Streptococcus mutans transports and subsequently ferments various carbohydrates, including galactose, to form lactic acid, which initiates dental caries formation. Previous work involving large chromosomal deletions in the region of the multiple-sugar metabolism (msm) genes has implied that they may have a potential role in the transport of galactose in this organism. Objective: To determine if these genes are involved with galactose transport in S. mutans, a polar interposon insertion and defined gene interruptions of the two putative sugar-binding proteins located within the msm operon were generated using recombinant DNA techniques. Methods: Analysis of these mutants by growth on Purple Broth Base agar plates with galactose as the sole fermentable carbon source revealed that all mutants with in vitro generated mutations within the msm genes still retained the ability to utilize galactose. To determine if the rate of galactose utilization was affected, growth curves were done using the mutant containing the polar interposon insertion within the msm operon and the parental strain. Results: Analysis of the growth curves revealed that the msm mutant grew much slower than the parental strain in the presence of galactose. Thus, the msm operon does appear to transport galactose. Conclusion: However, because galactose fermentation still occurs in these msm mutants, the msm genes are not the only mechanism for galactose transport. The ability to transport specific carbohydrates by multiple mechanisms appears to be a common feature in S. mutans. At this time, this secondary mechanism is not known and studies are currently underway to identify additional genetic loci involved with galactose transport in Streptococcus mutans. This work was suported by funding from NIH NIDCR DE10890.

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