The gut uses two barriers to help prevent pathogens from invading the body: the mechanical barrier and the immune barrier. The friendly microbes residing within our gut help to support these defensive barriers by resisting the invasion of pathogens and helping the development of the body’s immune system. The following information draws from a review (1) conducted by Yu-Jie Zhang et el. which discusses the roles and potential mechanisms of gut bacteria in human health and diseases.

The Mechanical Barrier

The mechanical barrier consists of a single layer of intestinal epithelial cells, the enterocytes and mucus. Gut microbes inhibit the invasion of harmful bacteria at this barrier through “colonisation resistance”, basically a crowding out of pathogens in which our beneficial microbes compete for nutrients and attachment sites on the mucosal surface of the colon. Microbes also keep pathogens at bay by changing the conditions of their environment, making it less hospitable to the bad guys. One way they do this is through the production of short-chain fatty acids (SCFAs) which helps reduce the intestinal pH. Our microbes can also resist colonisation by producing toxic metabolites to prevent the growth of or even kill potentially harmful bacteria.

The Immune Barrier

The immune barrier is composed of an array of immune cells including macrophages, neutrophils and natural killer cells. Gut bacteria prime the gut immune system’s immune cells to fend off pathogens. They enable the normal development of gut-associated lymphoid tissue, and the somatic diversification of antibody genes. They also induce the immune system to produce antibodies which help regulate gut bacterial communities in the small intestine. Metabolites produced by microorganisms during starch fermentation may also facilitate generation of certain immune cells which act to suppress immune response, thus enabling the body to maintain homeostasis and self-tolerance.

Studies of animals bred under germ-free conditions highlight the importance of gut bacteria in the immune barrier. One study showed that germ-free animals presented with structural and functional abnormalities including reduced production of important players pertaining to the immune system compared to conventionally-grown animals. Another study showed that animals which obtained cells from germ-free mice developed an earlier onset of colitis and, furthermore,  particular cells required to rectify the condition obtained from the germ-free mice were unsuccessful compared with mice who had been restored with immune cells from conventionally-housed animals.

Upholding the Barriers

Scientific evidence suggests that gut microbial communities are regulated not only by our genes, but also the environment in which we live and the diet that we consume.

Microbial diversity is king when it comes to our health. Whilst we can’t change our genetic makeup, we can influence the types of “good” microbes inhabiting our guts by making considered choices about the food we eat and the lifestyle choices we make. In doing so, we help to keep gut barriers functioning optimally, protecting ourselves from unwanted guests.

Brigid xx

References

(1) Zhang, Yu-Jie et al. “Impacts of gut bacteria on human health and diseases.” International journal of molecular sciences vol. 16,4 7493-519. 2 Apr. 2015, doi:10.3390/ijms16047493