Studies on the Microbiology of Early Enamel Demineralisation

Two theories exist regarding the microbial aetiology of dented caries, namely the Specific and Non Specific Plaque Hypotheses. Almost all human studies have involved either cross-sectional or longitudinal designs, with consequent limitations associated with both techniques. Therefore, an in situ model was designed to overcome some of these problems, and while most studies have examined only the relationship between plaque microflora and enamel demineralisation, in this study, many other factors thought to influence the development of caries were also investigated. These included diet, salivary counts of Strep. mutans and lactobacilli, salivary flow rate and buffering capacity, as well as previous caries experience. Preliminary studies examined the reproducibility of experimental methods, compared plaque microflora obtained from enamel specimens mounted on an intra-oral appliance to that from the natural dentition, and examined the susceptibility of enamel to demineralisation. Results showed little variation between repeat identifications from numerous sub-divisions of one original plaque sample. Further, no qualitative difference in microflora between plaques associated with natural and exogenously derived tooth surfaces was found. Since enamel susceptibility was found to be as variable within one tooth as between teeth, it was considered appropriate to employ sections from different teeth in subsequent studies. As the initial work showed that factors such as subject and sequence of experimental runs affected the results to some extent, the statistical package chosen for analysis of the effect of different treatment protocols and plaque microflora on enamel demineralisation, took these other variables into account. The in situ study was performed using seven volunteers, and attempted to determine the relationship between plaque microflora and enamel demineralisation, under normal conditions and with extra-oral sucrose stressing, both with and without inoculation of the subject's own Strep. mutans. Results showed that considerable inter-subject variation existed, in terms of plaque microflora and demineralisation. Sucrose caused no significant effect on plaque microflora composition but, overall, was associated with a slight increase in demineralisation, compared to unstressed plaque. Success of Strep. mutans implantation was very variable, as shown by proportional counts of this organism in plaque samples, although in all subjects, isolation frequency rose following implantation. Overall, the combination of implantation and sucrose application resulted in significantly greater demineralisation. In general, the isolation frequency of Strep. mutans was significantly higher in plaque associated with greater amounts of mineral loss, with mean and median proportions showing a similar trend. Lactobacillus spp. proportions were significantly higher in plaque associated with the greatest amount of demineralisation. Veillonella fell in mean proportion with increasing demineralisation, and no trend was seen, in this regard, in relation to Actinomyces. However, these results relate only to microbial counts at the end of the three week experimental period. Hence, a study on the microflora of developing plaque during this period was performed. As the total microbial count increases during the early stage of plaque growth, enamel slabs, from which absolute counts can be obtained, were used, as proportional counts alone can be misleading when the total count is varying. These early studies showed a change from a Streptococcus - to an Actinomyces - dominated plaque with time, and found no difference between sucrose and unstressed plaque with regard to proportional and final absolute bacterial counts, but demonstrated that the maximum bacterial mass was achieved more rapidly in sucrose plaque. The results of the studies in this thesis are in keeping with those of other workers, and suggest that Strep. mutans has a major role in initiation of demineralisation, while lactobacilli are associated with more extensive lesions, and may be important in the progression of established lesions. (Abstract shortened by ProQuest.)