The Role of Bacterial Exopolysaccharides in the Maintenance of Healthy Human Gut Bacteria

近年来，基于益生菌对促进健康和多样化肠道微生物群的益处，发酵健康食品和饮料越来越受欢迎。益生菌是活的微生物，当以足够的量食用时，赋予宿主健康益处。在这些保健品中发现的微生物是乳酸菌（LAB），如乳杆菌和双歧杆菌，它们也是微生物群的正常成员。然而，这些微生物在成人肠道定植的能力很差，它们的作用是短暂的;只有当个体继续食用益生菌产品时，益处才会明显。在增强这些有益肠道微生物的影响的背景下，很少关注乳酸菌产生和分泌的结构多样的聚糖胞外多糖（EPS）。EPS是高摩尔质量的多糖，其松散地附着于细胞表面或分泌到环境中。它们被分为两个不同的组;同多糖和杂多糖。同多糖由单一类型的单糖重复单元形成。相比之下，杂多糖由两种或多种类型的单糖组成。大多数LAB衍生的EPS属于杂多糖组。因为这些聚糖为微生物群的其他成员提供了潜在的碳源。因此，它们对肠道微生物种群具有复杂的多重级联效应。拟杆菌门是微生物组中发现的主要多糖降解菌，主要针对膳食多糖。它们采用多糖利用位点（PUL）共调节基因，所述共调节基因在特定聚糖的存在下上调，用于其感测、捕获、降解和转运。拟杆菌和它们各自的普尔斯在膳食多糖分解方面已经被广泛研究，例如膳食碳水化合物木聚糖和鼠李糖半乳糖醛酸聚糖II如何被该门的成员降解。然而，几乎没有生物化学数据来描述拟杆菌如何分解来自其他微生物的多糖。EPS的结构与膳食聚糖不同，为发现具有潜在生物技术和治疗相关性的新型酶提供了潜力。婴儿肠道的早期定居者双歧杆菌和断奶前出现的次级定居者拟杆菌之间的关系具有重要意义。了解拟杆菌如何降解和利用EPS可以提供关于拟杆菌进入婴儿肠道微生物群和不同成人微生物群发展的见解，并提供治疗干预的途径。该项目将利用生长研究、微生物群落分析、重组蛋白技术以及降解产物和酶活性的生化分析来评估拟杆菌降解LAB EPS的能力。通过与英国和美国的赛默飞世尔科技公司合作，利用最先进的质谱分析方法将能够对复杂碳水化合物进行鉴定和表征。该项目的信息将有助于阐明人类肠道复杂环境中碳水化合物介导的关系。

Fermented health foods and beverages have gained popularity in recent years based on the benefits of probiotics for encouraging a healthy and diverse gut microbiota. Probiotics are live microorganisms that, when consumed in adequate amounts, confer a health benefit to the host. The microbes found in these health products are lactic acid bacteria (LAB) such as Lactobacillus and Bifidobacterium, which are also normal members of the microbiota. However, the ability of these organisms to colonise the adult gut is poor and the effects they have are transient; benefits are only apparent if the individual continues to consume the probiotic product. Within the context of enhancing the impact of these beneficial gut microbes, little attention has been given to the structurally diverse glycan exopolysaccharides (EPS) produced and excreted by lactic acid bacteria. EPS are high molar mass polysaccharides that are either loosely attached to the cell surface or excreted into the environment. They are classed into two distinct groups; homopolysaccharides and heteropolysaccharides. Homopolysaccharides are formed of a single type of monosaccharide repeating unit. By contrast, heteropolysaccharides are made up of two or more types of monosaccharide. Mostly LAB derived EPS belong to the heteropolysaccharide group. Since these glycans provide a potential carbon source for other members of the microbiota. Therefore, they have complex, multiple cascading effects on gut microbial populations. The Bacteroides phyla are the main polysaccharide degraders found in the microbiome, targeting mainly dietary polysaccharides. They employ polysaccharide utilisation loci (PUL) coregulated genes that upregulate in the presence of a particular glycan for their sense, capture, degradation and transport. Bacteroides and their respective PULs have been extensively studied in terms of dietary polysaccharide breakdown, such as how dietary carbohydrates xylan and rhamnogalacturonan II are degraded by members of this phyla. However, there is little biochemical data to describe how Bacteroides breakdown polysaccharides derived from other microbes. Structures of EPS are distinct from dietary glycans, offering the potential for discovery of novel enzymes, with potential biotechnological and therapeutic relevance. The relationship between the Bifidobacterium, the early colonisers of the infant gut, and the Bacteroidetes, secondary colonisers that appear before weening, is of significant interest. To understand how Bacteroides degrade and utilise EPS may provide insights into the arrival of Bacteroides into the infant gut microbiota and the development of the diverse adult microbiota as well as provide a route for therapeutic intervention. The project will assess the ability of Bacteroides to degrade LAB EPS using growth studies, microbial community profiling, recombinant protein technology and biochemical analysis of degradation products and enzyme activities. In collaboration with ThermoFisher Scientific in the UK and US, access to state of the art mass spectrometry methods will allow for the identification and characterisation of the complex carbohydrates. The information from this project will help to elucidate the carbohydrate-mediated relationships in the complex environment of the human gut.