Mature dental plaque is composed of a highly complex community of microbes, with the population of microbes varying from person to person and between different sites within the mouth. Classical microbiological techniques have estimated that plaque contains 800 distinct oral species, with a healthy individual possessing 50 to 100 different species at any one time.10,14,15 However, a powerful new molecular technique tool called pyrosequencing, which analyses ribosomal RNA, has estimated at least 19,000 phylotypes (assuming a 6% difference in RNA sequence to constitute a new species).15 These populations of bacteria form their own microbial ecosystem in dental plaque. Just like any other ecosystem, the plaque microbial ecosystem can both influence its environment and be influenced by its environment, which in this case is the mouth.
Production of acid by the microbes in dental plaque as they ferment consumed sugars lowers plaque pH, which causes the localized environment to change. The lowering of plaque pH causes a corresponding shift in plaque ecology, in which acid-sensitive bacteria such as S. sanguinis are less able to survive, but aciduric bacteria such as S. mutans and Lactobacilli will thrive. The end result is disruption in the natural balance between dental plaque and the tooth surface, more acid production and increased demineralization.14,16,17 On the other hand, when pH remains neutral, acid-sensitive bacteria like S. sanguinis can survive, keeping acid production low and increasing remineralization.18 This concept of the oral environment being able to cause a shift in dental plaque ecology that can either lead to good oral health or disease, such as caries and gingivitis, is referred to as the “ecological plaque hypothesis.”10,14,17
What drives the shift in plaque ecology is not the presence of sugars per se, but rather the acid formed by their fermentation that can cause pH to drop from a neutral 7 to a pH of lower than 5.5. At a pH of 5.5, the plaque community remains stable, but as pH drops lower to 4.5, the numbers of S. mutans and Lactobacilli increase. When plaque pH drops below 4.5, this is considered an environmental catastrophe for plaque microflora, like S. sanguinis, that normally inhabit a healthy mouth. That is because these acid-sensitive species can be inhibited or killed, while acid-tolerant species proliferate (Figure 4).