Post-translational modification by the Pgf glycosylation machinery modulatesStreptococcus mutansphysiology and virulence

Abstract

Streptococcus mutans is a keystone pathogen of dental caries, and the ability to form biofilms is essential for its pathogenicity. We identified a glycosylation machinery (Pgf) in S. mutans that post-translationally modifies two surface-associated adhesins, Cnm and WapA. The four pgf genes (pgfS, pgfM1, pgfE, and pgfM2) are part of the S. mutans core genome and we hypothesized that the scope of Pgf goes beyond Cnm and WapA. By inactivating each pgf gene individually or creating a quadruple pgf mutant in S. mutans OMZ175, we showed that the Pgf machinery is important for biofilm formation. Compared to OMZ175, differences in surface charge, membrane stability, and genetic competence were also observed for most mutants. Importantly, in silico analyses and tunicamycin MIC assays suggest a functional redundancy between the Pgf machinery and the rhamnose-glucose polysaccharide synthesis pathway. Using a rat oral colonization model, we showed a 10-fold reduction in recovered CFUs for the pgf quadruple mutant compared to OMZ175. Finally, using Cnm as a model, we showed by glycoproteomics analyses that Cnm is heavily modified with N-acetyl hexosamine in OMZ175 whereas phosphorylations were observed for the pgfS mutant. Our findings indicate that the Pgf machinery participates in important aspects of S. mutans pathobiology.