Award for Research in Oral Science

¹ Forsyth Dental Center, Boston, Massachusetts

It is now recognized that certain specific streptococci have the ability to induce plaque formation and dental caries when introduced into the mouths of hamsters or gnotobiotic rats maintained on a sucrose-containing diet. Other closely related streptococci have not been observed to produce dental plaque or caries when tested under similar conditions.

Cariogenic streptococci isolated from both rodents and man have been found to synthesize large quantities of extracellular polysaccharides when provided with sucrose, whereas noncariogenic streptococci synthesize little or none. Cultures of cariogenic organisms grown in sucrose broth reflect polysaccharide synthesis for the organisms adhere to the walls of culture vessels forming gelatinous masses. This suggests that the production of these gelatinous polymers could enable cariogenic bacteria to form plaque.

The extracellular polysaccharide formed by most cariogenic streptococci is a dextran-like polymer for it is composed almost entirely of glucose. Solutions of the polysaccharide react with antidextran antiserum, and the polymer is hydrolyzed by preparations of dextranase. Dextran has been found to have a number of characteristics which are of possible importance for the initiation of dental plaque. First, it is able to adhere to untreated or saliva-coated powdered hydroxyapatite; secondly, it forms a precipitate with saliva; thirdly, it is relatively resistant to attack by plaque bacteria. Samples of pooled human plaque have been found to contain a constituent which is antigenically identical to dextran, indicating that at least part of the matrix of plaque is composed of a polysaccharide which appears identical to the extracellular dextran formed by cariogenic streptococci.

The production of these extracellular polysaccharides therefore seems to be at least partially responsible for the ability of bacteria to induce plaque. This hypothesis helps to explain the unusual specificity which cariogenic bacteria possess in the caries test systems currently in use.