MgrA Represses Biofilm Formation in Staphylococcus aureus

MgrA is a pleiotropic regulator that controls autolysis, virulence, and efflux pump activity in Staphylococcus aureus. We recently found that mgrA mutants of strains RN6390, SH1000, and MW2 also displayed enhanced biofilm formation compared with their respective parents. The biofilms formed by mgrA mutants of RN6390 and MW2 are independent of sigB and ica loci, two genetic elements that have been previously associated with biofilm formation in S. aureus. Biofilms formed by mgrA mutants are dependent on the expression of surface proteins mediated by the sortase gene srtA. Extracellular DNA was also a crucial component of the early biofilm of mgrA mutants. Genetic analysis indicated that biofilm formation in mgrA mutants is mediated in part by agr RNAIII, a genetic locus regulated by mgrA. Additionally, SarA is important to biofilm formation in mgrA mutants since the double sarA mgrA mutants failed to form biofilms compared to single mgrA mutants of RN6390 and MW2. However, the SarA-mediated effect is independent of agr and proteases such as V8 protease and aureolysin. Collectively, our data showed MgrA to be a repressor of biofilm formation, and biofilms formed by mgrA mutants have features that are distinct from other reported biofilm types in S. aureus.