Streptococcus mutans-derived extracellular vesicles promote herpes simplex virus infection in oral epithelia

The oral cavity contains the second most diverse bacterial community after the intestines, with bacteria and viruses coexist. Streptococcus mutans is a major pathogenic bacterium in the oral cavity, commonly associated with dental caries. We investigated the effects of S. mutans-derived extracellular vesicles (Sm EVs) on herpes simplex virus 1 (HSV-1) infection, which is prevalent in the oral cavity. We performed our experiments in human oral keratinocyte (HOK) cells and mucosal tissue-derived organoids, and analyzed human whole saliva (n = 50) for associations between S. mutans and HSV-1 envelope glycoprotein D (gD) mRNA levels by qPCR. Sm EVs significantly enhanced HSV-1 production in mucosal organoids. Indeed, mRNA and/or protein levels of type I (IFN-α and IFN-β), type II (IFN-γ), and type III (IFN-λ₁, IFN-λ₂, and IFN-λ₃) interferons were significantly lower in Sm EV-treated mucosal organoids compared with the vehicle control under mock-infection. When HSV-1 was introduced after Sm EV pretreatment, these IFN levels showed a general trend of statistically significant reduction compared with those in the vehicle control. Moreover, Sm EVs suppressed IFN mRNA and protein levels by upregulating the EGFR - ERK pathway in mucosal cells, creating an environment that enhances HSV-1 production. Interestingly, a positive correlation was noted between S. mutans and HSV-1 detected in human whole saliva samples. These results suggest that S. mutans can negatively modulate the host innate antiviral responses by secreting EVs, thereby enhancing viral production. This study might provide a new perspective for controlling viral infections in humans.