<i>Streptococcus mutans yidC1</i> and <i>yidC2</i> Impact Cell Envelope Biogenesis, the Biofilm Matrix, and Biofilm Biophysical Properties

Abstract

Proper envelope biogenesis of <i>Streptococcus mutans</i>, a biofilm-forming and dental caries-causing oral pathogen, requires two paralogs (<i>yidC1</i> and <i>yidC2</i>) of the universally conserved YidC/Oxa1/Alb3 family of membrane integral chaperones and insertases. The deletion of either paralog attenuates virulence <i>in vivo</i>, but the mechanisms of disruption remain unclear. Here, we determined whether the deletion of <i>yidC</i> affects cell surface properties, extracellular glucan production, and/or the structural organization of the exopolysaccharide (EPS) matrix and biophysical properties of <i>S. mutans</i> biofilm. Compared to the wild type, the Δ<i>yidC</i>2 mutant lacked staining with fluorescent vancomycin at the division septum, while the Δ<i>yidC1</i> mutant resembled the wild type. Additionally, the deletion of either <i>yidC1</i> or <i>yidC2</i> 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, the deletion of <i>yidC2</i> resulted in a significant reduction in biofilm biomass and pronounced defects in the spatial organization of the EPS matrix, thus modifying the three-dimensional (3D) biofilm architecture. The defective biofilm harbored smaller bacterial clusters with high cell density and less surrounding EPS than those of the wild type, which was stiffer in compression yet more susceptible to removal by shear. Together, our results indicate that the elimination of either <i>yidC</i> 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.<b>IMPORTANCE</b> 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 <i>Streptococcus mutans</i> are required for proper envelope biogenesis and full virulence, indicating that these proteins may also contribute to optimal biofilm formation in streptococci. Here, we show that the deletion of either <i>yidC</i> results in changes to the structure and physical properties of the EPS matrix produced by <i>S. mutans</i>, ultimately impairing optimal biofilm development, diminishing its mechanical stability, and facilitating its removal. Importantly, the universal conservation of bacterial <i>yidC</i> orthologs, combined with our findings, provide a rationale for YidC as a possible drug target for antibiofilm therapies.