The role of the intestinal microbiota in enteric infection

Abstract

The consortia of microorganisms inhabiting the length of the gastrointestinal tract, the gastrointestinal microbiota, are vital to many aspects of normal host physiology. In addition, they are an active participant in the progression of many diseases, among them enteric infections. Healthy intestinal microbiota contribute to host resistance to infection through their involvement in the development of the host immune system and provision of colonization resistance. It is not surprising then that disruptions of the microbial community translate into alterations of host susceptibility to infection. Additionally, the process of the infection itself results in a disturbance to the microbiota. This disturbance is often mediated by the host inflammatory response, allowing the pathogen to benefit from the inflammation at the intestinal mucosa. Uncovering the mechanisms underlying the host-pathogen-microbiota interactions will facilitate our understanding of the infection process and promote design of more effective and focused prophylactic and therapeutic strategies.

Figure 1. The role of the intestinal microbiota in host immune development and infection progression

A, intestinal microbiota interact with the host gastrointestinal system to stimulate the proper maturation of mucosal immunity and to provide colonization resistance against invading pathogens. Examples of some microbiota components capable of contributing to the induction of several aspects of mucosal immunity are shown. SFB, segmented filamentous bacteria. B, perturbations to the intestinal ecosystem, either in the form of infection and associated inflammation or iatrogenic in nature (such as antibiotic treatment), influence the infection progression by affecting the availability of nutrients in the intestine, modifying the immune response to the invading pathogen, aggravating the gastrointestinal pathology and promoting colonization by opportunistic pathogens.

Figure 2. Host–pathogen-microbiota interactions and outcomes

A healthy host exists in a state of balance with the intestinal microbiota, wherein the microbiota contribute to normal host immunity, to homeostasis at the intestinal mucosa and to metabolism, and the host provides it with a convenient habitat, while keeping the microbial numbers in check. An invading pathogen interacts with both the host and the microbiota, enacting a disturbance in the host–microbiota balance through its virulence strategy. If both the host and the indigenous microbiota are resilient, the balance is restored with a consequent recovery at the intestinal mucosa. If the host–microbiota balance is disturbed too severely, leading to the complete disruption of intestinal homeostasis, death of the host will likely follow. If the pathogen carves out a niche for itself within the indigenous microbial community, a new equilibrium between the host and the microbiota will eventually be established, resulting in chronic carriage of the pathogen.