Mutation of luxSAffects Biofilm Formation in Streptococcus mutans

ABSTRACTQuorum 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 luxS-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 luxSin biofilm formation. In this study, we report for the first time that luxS-dependent quorum sensing is involved in biofilm formation of Streptococcus mutans. S. mutansis a major cariogenic bacterium in the multispecies bacterial biofilm commonly known as dental plaque. An ortholog of luxSfor S. mutanswas identified using the data available in the S. mutansgenome project (http://www.genome.ou.edu/smutans.html). 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 luxSnegative phenotype of Escherichia coliDH5α. It was also shown that AI-2 is indeed produced by S. mutans. AI-2 production is maximal during mid- to late-log growth in batch culture. Mutant strains devoid of the luxSgene 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 luxSmutants. Microscopic analysis of in vitro-grown biofilm structure revealed that the luxSmutant 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 S. mutans.