Contributions of Candida albicans Dimorphism, Adhesive Interactions, and Extracellular Matrix to the Formation of Dual-Species Biofilms with Streptococcus gordonii.

Fungal and bacterial populations coexist in the oral cavity, frequently forming mixed-species biofilms that complicate treatment against polymicrobial infections. However, despite relevance to oral health, the bidirectional interactions between these microbial populations are poorly understood. In this study, we aimed to elucidate the mechanisms underlying the interactions between the fungal species and the bacterial species as they coexist in mixed-species biofilms. Specifically, the interactions of different mutant strains deficient in filamentation (Δ/Δ and Δ/Δ), adhesive interactions (Δ/Δ and Δ/Δ), and production of matrix exopolymeric substances (EPS) (Δ/ΔΔ/Δ, Δ/Δ, and Δ/Δ) were evaluated with under different conditions mimicking the environment in the oral cavity. Interestingly, our results revealed that growth of the biofilm-deficient and mutant strains in synthetic saliva or with restored their biofilm-forming ability. Moreover, challenging previous observations indicating an important role of morphogenetic conversions in the interactions between these two species, our results indicated a highly synergistic interaction between and the filamentation-deficient and deletion mutants, which was particularly noticeable when the mixed biofilms were grown in synthetic saliva. Importantly, dual-species biofilms were found to exhibit increase in antimicrobial resistance, indicating that components of the fungal exopolymeric material confer protection to streptococcal cells against antibacterial treatment. Collectively, these findings unravel a high degree of complexity in the interactions between and in mixed-species biofilms, which may impact homeostasis in the oral cavity. Microbial communities have a great impact in health and disease. interacts with multiple microorganisms in the oral cavity, frequently forming polymicrobial biofilms. We report on the synergistic interactions between and the Gram-positive bacterium , for which we have examined the different contributions of adhesive interactions, filamentation, and the extracellular matrix to the formation of dual-species biofilms. Our results demonstrate that growth in the presence of the bacterium can restore the biofilm-forming ability of different mutant strains with defects in adhesion and filamentation. The mixed-species biofilms also show high levels of resistance to antibacterial and antifungal antibiotics, and our results indicate that the fungal biofilm matrix protects bacterial cells within these mixed-species biofilms. Our observations add to a growing body of evidence indicating a high level of complexity in the reciprocal interactions and consortial behavior of fungal/bacterial biofilms.