Hypertensive pregnancy disorders (HPDs) are associated with an increased risk of adverse pregnancy outcomes. Pregnancies complicated by hypertension are increasingly prevalent, with these women less likely to undergo successful cardiovascular adaptation during pregnancy which is critical in fetoplacental development. Nitrate, found in green leafy vegetables, has emerged as a possible intervention for cardiovascular complications. Reduction of dietary nitrate to nitrite, in particular by commensal oral bacteria in sessile biofilms on the dorsal surface of the tongue, can increase nitric oxide (NO) within the vasculature, reducing systemic blood pressure (BP). In this project, it was hypothesised that there would be altered oral bacterial profiles in women with hypertension during pregnancy, with hypertensive women displaying reduced levels of known high nitrate reducing oral bacteria. These microbial differences would explain variability in the efficacy of dietary nitrate supplementation, namely in its effectiveness to reduce BP. To understand whether oral nitrate metabolism plays a role in the regulation of BP in pregnancy and whether targeting this pathway may have therapeutic potential, this study aimed to 1) investigate nitrate reductase (NaR) activity and microbiota composition in normotensive versus hypertensive women and 2) determine using an in vitro biofilm model whether nitrate supplementation can alter the oral microbiota-derived microcosms to favour nitrate-reducing species. Plasma and salivary samples were collected in fasted pregnant and non-pregnant participants, with or without hypertension, with participants provided with a single dietary nitrate challenge in the form of a beetroot juice. Biochemical and physiological measurements were analysed pre- and post- the nitrate challenge. Oral NaR activity and salivary nitrite were determined using the Griess reaction and the composition of the oral microbiota analysed with 16S rRNA gene sequencing. Hypertensive individuals demonstrated significantly lower levels of salivary nitrite at baseline and a trend towards a reduction in salivary NaR activity. Abundances of the nitrate-reducing taxon Veillonella were significantly lower in hypertensive participants. There were varied BP responses to the acute nitrate challenge in both normotensive and hypertensive participants. There appears to be evidence of altered nitrate reduction in hypertensive individuals, but this does not appear to contribute towards an acute response to a single nitrate challenge. Representative oral biofilm microcosms were established in a continuous culture flow-through system and challenged with nitrate acutely. Biofilm communities were analysed in their microbial structure using selective agar plating and 16S rRNA gene sequencing, in addition to NaR assays. In vitro, an acute challenge with 15mM inorganic nitrate increased NaR activity 2-fold, combined with elevated nitrite. Increased abundances of both Veillonella and Neisseria were noted in the nitrate treated microcosms. This data suggests that nitrate challenges can shift community structure to favour high-nitrate reducing bacterial taxa accompanied with increased NaR activity in vitro. This data indicates an association between BP status and oral NaR activity, confirming previous findings in non-pregnant adults. In vitro modelling data demonstrates the feasibility of modelling oral microcosm responses to nitrate and suggest that NaR activity can be modulated with an acute inorganic nitrate challenge. Modulation of the nitrate-nitrite-NO pathway may be a feasible intervention in the future for the management of hypertensive pregnancies. The data generated from this project will inform the design of future interventional trials which would use nitrate supplementation and/or probiotic supplementation to improve pregnancy outcomes in HPDs. Ultimately, these studies would aim to reduce the incidence and severity of HPDs, to benefit both mother an
Investigating Nitrate Metabolism and the Oral Microbiome in Hypertensive Disorders of Pregnancy
Willmott, T. (Author). 31 Dec 2022
Student thesis: Phd