Interactions between diet and oral bacteria in the regulation of cardiovascular and oral health

• Main Author: Burleigh, Mia Cousins
• Format: Dissertation
• Language: English
• Published: University of the West of Scotland 2020

Nitric oxide (NO) is a near ubiquitous signalling molecule which is important for cardiovascular health and many other physiological processes. A portion of the NO humans require can be generated endogenously from the nitric oxide synthase (NOS) enzymes which catalyse NO production from L-arginine. In addition to the NOS pathway, certain species of nitrate (NO₃⁻) and nitrite (NO₂⁻) reducing oral bacteria can convert NO₃⁻ to bioactive NO in a stepwise fashion in the NO₃⁻ - NO₂⁻ - NO pathway. Despite their importance, little is known regarding the effect of dietary interventions on the structure and function of bacterial communities important to NO₃⁻ reduction. Using metagenomic next generation sequencing, this series of research explored how NO₃⁻ reducing bacteria regulate systemic [NO₂⁻] as a substrate for NOS-independent NO generation and how the oral microbiome responds to different dietary interventions. The overarching aim of the PhD was to enhance our understanding of how NO₃⁻ reducing bacteria contribute to cardiovascular and oral health. A mix of males and females participated in in the cross-sectional study described in Chapter 3. The oral microbiome was sampled at baseline, a NO₃⁻ dose was then administered and NO metabolites and blood pressure (BP) were monitored for 3.5 h. The abundance of NO₃⁻ reducing bacteria was significantly correlated with the change in salivary NO₂⁻ (P = 0.03, r = 0.44, Fig. 3A) but not with any other variable (all P > 0.05). The data shows that the rate and magnitude of NO₂⁻ production in the saliva was greater in individuals with higher abundance of oral NO₃⁻ reducing bacteria. The next study (Chapter 4) examined the response of the oral microbiome and salivary pH to seven days of NO₃⁻ supplementation in young healthy males. Dietary supplementation with NO₃⁻ increased salivary pH and reduced the abundance of some bacteria previously identified as important to NO generation whilst increasing the abundance of others, Neisseria (from 2% ± 3%-9% ± 5%, P < 0.001), Prevotella (from 34% ± 17%-23% ± 11%, P = 0.001) and Actinomyces (from 1% ± 1%-0.5% ± 0.4%). These changes did not enhance the cardiovascular responsiveness to a NO₃⁻ dose in young healthy males. Chapter 5 shows that ingesting a probiotic designed to improve oral health increased plasma NO₂⁻ Δ50.24 ± 51.23 nM but did not alter blood pressure in young healthy males (P > 0.05). The increase in plasma NO₂⁻ was not related to abundance changes in NO₃⁻ reducing bacteria, and the mechanism remains unclear. The final study (Chapter 6) examined the effects of a single dose of NO₃⁻ on salivary pH in a group of trained male endurance runners who consumed carbohydrate at rest and after exercise. Compared to the negative-control (ingestion of water), salivary-pH was significantly reduced following the ingestion of carbohydrate in the positive-control and placebo trials (both P < 0.05). Salivary-pH was similar between the negative-control and NO₃⁻-trials before and after exercise despite ingestion of carbohydrate in the NO₃⁻-trial (both P≥0.221). These results showed that NO₃⁻ increases salivary pH in the hours following consumption and prevented the expected drop in salivary pH following carbohydrate supplementation and endurance exercise. The collective results of this thesis confirm previous findings that oral bacteria play an integral role in NO₃⁻ metabolism and are important for cardiovascular health. The data presented extends these findings by demonstrating for the first time that the structure of the bacterial community is related to the efficiency of NO₃⁻ metabolism and that probiotics designed to improve oral health increase plasma NO₂⁻. The data also suggests that NO₃⁻ alters the abundance of specific oral bacteria and mitigates the detrimental effects of oral acidity by increasing salivary pH.