The <i>Streptococcus mutans</i> collagen-binding protein Cnm enhances early biofilm formation with <i>Candida albicans</i>

ABSTRACT Streptococcus mutans strains expressing the collagen-binding protein Cnm are strongly associated with severe and recurrent dental caries, yet the mechanistic basis for their clinical enrichment alongside Candida albicans remains unclear. Here, we investigated whether Cnm contributes to early cross-kingdom interactions that promote dual-species biofilm development. Using purified proteins, we found that C. albicans binds robustly to Cnm at levels comparable to those of glucosyltransferase B (GtfB) and higher than those of other S. mutans adhesins tested. Preincubation of collagen with Cnm inhibited fungal binding, indicating that Cnm cannot simultaneously engage collagen and C. albicans , and suggesting competition for the same or adjacent binding regions. Cnm expression significantly enhanced coaggregation with C. albicans in both collagen-free and collagen-rich environments. In early attachment assays, Cnm increased S. mutans adherence to collagen-coated surfaces and promoted C. albicans recruitment on uncoated surfaces, consistent with ligand-dependent binding specificity. Pre-coaggregation in saliva further enhanced the ability of Cnm + S. mutans to attach to C. albicans . At 24 h, Cnm expression increased biomass and S. mutans CFUs exclusively on collagen-coated surfaces, regardless of sucrose availability. Together, these findings identify Cnm as a dual-binding adhesin that associates with either collagen or C. albicans, depending on environmental context, thereby accelerating coaggregation, early colonization, and biofilm maturation. This mechanism provides a biological explanation for the co-enrichment of Cbp + S. mutans and C. albicans seen in dental caries and highlights Cnm as a key mediator of cross-kingdom synergy in early biofilm formation. IMPORTANCE Dental caries is a multifactorial and polymicrobial disease in which the consumption of fermentable carbohydrates favors acidogenic and aciduric microorganisms at the expense of beneficial commensal bacteria, creating a dysbiotic environment. Streptococcus mutans and Candida albicans establish a synergistic relationship that exacerbates dysbiosis, thereby promoting caries development and progression. The current paradigm of this cross-kingdom synergism centers on increased extracellular polysaccharide production and enhanced biofilm biomass in the presence of sucrose. Here, we show that the collagen-binding protein Cnm, produced by approximately 20% of S. mutans isolates, promotes interspecies co-aggregation with C. albicans , facilitating interspecies attachment and early biofilm formation. Our findings expand the current paradigm by demonstrating that Cnm recruits C. albicans to the developing biofilm. This interaction may play a crucial role in the stability and virulence of early biofilm communities, particularly under low-sucrose conditions.