Association Between Oral Microbiome Composition and Depression in Smokers and Nonsmokers

Background: While a substantial body of research has been dedicated to exploring the influence of alterations in the brain-gut-microbiome axis on conditions like depression and other mental disorders, there has been comparatively less focus on understanding how the oral microbiome affects brain function, particularly in response to smoking. Depression has been found to have a potential connection with oral dysbiosis, a condition that has shown associations with various disorders. Methods: We conducted an analysis of the oral saliva microbiome data (accession number: PRJEB44565) from a cohort of 44 adult participants to explore the link between oral microbiome composition and depression, with a particular focus on distinguishing between smokers and nonsmokers. Our study involved a comparison of salivary microbiome data between individuals with depression (n = 22) and a matched control group (n = 22). Within the depressed cohort, we further stratified participants into two subgroups: smokers (n = 11) and nonsmokers (n = 11). To analyze the microbiome data, we employed the SRA Toolkit for gut microbiome data retrieval and conducted taxonomic classification and diversity assessments using the EZBioCloud server. Results: We observed significant disparities in both the abundance and diversity of the oral microbiome when comparing smokers to nonsmokers. In the salivary microbiome of smokers who were part of the depressed cohort, the predominant phyla were Bacteroidetes (37.91%), Firmicutes (24.43%), Proteobacteria (22.70%), Fusobacteria (7.26%), and Actinobacteria (5.88%). Conversely, among nonsmokers, the dominant phyla were Bacteroidetes (34.57%), Proteobacteria (28.33%), Firmicutes (23.17%), Actinobacteria (6.93%), and Fusobacteria (5.79%). Moreover, we observed significant disparities in oral microbiota diversity between the two groups, with smokers exhibiting a lower level of diversity in their salivary microbiome. Conclusion: This study unveils a potential link between the oral microbiome's composition and the presence of depression, both in smokers and nonsmokers. Changes in the oral microbiota, particularly the prevalence of specific bacterial species, could be associated with the development of depression.