The bacteriology of caries

The role of bacteria in plaque formation and activity is highly significant in the development of demineralisation of the tooth surface, both enamel and dentine. Streptococci are the first bacterial species to adhere to teeth and assist in plaque formation. Other species progressively infiltrate the biofilm and after a few days of unimpeded growth, gram negative bacilli predominate. The most cariogenic organisms are adherent streptococci such as Streptococcus mutans, Strep. sobrinus and Lactobacillus. These organisms not only produce organic acids rapidly from refined carbohydrates, that is they are acidogenic, but they are also able to withstand highly acidic environments, that is they are aciduric. Strep. sobrinus is the most rapid acid producer, though it is usually present in much reduced numbers relative to Strep. mutans.

The lactobacillus in particular flourishes in acidic environments and is one of the predominant organisms in already carious dentine. That is to say it is unlikely to initiate a caries lesion but it will be present in significant numbers in an active dentine lesion. The presence of a high count of lactobacillus in a saliva test suggests that there is active disease present.

The polysaccharides secreted by Strep. mutans and other plaque bacteria provide adherence to the tooth surface via pellicle and will produce further carbohydrate for bacterial metabolism when dietary sources have been exhausted. Bacterial metabolism of high levels of refined carbohydrate in plaque can cause an immediate 2-4 point drop in pH at the tooth surface. The degree of fall depends on plaque thickness, the number and mix of plaque bacteria and the efficiency of salivary buffering. Recovery to normal resting pH takes from 20 minutes for the average patient to several hours for those with a high susceptibility to caries. A very high salivary flow rate may return the pH towards neutral quite rapidly, but local retention of sticky foods may delay the rise in pH until the food is completely dissolved or removed. Conversely, frequent intake of cariogenic food or drink will maintain the low pH levels over long periods of time. Carious demineralisation is proportional to the pH level and the duration of contact of low pH plaque with the tooth surface.