Mutation of<i>luxS</i>Affects Biofilm Formation in<i>Streptococcus mutans</i>

<jats:title>ABSTRACT</jats:title><jats:p>Quorum sensing is a bacterial mechanism for regulating gene expression in response to changes in population density. Many bacteria are capable of acyl-homoserine lactone-based or peptide-based intraspecies quorum sensing and<jats:italic>luxS</jats:italic>-dependent interspecies quorum sensing. While there is good evidence about the involvement of intraspecies quorum sensing in bacterial biofilm, little is known about the role of<jats:italic>luxS</jats:italic>in biofilm formation. In this study, we report for the first time that<jats:italic>luxS</jats:italic>-dependent quorum sensing is involved in biofilm formation of<jats:italic>Streptococcus mutans. S. mutans</jats:italic>is a major cariogenic bacterium in the multispecies bacterial biofilm commonly known as dental plaque. An ortholog of<jats:italic>luxS</jats:italic>for<jats:italic>S. mutans</jats:italic>was identified using the data available in the<jats:italic>S. mutans</jats:italic>genome project (<jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="url" xlink:href="http://www.genome.ou.edu/smutans.html">http://www.genome.ou.edu/smutans.html</jats:ext-link>). Using an assay developed for the detection of the LuxS-associated quorum sensing signal autoinducer 2 (AI-2), it was demonstrated that this ortholog was able to complement the<jats:italic>luxS</jats:italic>negative phenotype of<jats:italic>Escherichia coli</jats:italic>DH5α. It was also shown that AI-2 is indeed produced by<jats:italic>S. mutans</jats:italic>. AI-2 production is maximal during mid- to late-log growth in batch culture. Mutant strains devoid of the<jats:italic>luxS</jats:italic>gene were constructed and found to be defective in producing the AI-2 signal. There are also marked phenotypic differences between the wild type and the<jats:italic>luxS</jats:italic>mutants. Microscopic analysis of in vitro-grown biofilm structure revealed that the<jats:italic>luxS</jats:italic>mutant biofilms adopted a much more granular appearance, rather than the relatively smooth, confluent layer normally seen in the wild type. These results suggest that LuxS-dependent signal may play an important role in biofilm formation of<jats:italic>S. mutans</jats:italic>.</jats:p>