Genetic diversity within Streptococcus mutans evident from chromosomal DNA restriction fragment polymorphisms

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Abstract
Attempts to study the acquisition, transmission, and other aspects of the natural history of Streptococcus mutans infections in humans have been hampered by limitations and inconsistencies in methods by which phenotypic characteristics of individual isolates are examined. Because most mutans streptococci associated with human dental caries fall within the biotype I (serotypes c and f) grouping, designated S. mutans, these typing methods are of little value in distinguishing individual isolates. Here we show that strains of S. mutans obtained from over 30 individuals demonstrate unique "fingerprints" of chromosomal DNA digested with restriction endonuclease HaeIII. To demonstrate that this polymorphism in restriction fragments can be used to study the acquisition and transmission of this organism, we examined isolates of S. mutans from three mother-infant pairs obtained at the time the infant first became colonized by this organism. Results indicate that strains of S. mutans found in infants exhibit restriction fragment profiles identical to those of their mothers, strongly supporting the notion that mothers transmit this organism to their infants. Also, we show that strains of S. mutans with the same restriction fragment profile were stably maintained over a 3-year interval in the one mother-infant pair studied. Moreover, we found that mothers and their infants harbored only a few individual strains, suggesting that transmission of this organism is probably confined within discrete family cohorts. Collectively, these findings demonstrate the potential utility of genomic fingerprinting in studying the natural history of S. mutans infections in humans.