Biofilm formation by Streptococcus gordonii: Genes expressed and mechanism of protection by MsrA

Biofilms are ubiquitous multi-species microbial communities. They are widely studied because of the serious problems they cause in human health, medical environments, and industrial facilities. Dental plaque is a complex biofilm formed on tooth surfaces, and contributes to oral diseases including caries and periodontal disease. Streptococcus gordonii is one of the early colonizers in dental plaque, and is studied in an in vitro model using saliva-coated hydroxyapatite (sHA) to simulate the tooth. To determine the genes that are uniquely expressed during biofilm formation on sHA, IVET, a technology originally developed to detect host-induced genes in vivo, is used. By use of IVET, S. gordonii biofilms growing on damaged heart valves of rabbit in vivo expressed the gene for methionine sulfoxide reductase (msrA). Then we focused on the functional mechanism of MsrA, which was hypothesized to contribute to the maintenance of adhesins in three non-oral pathogens. An msrA-defective mutant of S. gordonii was constructed and compared with the wild-type for anti-oxidant activity by a peroxide inhibition assay. To determine the cell morphological effects of the loss of protection against oxidation, wild-type and mutant strains were compared by Field Emission Scanning Electron Microscopy (FESEM). To assess functional consequences of the msrA mutation, the wild-type and mutant strains were compared for adhesion to sHA, biofilm formation on polystyrene surfaces, peroxide production, msrA-specific mRNA expression, and the possible target proteins of MsrA. To learn the location of MsrA functions in S. gordonii, msrA gene was fluorescence-labeled and observed by fluorescent microscopy. This project showed how msrA, among many genes that are regulated in S. gordonii biofilms, protects the adhesion and biofilm functions of the organism.