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

近年来，发酵的健康食品和饮料越来越受欢迎，因为益生菌有助于促进健康和多样化的肠道微生物区系。益生菌是活的微生物，当适量食用时，对宿主的健康有益。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，它们也是微生物群中的正常成员。然而，这些微生物在成人肠道中定植的能力很差，它们的影响是暂时的；只有当个人继续食用益生菌产品时，益处才明显。在加强这些有益肠道微生物的影响的背景下，很少有人关注乳酸菌产生和排泄的结构多样化的多糖胞外多糖(EPS)。EPS是一种高摩尔质量的多糖，要么松散地附着在细胞表面，要么被排泄到环境中。它们被分为两个不同的类别：均多糖和杂多糖。多糖是由单一类型的单糖重复单位形成的。相比之下，杂多糖是由两种或两种以上的单糖组成的。实验室提取的EPS大多属于杂多糖类。因为这些多糖为微生物区系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、多重的级联效应。类杆菌是微生物群中发现的主要多糖降解物，主要针对饮食中的多糖。他们利用多糖利用位点(PUL)共同调节的基因，在特定的多糖存在的情况下上调它们的感觉、捕获、降解和运输。类杆菌及其各自的PULs在膳食多糖分解方面已被广泛研究，例如这一门的成员如何降解膳食碳水化合物木聚糖和鼠李半乳糖醛酸酶II。然而，几乎没有生化数据来描述类杆菌是如何分解来自其他微生物的多糖的。EPS的结构不同于膳食多糖，为发现新的酶提供了潜力，具有潜在的生物技术和治疗意义。双歧杆菌和类杆菌之间的关系非常有趣，双歧杆菌是婴儿肠道的早期定殖者，类杆菌是在断奶前出现的次要定殖者。了解类杆菌是如何降解和利用EPS的，可能会为类杆菌进入婴儿肠道微生物区系和各种成人微生物区系的发展提供洞察，并为治疗干预提供一条途径。该项目将通过生长研究、微生物群落特征分析、重组蛋白质技术以及降解产物和酶活性的生化分析来评估类杆菌降解实验室EPS的能力。与英国和美国的ThermoFisher Science合作，使用最先进的质谱学方法将能够识别和表征复杂的碳水化合物。来自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.