The effect of nitrate supplementation on Porphyromonas gingivalis abundance in the oral microbial community

Alzheimer’s disease, the most common type of dementia, is characterized by a gradual decline in cognitive function. As a neurodegenerative disease, Alzheimer’s is distinguished by neuronal loss and atrophy in regions of the brain responsible for learning and memory. Despite many hypotheses to explain the potential causes of Alzheimer’s disease, its pathogenesis remains poorly understood. Although risk factors such as aging and poor cardiovascular health have been historically well-established in Alzheimer’s disease pathology, researchers are continually exploring new hypotheses to explain its onset and progression. Porphyromonas gingivalis, a gram-negative bacterium that is the main etiological agent in periodontal disease, has recently been attributed to Alzheimer’s disease pathogenesis. Researchers suggest that P. gingivalis can travel from the oral cavity into the brain, where it may contribute to the development of Alzheimer’s disease. As a result, treating P. gingivalis colonization in the oral cavity could potentially offer protection against Alzheimer’s disease. One proposed treatment is a dietary nitrate supplement, which may reduce the abundance of P. gingivalis in the oral cavity and improve overall vascular health. In this study, we administered a 10-day potassium-nitrate supplement of 4.84mmol nitrate per day to ten healthy participants and assessed changes in oral microbiome composition and vascular health before and after the intervention. Participants attended the laboratory twice:once before and once after the 10-day nitrate supplementation. At each lab visit, we took oral samples from saliva, hard tissue (floss), and soft tissue (tongue swab) to evaluate the oral microbiome composition. Additionally, we took measurements of endothelial function and blood pressure as markers of vascular health. We assessed the composition of the oral microbiome, specifically the relative abundance of Porphyromonas, using 16S rRNA gene sequencing. Blood pressure was measured using photoplethysmography in the finger and endothelial function was analyzed using a flow-mediated dilation test. Paired t-tests were used to analyze changes in the abundance of Porphyromonas, blood pressure,and flow-mediated dilation responses from pre to post nitrate supplementation. We hypothesized there would be a decrease in the relative abundance of Porphyromonas in the oral cavity, a decrease in blood pressure, and an improvement in FMD response post supplementation. We found that there were no clear changes in bacterial composition and no significant shifts in the abundance of Porphyromonas from pre to post nitrate supplementation from the different sample sites. However, we did find that the oral microbiome associated with each sample site was closely related among individuals, and that the oral microbiome composition remained relatively stable over time. Furthermore, we found the highest relative abundance of Porphyromonas in the subgingival sulcus (i.e., the floss sample type) as compared to other areas of the oral cavity. As for vascular health, there were no significant changes in blood pressure or flow-mediated dilation response. Although we did not detect significant changes in oral microbiome composition or markers of vascular health after a short-term nitrate supplementation, we did detect differences in the prevalence of Porphyromonas in the oral cavity. The methodology and results presented here lay a foundation for future work and have the potential to be developed into a larger scale study. If future studies see improvements post supplementation in both oral microbiome composition and vascular health, the novel use of a dietary nitrate supplement could be administered to people suffering from cognitive impairment, with the hope of potentially providing protection against Alzheimer’s disease.