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J. Bacteriol. doi:10.1128/JB.01183-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Population Structure of Plasmid-Containing Strains of Streptococcus mutans, a Member of the Human Indigenous Biota

College of Dentistry, School of Medicine, Department of Biology, New York University, New York

^* To whom correspondence should be addressed. Email: pwc2{at}nyu.edu,

	Abstract

Streptococcus mutans, a member of the human indigenous biota and transmitted mostly mother to child, suggests that its phylogeny might parallel the evolutionary history of its human host. The relatedness and phylogeny of plasmid-containing strains of S. mutans was examined based on chromosomal DNA fingerprints (CDF), a hypervariable region (HVR) of a 5.6-kb plasmid, the ribosomal DNA intergenic spacer region (IGSR), serotypes, and the genotypes of mutacin I and II. Plasmid-containing strains were studied because their genetic diversity was twice as great as that of plasmid-free strains. The CDF of S. mutans from unrelated human hosts were unique, except those from Caucasians, which were essentially identical. The evolutionary history of IGSR, with or without the serotype and mutacin characters, clearly delineated an Asian clade. Also, a continuous association with mutacin II could be reconstructed through an evolutionary lineage with IGSR, but not for serotype e. DNA sequences from the HVR of the plasmid produced a well-resolved phylogeny that differed from the chromosomal phylogeny, indicating that the horizontal transfer of the plasmid may have occurred multiple times. The plasmid phylogeny was more congruent with serotype e than with mutacin II evolution, suggesting a possible functional correlation. Thus, the history of this three-tiered relationship between human, bacterium and plasmid, supported both coevolution and independent evolution.