It has been recognised for many years that the indigenous microbiota has an important role to play in maintaining the health of an individual by preventing colonisation by exogenous pathogens. However, this “barrier” function is far from being the only contribution that the microbiota makes to the well–being of its host (Table 9.1). It is well established that resident microbes contribute significantly to human development and nutrition and are an effective means of neutralising potentially toxic dietary components. Most (approximately 1014) of the microbial inhabitants of a human being are located in the colon, which also harbours one of the most diverse of the resident microbial communities. The activities of this microbiota, therefore, are likely to have the greatest impact on human development and health, and have been the most extensively investigated. Studies of the role played by microbial communities inhabiting other sites in maintaining human health have largely been devoted to their ability to prevent colonisation by exogenous pathogens. Ethical considerations limit the type of experiments that can be carried out using humans; therefore, much of our knowledge of the role of the indigenous microbiota on mammalian development and health have come from studies involving animals.

Colonisation resistance

Exclusion of exogenous microbes

Co–evolution has ensured that each exposed surface of a human being is colonised by microbes exquisitely adapted to that particular environment.