Oral Streptococcus and Vitamin B12 Deficiency in E-Cigarette Users: A Microbial–Nutrient–Neurotransmitter Axis Linking to Mood Disturbances

Background: With the rising popularity of e-cigarettes, questions have emerged regarding their broader influence on oral ecology, nutrient absorption, and psychological health. Disruptions in the oral microbiome may alter vitamin B12 dynamics. This, in turn, may indirectly influence serotonin biosynthesis and mood regulation. Methods: An observational cross-sectional study involved 150 young adults segregated into groups of 50 each as vapers, smokers, or non-users. Streptococcus sanguinis and vitamin B12 metabolism-related variable statuses methylmalonic acid and holotranscobalamin were quantified in samples, along with inflammatory markers interleukin-6, kynurenine/tryptophan ratio, and serotonin. Statistical techniques, including ANOVA and the Kruskal-Walli’s test, correlation, and serial mediation strategies, were employed. Results: Vapers had significantly higher S. sanguinis counts, lower abundances of vitamin B12 and HoloTC, higher MMA levels, reduced serotonin, higher levels of IL-6, and Kyn/Trp ratios compared to other groups; p < 0.05 for all. About 36% of vapers had biochemical B12 deficiency, while approximately 30 % had moderate-to-severe levels of depressive symptoms. A significant serial mediation pathway involving S. sanguinis affecting B12 and serotonin, which in turn influenced depression. These findings indicate an interconnected microbial-nutrient-neurotransmitter mechanism. Conclusion: The findings propose a biologically plausible connection between vaping-induced oral dysbiosis, vitamin B12 depletion, and altered serotonergic activity. This integrated microbial–nutrient–neurochemical framework underscores that vaping extends beyond nicotine exposure, influencing metabolic and psychological dimensions that merit further longitudinal investigation.