Post-translational modification by the Pgf glycosylation machinery modulates Streptococcus mutans physiology and virulence

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 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. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=164 SRC="FIGDIR/small/511621v1_ufig1.gif" ALT="Figure 1"> View larger version (53K): org.highwire.dtl.DTLVardef@146f37eorg.highwire.dtl.DTLVardef@11c0e45org.highwire.dtl.DTLVardef@ece1eaorg.highwire.dtl.DTLVardef@1e3f221_HPS_FORMAT_FIGEXP M_FIG C_FIG Abbreviated summaryIn this study, we demonstrate that the Pgf glycosylation machinery of Streptococcus mutans, a keystone pathogen of dental caries, regulates several aspects of bacterial pathophysiology that ultimately contribute to S. mutans fitness in oral colonization experiments. Using the heavily glycosylated Cnm adhesin as a model, we found that inactivation of the glycosyltransferase PgfS results in loss of Cnm glycosylation, but instead, Cnm became heavily phosphorylated, suggesting a crosstalk/competition between these two post-translational modification mechanisms.