Streptococcus mutans yidC1 and yidC2 impact cell-envelope biogenesis, biofilm matrix and biophysical properties

Abstract

Proper envelope biogenesis of Streptococcus mutans, a biofilm-forming and dental caries causing oral pathogen, requires two paralogs (yidC1 and yidC2) of the universally conserved YidC/Oxa1/Alb3 family of membrane integral chaperones and insertases. Deletion of either paralog attenuates virulence in vivo but the mechanisms of disruption remain unclear. Here, we determined whether deletion of yidC affects cell surface properties, extracellular glucan production, and/or the structural organization of EPS matrix and biophysical properties of S. mutans biofilm. Compared to wild type, the ΔyidC2 mutant lacked staining with vancomycin at the division septum, while the ΔyidC1 mutant resembled wild-type. Additionally, deletion of either yidC1 or yidC2 resulted in less insoluble glucan synthesis, but produced more soluble glucans, especially at early and mid-exponential growth phases. Alteration of glucan synthesis by both mutants yielded biofilms with less dry-weight and insoluble EPS. In particular, deletion of yidC2 resulted in significant reduction of biofilm biomass and pronounced defects in the spatial organization of the EPS matrix, thus modifying the 3D biofilm architecture. The defective biofilm harbored smaller bacterial clusters with high cell density and less surrounding EPS compared to wild type, which was stiffer in compression yet more susceptible to removal by shear. Together, our results indicate that elimination of either yidC paralog results in changes to the cell envelope and glucan production that ultimately disrupts biofilm development and EPS matrix structure-composition, thereby altering the physical properties of the biofilms and facilitating their removal. YidC proteins, therefore, represent potential therapeutic targets for cariogenic biofilm control.

Importance. YidC proteins are membrane localized chaperone insertases that are universally conserved in all bacteria, and are traditionally studied in the context of membrane protein insertion and assembly. Both YidC paralogs of the cariogenic pathogen Streptococcus mutans are required for proper envelope biogenesis and full virulence, indicating these proteins may also contribute to optimal biofilm formation in streptococci. Here we show that deletion of either yidC results in changes to the structure and physical properties of the EPS matrix produced by S. mutans, ultimately impairing optimal biofilm development, diminishing its mechanical stability, and facilitating its removal. Importantly, the universal conservation of bacterial yidC orthologs, combined with our findings, provide a rationale for YidC as a possible drug target for anti-biofilm therapies.

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ORCID for Paul Stoodley: orcid.org/0000-0001-6069-273X

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