SMU.940 regulates dextran-dependent aggregation and biofilm formation in Streptococcus mutans

Summary

The oral bacterium Streptococcus mutans is the principal agent in the development of dental caries. Biofilm formation by S. mutans requires bacterial attachment, aggregation and glucan formation on the tooth surface under sucrose supplementation conditions. Our previous microarray analysis of clinical strains identified 74 genes in S. mutans that were related to biofilm morphology; however, the roles of almost all these genes in biofilm formation are poorly understood. We investigated the effects of 21 genes randomly selected from our previous study regarding S. mutans biofilm formation, regulation by the complement pathway, and responses to competence-stimulating peptide (CSP). Eight CSP-dependent genes were identified, and their roles in biofilm formation and aggregation were examined by mutational analyses of the S. mutans UA159 strain. Of these 8 genes, the inactivation of the putative hemolysin III family SMU.940 gene of S. mutans UA159 promoted rapid dextran-dependent aggregation and biofilm formation in tryptic soy broth without dextrose (TSB) with 0.25% glucose and slightly reduced biofilm formation in TSB with 0.25% sucrose. SMU.940 mutant showed higher expression of GbpC and gbpC gene than wild type. GbpC is known to be involved in the dextran-dependent aggregation of S. mutans. An SMU.940-gbpC double mutant strain was constructed in the SMU.940 mutant background. The gbpC mutation completely abolished the dextran-dependent aggregation of the SMU.940 mutant. In addition, the aggregation of the mutant was abrogated by dextranase. These findings suggest that SMU.940 controls GbpC expression, and contributes to the regulation of dextran-dependent aggregation and biofilm formation.

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