Enterococcus faecalis induces H₂O₂-mediated epithelial cell death and enhances Candida albicans virulence in oropharyngeal candidiasis.

In immunosuppressed humans with oropharyngeal candidiasis (OPC) and in mice with experimental OPC, Candida albicans infection is associated with a bacterial imbalance characterized by significantly reduced oral microbiome diversity and the expansion of enterococcal and streptococcal species, which may exacerbate oral mucosal pathology. In this study, we applied an unbiased genome-wide transcriptomic profiling approach to shed further mechanistic light on the role of indigenous enterococcal communities in mucosal infection in a mouse model of cancer chemotherapy-associated OPC. Transcriptomic profiling of tongue tissues revealed a wide-ranging, barrier-compromising molecular activity of resident enterococci that explains the previously observed attenuation of fungal mucosal invasion with antibiotic treatment in this mouse model. Mechanistically, we validated the pathogenic potential of resident bacteria by showing that enterococci isolated from mice with OPC produce hydrogen peroxide (H(2)O(2)) and induce oral epithelial cell death through apoptosis and necrosis in vitro. We also discovered that C. albicans increased enterococcal H(2)O(2) production. These findings uncover a novel mechanism of pathogenic synergy between C. albicans and Enterococcus faecalis, which may be responsible for increased epithelial barrier damage and mucosal invasion by C. albicans hyphae during cancer chemotherapy. IMPORTANCE: Chemotherapy-induced mucosal barrier injury and immune suppression increase susceptibility to oropharyngeal candidiasis (OPC), a debilitating fungal infection. Our study uncovers a previously unknown pathogenic interaction between Candida albicans and Enterococcus faecalis, by showing that indigenous enterococci produce H(2)O(2), which contributes to oral epithelial cell death during fungal infection. By integrating transcriptomics with functional assays, we demonstrate that enterococci compromise epithelial integrity independently of fungal burdens, highlighting the role of the bacterial microbiota in driving tissue damage. These findings emphasize the need to consider bacterial-fungal interactions in managing OPC and suggest that targeting the microbial crosstalk could be a promising adjunctive strategy in immunocompromised hosts.