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Mutans Group

8.2.5.1 Streptococcus mutans and Streptococcus sobrinus

The two species Streptococcus mutans and Streptococcus sobrinus are found in the oral cavity, colonize exclusively the tooth surface of humans and some animals, and are responsible for dental caries. The genome of a serotype c strain of S. mutans has been sequenced and has a genome size of 2 Mbp and a G+C content of 36.8% [74]. Although considered an oral pathogen, it lacks virtually all virulence functions of other pathogenic streptococci. Its primary virulence mechanisms include glucosyltransferase enzymes capable of synthesizing adherent glucans, which bind to the smooth surfaces of teeth, glucan binding proteins, and its acido-genic potential as a lactic acid producer. Additionally, it has the ability to metabolize a wide variety of carbohydrates and has pathways for the synthesis of all of its required amino acids. Approximately 15% of the S. mutans genome is devoted to genes for various transport systems, most of which are devoted to the carbohydrates it uses. It also possesses genes for proteases, peptidases, and other exoen-zymes for action on various food substrates present in the oral cavity. Horizontal gene transfer occurs via transformation in S. mutans as it has a fully active competence system. However, no bacteriophages have been identified in its genome. The alignment of the genomes of S. mutans and S. mitis indicates a lesser degree of homology than with organisms in the same species.

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