Oral Microbiota Taxa and Pri-miRNA Expression in Bipolar Disorder: A Case–Control Study

Background/Objectives: Emerging evidence suggests a role for oral microbiota in mood disorders, particularly bipolar disorder (BD), complementing established links between gut dysbiosis and psychiatric symptoms. This study investigates the composition of oral microbial taxa and the expression of inflammation-related pri-miRNAs (146a and 155) in individuals with BD, aiming to explore their potential as biomarkers in the oral–gut–brain axis. Methods: A matched case–control design was implemented, recruiting 25 BD patients and 46 controls matched by age and sex. Salivary samples were collected, and microbial profiling was conducted via real-time qPCR targeting major bacterial phyla and genera. Pri-miRNA 146a and 155 expression was evaluated through RT-qPCR using validated primers. Statistical comparisons between groups were performed using Fisher’s exact test and non-parametric tests for continuous variables. Results: Microbial analysis revealed significant reductions (p < 0.01) in α-Proteobacteria, γ-Proteobacteria, and Actinobacteria in BD patients versus controls. A shift toward a higher Firmicutes/Bacteroidetes ratio was observed in the BD cohort, suggesting differences in the oral biotic status between the two groups. However, pri-miRNA 146a and 155 expression levels did not differ significantly between the groups and exhibited high inter-individual variability. Conclusions: The findings indicate that oral microbiota composition differs in BD patients, potentially influencing systemic homeostasis through interactions with gut microbial communities and SCFA pathways. These findings should be interpreted as preliminary and hypothesis-generating given the modest sample size. While pri-miRNAs 146a and 155 did not distinguish BD status, the observed microbial taxa alterations should be regarded as exploratory and hypothesis-generating. Larger, longitudinal studies are required to clarify their potential role in BD pathogenesis and risk assessment.