS. Caries development in the absence or presence of MS as well as different bacterial successions in plaque can be explained readily by the dynamic and positive relationship among the factors carbohydrate consumption, plaque flora composition, plaque acidogenic potential, and caries activity.

The pH of Dental Plaque in its Relation to Early Enamel Caries and Dental Plaque Flora in Humans

Dental caries appears to result from the action of multiple, interrelated factors. A companion study dealt with the plaque-flora/caries relationship (van Ruyven et al., 2000). The plaque-pH/caries relationship is the subject of this study. Since both studies involve the same subjects, plaques, and tooth surfaces, data on the examined factors have also been integrated. In vivo plaque pH determinations (microelectrode) were done on buccal sound (s) and white-spot (ws) caries surfaces in a selected dentition area in a low-caries (no ws) and higher-caries subject group. The pH response to sugar was evaluated before and after a sugar rinse, a local sugar application, or sucking on a sugary lozenge. pH profiles with sugar rinsing and normal or limited salivary flow conditions, showed progressively decreasing plaque pH values at various time points in the order of: low-caries subjects (s sites), higher-caries subjects (s sites), higher-caries subjects (s + ws sites), and higher-caries subjects (ws sites). The minimum pH values showed the same trend. Analyses of all data indicated only a statistical difference for minimum values for s sites in low-caries subjects vs. ws sites in higher-caries subjects, and for s and ws sites in the latter. Local sugar application and sucking on a sugary lozenge induced smaller pH drops than sugar rinsing; such suboptimal sugar exposure caused a disappearance of the difference between the minimum pH values for s and ws sites observed with sugar rinsing in the higher-caries subjects. Initial plaque pH values were similar regardless of subject or tooth caries status. The values were also not correlated with the plaque levels of strongly iodophilic polysaccharide-storing bacteria. Collectively, both studies indicate that increasing subject caries status is characterized by increasing plaque levels of highly-acid-tolerant, acidogenic bacteria and an increasing plaque-pHlowering potential and support the dynamic relationship between these parameters.

Mutans Streptococci and Non-mutans Streptococci Acidogenic at Low pH, and in vitro Acidogenic Potential of Dental Plaque in Two Different Areas of the Human Dentition

Samples of human dental plaque were obtained from sound tooth surfaces in the lower anterior and upper posterior areas of each of 11 subjects with various degrees of caries experience. Both types of plaque were compared for: (1) their pH-loweringpotential [pH at 10 and 60 min after sugar addition and the pH drop between 0 and 10 min (delta pH)] with an in vitro method involving dispersed plaque suspensions and excess glucose supply; (2) the proportions ofmutans streptococci; and (3) the distribution of the predominant non-mutans streptococci according to their final pH in glucose broth. Compared with plaque from the lower anterior area, plaque from the upper posterior area exhibited a significantly higher pH-lowering potential, i.e., a lower pH at 10 and 60 min and a greater delta pH and significantly higher levels of mutans streptococci. The final pH values for the non-mutans streptococci exhibited a wide range from about 4.4 to over 5.0. The proportions of such organisms designated as capable of acidogenesis at low pH (final pH < 4.6), whether expressed as a percentage of the total non-mutans streptococci or of the total plaque flora, were significantly increased in plaque from the upper posterior area. The proportions of non-mutans streptococci capable of acidogenesis at low pH in plaque from the upper posterior area were also significantly increased, with decreasing pH values at 10 and 60 min. The findings strengthen the link between the capacity of the plaque flora for acidogenesis at a low pH and its pH-loweringpotential as well as the role of bacterial acidogenesis at a low pH as an oral ecological determinant.

Association of the Microbial Flora of Dental Plaque and Saliva with Human Root-surface Caries

Dental plaque was obtained from one or two sound root surfaces of subjects with different degrees of root-surface caries experience. From subjects with root-surface caries, plaque samples were also obtained from either one incipient or one more advanced lesion. Proportions of the total flora were determined for total streptococci and different streptococcal species, total and different Actinomyces species, and lactobacilli. A sample of saliva was obtained from about one-third of the subjects for determination of the concentrations of mutans streptococci and lactobacilli. The main observations were: (1) Subjects without root-surface caries or restorations (group I), as compared with subjects with root-surface caries with or without restorations (group II), were characterized by having a lower prevalence and proportion of mutans streptococci and a higher prevalence and proportion of A. naeslundii in plaque on sound root surfaces; (2) subjects in group I also tended to have a lower salivary concentration of mutans streptococci and lactobacilli than subjects in group II; (3) dental plaque on sound surfaces in group II subjects contained a lower proportion of mutans streptococci than plaque associated with incipient or advanced lesions; and (4) the prevalence and proportion of lactobacilli in plaque associated with sound as well as carious root surfaces were very low. The data reinforce findings from other studies and indicate that, as for coronal caries, the plaque and saliva populations of mutans streptococci specifically are correlated positively with the presence of root-surface caries.