The tongue microbiome and nitric oxide bioavailability across the human lifespan

人类消化系统包含数万亿的微生物，统称为肠道微生物群。通过更好地理解肠道微生物群在维持身心健康方面的作用，已经取得了重要的发现。口腔微生物群落的不平衡和牙齿健康状况不佳与心血管健康受损有关，但口腔微生物群作为人类健康的潜在调节剂的作用尚未得到很好的理解。可能将口腔微生物群与健康联系起来的一种机制是硝酸盐-亚硝酸盐-一氧化氮（NO）还原途径。这是第一步，关键在于通过饮食摄入的硝酸盐被口腔中的细菌还原为亚硝酸盐。在许多其他生理作用中，NO调节血管内皮功能，并因此调节血压（BP）。在英国，至少25%的成年人和超过50%的60岁以上的人血压升高，这被认为是过早发病和死亡的重要原因。流行病学研究表明，更多地食用高硝酸盐食品（特别是绿色多叶蔬菜和一些水果，如“地中海”或“每日5份”饮食）可以预防不良心血管事件，并且已知膳食硝酸盐补充剂可显著降低年轻人和老年人的血压。个体之间在将硝酸盐还原为亚硝酸盐的能力以及对血压的影响方面的差异表明，高硝酸盐饮食的益处可能取决于个体是否具有“正确”的口腔微生物群。通过涉及一氧化氮合酶（NOS）酶的经典途径在体内产生NO的能力在老年人中受损，导致血压逐渐升高。我们认为，硝酸盐-亚硝酸盐-NO途径具有显著的潜力，以弥补功能失调的NOS在老年人。然而，关于人类宿主与口腔微生物群之间的共生关系如何随着衰老而变化，以及微生物群如何在衰老过程中维持心血管健康，人们知之甚少。宗旨和目标：我们的目的是调查是否存在特定的硝酸盐还原菌居住在舌头表面的影响，可能来自高硝酸盐饮食的消费，以及是否有年轻人和老年人之间的差异心血管的好处。这建立在我们的初步数据的基础上，这些数据表明年轻人和老年人之间口腔微生物组的差异，以及个体中特定口腔细菌的患病率与其将标准化饮食硝酸盐推注减少为亚硝酸盐的能力以及观察到的BP降低之间的相关性。这表明，通过操纵饮食和/或口腔微生物组来增强这种硝酸盐-亚硝酸盐-NO途径可能会补偿随着年龄的增长内源性产生NO的能力降低。目的：1）确定负责将人体口腔中的无机硝酸盐还原为亚硝酸盐的细菌种类。2）比较年轻人和老年人的口腔微生物组和硝酸盐还原能力。3）确定饮食中硝酸盐补充剂和抗菌漱口水的体内影响对口腔微生物组、NO生物利用度/生物活性和心血管健康指数的影响（血压、动脉僵硬度、血流介导的扩张）。4）评估硝酸盐、抗菌漱口水和甜味剂木糖醇对体外生物膜模型中口腔细菌的数量和相对丰度的影响。这项工作的结果将与工业赞助商杜邦营养与健康公司合作进行，最终可能导致益生元和/或益生菌干预措施的发展，有助于在整个人类寿命期间维持NO稳态和心血管健康。

Context: The human digestive system contains trillions of microbes collectively called the gut microbiota. Important discoveries have been made through better understanding the role the gut microbiota has in maintaining physical and mental health. Imbalances in the oral microbial community, and poor dental health, have been associated with impaired cardiovascular health, but the role of the oral microbiota as a potential modulator of human health is not well understood. One mechanism that may link the oral microbiota to good health is the nitrate-nitrite-nitric oxide (NO) reduction pathway. This, for its first step, crucially relies on the reduction of nitrate, which has been ingested through the diet, to nitrite by bacteria residing in the mouth. Among many other physiological roles, NO regulates vascular endothelial function, and therefore blood pressure (BP). In the UK, at least 25% of adults and more than 50% of people over 60 years old have elevated BP and this is considered a significant cause of premature morbidity and mortality. Epidemiological studies indicate that a greater consumption of high nitrate foodstuffs (specifically, green leafy vegetables and some fruits, such as in the 'Mediterranean' or '5 a day' diets) may protect against adverse cardiovascular events, and it is known that dietary nitrate supplementation significantly reduces BP in both young and older adults. Differences between individuals in the ability to reduce nitrate to nitrite and in the effects on BP indicate that the benefits derived from a high nitrate diet may be contingent on individuals having the 'right' oral microbiota.The ability to produce NO in the body through the classical pathway involving nitric oxide synthase (NOS) enzymes is impaired in older age, leading to a gradual increase in BP. We propose that the nitrate-nitrite-NO pathway has significant potential to compensate for dysfunctional NOS in older age. Little is known, however, about how the symbiotic relationship between the human host and the oral microbiota might change with ageing and how the microbiota could be harnessed to maintain cardiovascular health during ageing. Aims and Objectives: We aim to investigate whether the prevalence of specific nitrate-reducing bacteria residing on the surface of the tongue influences the cardiovascular benefits that may be derived from the consumption of a high nitrate diet and whether there are differences between young and older adults. This builds on our preliminary data which indicate differences in the oral microbiome between young and older adults as well as correlations between the prevalence of specific oral bacteria in an individual and their ability to reduce a standardised dietary nitrate bolus to nitrite and the observed reduction in BP. This suggests that augmentation of this nitrate-nitrite-NO pathway through manipulation of diet and/or the oral microbiome might compensate for a reduced ability to generate NO endogenously with older age. Objectives:1) Identify the bacterial species responsible for the reduction of inorganic nitrate to nitrite in the human oral cavity.2) Compare the oral microbiomes and nitrate reduction ability of young and older adults.3) Establish the in vivo effects of dietary nitrate supplementation and antibacterial mouthwash on the oral microbiome, NO bioavailability/bioactivity and indices of cardiovascular health (blood pressure, arterial stiffness, flow-mediated dilation).4) Assess the effects of nitrate, antibacterial mouthwash and the sweetening agent, xylitol, on the quantities and relative abundances of oral bacteria in an in vitro biofilm model.Potential Applications and Benefits: The results of this work, which will be undertaken in collaboration with industrial sponsor DuPont Nutrition & Health, may ultimately lead to the development of prebiotic and/or probiotic interventions that help to maintain NO homeostasis and cardiovascular health across the human lifespan.