All species of the oral bacterium Streptococcus mutans vary in their innate ability to metabolize carbohydrates, produce acid, and thus induce tooth decay. About 2 years ago, scientists in the United Kingdom compared the genomes of 9 distinct S mutans strains originally isolated in Scandinavia, England, and the United States. They discovered tremendous heterogeneity in the gene content, supporting the idea that each species contains not only a “core” S mutans genome but also a “dispensable” genome consisting of genes acquired from other sources and which are not essential for the bacterium’s survival. They deduced that nearly one fifth of the protein-encoding open reading frames in the DNA of the S mutans reference species UA159 are dispensable. If gene content varies among S mutans species throughout the world, what about the gene content present in the same geographic location? Could the type of S mutans that colonizes a person’s mouth potentially contribute to the variability that dentists see in each patient’s susceptibility to tooth decay? Recently, a team of National Institute of Dental and Craniofacial Research (NIDCR) grantees provides a preliminary answer from a study of S mutans species present in the mouths of 9 randomly selected children from eastern Iowa. The children had from 2 to 9 confirmed strains of S mutans: 7 had little or no tooth decay and had multiple S mutans strains that were genetically identical, while 2 with a great deal of decay had isolates with unique genotypes. The team then assembled a total of 44 S mutans species isolated from the children and compared them one by one against the standard UA159 reference sequence. They found 323 of the open-reading frames present in the UA159 were absent in one or more of the test strains. “Overall, gene content diversity among S mutans has a fluid and rapidly evolving genome structure that is likely to be similar to that found in other Streptococci,” the authors noted.
(Source: NIDCR, Science News in Brief, June 11, 2009)
