The global distribution of individual species of oral bacteria demonstrates their ability to survive among their human hosts. Such an ubiquitous existence is the result of efficient transmission of strains and their persistence in the oral environment. Genetic analysis has identified specific clones of pathogenic bacteria causing infection. Presumably, these express virulence-associated characteristics enhancing colonization and survival in their hosts. A similar situation may occur with the oral resident flora, where genetic variants may express specific phenotypic characteristics related to survival. Survival in the mouth is enhanced by dental plaque formation, where persistence is associated with the bacteria's capacity not only to adhere and grow, but also to withstand oxygen, wide fluctuations in pH and carbohydrate concentration, and a diverse array of microbial interactions. Streptococcus mutans has been discussed as a 'model' organism possessing the biochemical flexibility that permits it to persist and dominate the indigenous microflora under conditions of stress.

A total of 45 strains of black‐pigmented bacteroides, including Bacteroides melaninogenicus subsp. melaninogenicus, Bacteroides melaninogenicus subsp. intermedius and Bacteroides melaninogenicus subsp. asaccharolyticus, have been examined for morphological and physiological characteristics. They were also tested for the range of acidic metabolites, the typical basic amino acid of the mucopeptide, the base composition of DNA, the electrophoretic mobility of malate dehydrogenase and the susceptibility to certain antibiotics. The subspecies most commonly isolated from supragingival human dental plaque are B. melaninogenicus subsp. intermedius and B. melaninogenicus subsp. melaninogenicus. A list of tests for the differentiation of the three subspecies is given, but the separation of B. melaninogenicus subsp. asaccharolyticus from the other two subspecies of B. melaninogenicus is nevertheless recommended.

Summary. Three hundred and sixty-seven strains of pioneer streptococci isolated from the mouths of 40 healthy, full-term infants during the first month of life were examined by two taxonomic schemes that incorporated biochemical and physiological characteristics, IgAl protease production and glycosidase activities. Streptococcus mitis biovar 1 and S. oralis comprised 55-0 YO of the pioneer streptococci isolated from neonates. S. salivarius constituted 25.3% of the isolates, while S. anginosus, S. mitis biovar 2, S. sanguis and S. gordonii accounted collectively for 1 1.4 % . Difficulties in identifying streptococci were encountered and 8.4% of the 367 isolates could not be assigned to a recognised species.

The amounts and types of nutrients in the environment influence the development and final bacterial and chemical composition of biofilms. In oligotrophic environments, organisms respond to nutrient stress by alterations in their cell morphology and cell surfaces, which enhance adherence. Little is known of the responses to stress by bacteria in the animal oral cavity. The environment in the oral cavity is less extreme, and saliva provides a constant source of nutrients. Catabolic cooperation among oral bacteria allow carbon and nitrogen from salivary glycoproteins to be utilized. Modification of growth environments of oral bacteria can influence their cell surfaces and adhesion. Studies in experimental animals have shown that feeding either glucose or sucrose diets or fasting has little effect on the initial stages of development of oral biofilms. However, diet can influence the proportions of different bacterial species later in biofilm development. Studies of competition among populations in communities of oral bacteria in vitro and in vivo have shown the significance of carbon limitation and excess and changes in environmental pH. Relatively few studies have been made of the role of a nitrogen metabolism in bacterial competition in biofilms. In keeping with biofilms in nature, oral biofilms provide a sequestered habitat, where organisms are protected from removal by saliva and where interactions among cells generate a biofilm environment, distinct from that of saliva. Oral biofilms are an essential component in the etiologies of caries and periodontal disease, and understanding the biology of oral biofilms has aided and will continue to aid in the prevention and treatment of these diseases.

Studies on the microbiology of root surface caries between 1970 and 1975 placed emphasis on Gram-positive pleomorphic filamentous rods, particularly Actinomyces viscosus and Actinomyces naeslundii. Both of these species had been shown to produce root surface caries in experimental animals. Since this time, studies have placed more emphasis on Streptococcus mutans, and S. mutans and Lactobacillus are significant in prediction of root surface caries risk in patients. Subsequent studies confirmed an association between S. mutans and 'soft' or 'initial' root lesions. Thus, it is important when determining the microflora of root surface lesions to make careful characterization of the state of the lesion. A second important aspect of the analysis of bacterial communities associated with root surface caries is better definition of the organisms. Most studies have concentrated on 'target organisms' S. mutans, S. sanguis, A. viscosus, A. naeslundii, Lactobacillus, and Veillonella. However, it has been known for 17 years that the Actinomyces associated with the lesions may be variants of A. viscosus and A. naeslundii. Such strains (intermediate strains) have been described in taxonomic studies of Actinomyces, yet little is known of the differences in physiology of these strains or their relationship to root surface caries. A similar situation exists with oral Streptococcus where new taxonomic divisions are being proposed. Recognition of the potential diversity within the 'target' genera of root surface caries could yield valuable data. Recent studies suggest that this is so, since samples from root surface lesions which contain S. mutans and Lactobacillus show a high isolation of S. mitis 1 and no isolations of A. naeslundii. Careful definition of the lesions of root surface caries and the flora will allow analysis to relate a specific bacterial community to the state fo the lesion and assist in monitoring the control of the lesion through fluoride and antibacterials.