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Scientists Explore Gene Variability in S. Mutans

June 11, 2009

S. mutans UA159 genomeScientists have long known that not all species of the oral bacterium Streptococcus mutans are created equal.  Each varies in its innate ability to metabolize carbohydrates, produce acid, and thus induce tooth decay.  This point was hammered home about two years ago when scientists in the UK compared the genomes - the complete set of genes within an organism - of nine distinct S. mutans strains originally isolated in Scandinavia, England, and the United States.  The researchers 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.  In fact, the scientists deduced that nearly one fifth of the protein-encoding open reading frames in the DNA of the S. mutans reference species UA159 are dispensable.  "This figure is very close to the size of the 'dispensable genome' in other species as diverse as Staphylococcus aureus, Helicobacter pylori, and Bordetella pertussis and streptococci . . ."  An open-reading frame helps to identify the individual genes that comprise a genome.

But this raised an important follow-up question.  If gene content varies among S. mutans species from throughout the world, what about those present in the same geographic location?  The implication being:  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?  In the June issue of the journal Oral Microbiology Immunology, a team of NIDCR grantees provides a preliminary answer from a study of S. mutans species present in the mouths of nine randomly selected children from Eastern Iowa.  The scientists found that each child had from two to nine confirmed strains of the bacterium in their mouths.  Interestingly, the seven children with little or no tooth decay had multiple S. mutans strains that were genetically identical, while the two children with a great deal of decay had isolates with unique genotypes.  The scientists 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.

  • Zhang L, Foxman B, Drake DR, Srinivasan U, Henderson J, Olson B, Marrs CF, Warren JJ, and Marazita ML. Comparative whole-genome analysis of Streptococcus mutans isolates within and among individuals of different caries status. Oral Microbial Immunol 2009: 24:197-203.   


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This page last updated: February 26, 2014