Abstract
Oral dysbiosis, defined as an imbalance in the oral microbiota, has been linked to systemic diseases, including cardiovascular, neurodegenerative, and gastrointestinal diseases. Here, dysbiosis refers to a shift in the oral microbiota away from a typical or presumed healthy state. Previous studies have shown that e-cigarette and tobacco smoking are associated with oral dysbiosis. However, cross-population trends and functional pathways of how e-cigarette and tobacco smoking lead to oral dysbiosis have yet to be identified. Here, we investigated the effect of smoking status on the composition and the predictive functional potential of the salivary microbiota across cohorts from China and the U.S.A. using diversity metrics, core microbiome, differential abundance, indicator species, and predictive functional analyses. We found that e-cigarette use affected both the richness and evenness of the salivary microbiota diversity, while tobacco use affected only the richness in the China cohort. Meanwhile, in the U.S.A. cohort, tobacco use was associated with higher richness and evenness compared to e-cigarette use. Differential abundance revealed that both e-cigarette and tobacco smoking were associated with significant differences in the abundance of phyla in both cohorts, namely Actinomycetota, Bacillota, and Pseudomonadota. Through indicator taxa and predictive functional analyses, we found that the salivary microbiota composition of the China cohort was associated with pathogenic genera and a predicted enrichment of pathways related to biofilm formation, efflux pumps, and proteins that are directly linked to oral disease and inflammation. These findings highlight the potential impact of e-cigarette and tobacco smoking on oral microbiota and the complexity of these changes.