The molecular basis of bacteria adhesion to gastrointestinal mucus

人体内存在着大量的微生物，其中大多数存在于我们的肠道中，它们被统称为人类肠道微生物区系。微生物区系含有大约10-100万亿个细菌，分属15,000~36,000个物种，其中结肠的密度最高，达到1.5公斤。事实上，我们携带的微生物数量大约是我们自己细胞的10倍，它们总共拥有的基因大约是我们的100倍。肠道微生物区系是人类健康发展和维持所必需的。这些细菌有助于消化我们的食物，产生营养，为危险物质解毒，保护我们免受有害细菌(病原体)的伤害，并帮助我们的免疫系统发展。然而，微生物群并不是无害的，在损害我们限制微生物群从肠道进入的能力的条件下，细菌物种可以入侵人体导致疾病。此外，微生物区系组成的变化，即所谓的生物失调，与炎症性肠道疾病有关，也越来越多地与肠道外的一些疾病有关。目前还没有深入了解是什么触发了微生物区系的这些变化。然而，我们正在开始解开让大多数细菌在我们肠道内和平共处的机制。研究人员最近发现，覆盖在肠道细胞上的保护性粘液层在维持微生物区系方面起着至关重要的作用。粘液是在结肠中大量产生的，而我们的肠道细菌大多存在于结肠中。它的组织结构对其保护功能至关重要；它分为致密层和疏松层，前者阻止细菌渗透到我们的体内(从而保护我们免受可能的入侵)，后者为我们的肠道细菌提供一个家园(这样我们仍然可以从它们的保护活动中受益，而不会有相关的入侵风险)。这个系统是基于一种叫做粘蛋白的大型蛋白质的排列，这种蛋白质含有一系列非常复杂的糖。粘液还含有很大比例的抗体，这些抗体加强了我们肠道细菌对肠道的限制。据认为，粘蛋白中的糖类为细菌提供了一个附着场所，有助于维持正常的肠道功能。然而，这些假说仍有待检验。我们的小组最近发现，一些生活在肠道中的细菌在其表面具有粘液结合蛋白(MUB)，这有助于它们与粘液层结合。然而，我们不知道它们到底识别粘液中的什么，以及这可能如何影响健康。这项工作的一个重要方面将是确定MUB结合的结构和如何结合。糖的分析很复杂，因此多年来我们一直不知道它们在生物系统中的确切作用和重要性。最近的技术进步将帮助我们确定哪些粘蛋白糖参与了相互作用。互补的生化分析将进一步深入了解组成MUB的多个蛋白质单位所呈现的结合的特异性和强度。利用结晶学和诱变技术，我们还将确定与粘蛋白和抗体相互作用的准确氨基酸，这将有助于了解有害细菌或保护性细菌与肠道相互作用的方式的差异。我们将把这项体外工作扩展到肠道细胞模型，以研究从细菌中纯化的MUB与携带MUB的细菌在生物学相关系统中的相互作用。我们将确定与细菌与粘液黏附的抗体相关的后果，以及这可能如何改变肠道细胞对细菌的反应方式。这项工作的结果将有助于我们理解如何与肠道细菌保持有益的关系，并可能导致重新调整微生物群落或防止生态失调的新策略的开发。

The human body is colonized by a vast number of microbes, most of them are present in our gut where they are collectively referred to as the human gut microbiota. The microbiota contains approximately 10-100 trillion bacteria belonging to 15,000~36,000 species with the greatest density populating the colon where they reach 1.5 kg. In fact, the microbes that we carry around outnumber our own cells by about 10-fold and collectively they have about 100-fold more genes than we do. The gut microbiota is required for the development and maintenance of human health. These bacteria help digest our food, produce nutrients, detoxify dangerous substances, protect us from harmful bacteria (pathogens) and help with the development of our immune system. However, the microbiota is not innocuous, and under conditions that compromise our ability to limit the microbiota's entry from the intestine, bacteria species can invade the body to cause disease. Furthermore, shifts in the composition of the microbiota, referred to as dysbiosis, have been linked to inflammatory bowel diseases and are also increasingly associated to a number of diseases outside the gut. There is currently no deep understanding of what triggers these changes in the microbiota. However we are starting to unravel the mechanisms that allow the majority of the bacteria to live in peaceful coexistence within our gut. Researchers recently showed that the protective mucus layer covering cells lining the gut plays a crucial role in the maintenance of the microbiota. Mucus is produced in large amounts in the colon where most of our gut bacteria are present. Its organisation is crucial to its protective function; it is divided into a dense layer which prevents the bacteria to penetrate into our body (thus protects us against a possible invasion) and a loose layer above it which provides a home for our gut bacteria (so that we can still benefit from their protective activities without the associated risk of an invasion). This system is based on the arrangement of large proteins called mucins which contain a very complex array of sugars. Mucus also harbours a large proportion of antibodies which reinforce the confinement of our gut bacteria into the gut. It is thought that the sugars present in mucins provide an attachment site for the bacteria that help maintain normal gut function. However these hypotheses remain to be tested. Our Group recently showed that some of the bacteria that live in the gut have mucus-binding proteins (MUB) on their surfaces which help them bind to the mucus layer. However we do not know what exactly they recognise in the mucus and how this may influence health. An important aspect of this work will be to identify the structures MUB bind to and how. Sugars are complex to analyze therefore their precise role and importance in biological systems has eluded us for many years. Recent technological advances will help us identify which mucin sugars are involved in the interaction. Complementary biochemical analyses will provide further insights into the specificity and strength of binding presented by the multiple protein units constituting MUB. Using crystallography and mutagenesis we will also determine the precise amino acids involved in the interaction with mucins and antibodies, this will help understand differences in the way harmful or protective bacteria interact with the gut. We will expand this in vitro work to intestinal cell models to study the interaction of MUB purified from the bacteria and of bacteria harbouring MUB in a biologically relevant system. We will determine the consequences of the association with antibodies to the adhesion of bacteria to mucus and how this may change the way the intestinal cells respond to bacteria. Results from this work will help us understand how to keep a beneficial relationship with our gut bacteria and may lead to the development of novel strategies to readjust microbial community or prevent dysbiosis.