Mucin O -glycans suppress quorum-sensing pathways and genetic transformation in Streptococcus mutans

Mucus barriers accommodate trillions of microorganisms throughout the human body while preventing pathogenic colonization 1 . In the oral cavity, saliva containing the mucins MUC5B and MUC7 forms a pellicle that coats the soft tissue and teeth to prevent infection by oral pathogens, such as Streptococcus mutans 2 . Salivary mucin can interact directly with microorganisms through selective agglutinin activity and bacterial binding 2 – 4 , but the extent and basis of the protective functions of saliva are not well understood. Here, using an ex vivo saliva model, we identify that MUC5B is an inhibitor of microbial virulence. Specifically, we find that natively purified MUC5B downregulates the expression of quorum-sensing pathways activated by the competence stimulating peptide and the sigX -inducing peptide 5 . Furthermore, MUC5B prevents the acquisition of antimicrobial resistance through natural genetic transformation, a process that is activated through quorum sensing. Our data reveal that the effect of MUC5B is mediated by its associated O -linked glycans, which are potent suppressors of quorum sensing and genetic transformation, even when removed from the mucin backbone. Together, these results present mucin O -glycans as a host strategy for domesticating potentially pathogenic microorganisms without killing them.