The Ecological Role of Mutacin Production in Competition between Streptococcus mutans and Streptococcus sanguis

Objective: The objective of this study is to determine the ecological role of mutacin production in the competition between the cariogenic bacterium, Streptococcus mutans, and the noncariogenic bacterium Streptococcus sanguis. Methods: Insertional mutagenesis, in vitro competition assays. Results: The S. mutans strain UA140 produces two types of mutacins, the lantibiotic mutacin I and the nonlantibiotic mutacin IV. By antibiotic cassette insertion, we mutagenized each mutacin producing genes, thus creating four isogenic strains: UA140wt (I⁺IV⁺), UA140DC (I^-IV⁺), UA140Dnlm (I⁺IV^-), and UA140DM (I^-IV^-). In vitrocompetition studies between the S. mutans strains and the S. sanguis strain ATCC10556 in TH broth demonstrated 1) in mixed culture of UA140 and ATCC10556, the former became dominant at 10 h after inoculation and eliminated the latter by 14 h after inoculation; 2) the double mutant UA140DM became dominant at 14 h, but never eliminated ATCC10556; 3) UA140DC had a similar effect on ATCC10556 as the wt UA140, and 4) UA140Dnlm had a similar effect on ATCC10556 as UA140DM. In addition, the initial ratio of the two competing strains had a large impact on the final outcome. At the ratio of 10:1 of ATCC10556 to UA140, UA140 never became predominant, although not eliminated either. However, at the same ratio, UA140MD was eliminated from the mixed culture at 8 h after the inoculation. Conclusion: mutacin production plays a critical role in competition between S. mutans and S. sanguis in mixed cultures. Mutacin IV is more important than mutacin I in liquid culture competition, because mutacin I is not produced under liquid culture conditions. In addition, the initial population size of the competing strains also determined the final outcome; the higher the initial population, the easier it became predominant, or the harder it was eliminated from the mixed population. This study was supported by NIH grant R01 DE09082.