The functions of the gut microbiota

You have now learned what the gut microbiome is and how it is composed. But what exactly does this community of microorganisms do? In this step, we will explore some of the functions of the gut microbiota, including digestion, metabolic activity, and immune function, and learn how microbes in the gut interact with the host and play a critical role in shaping human health and disease. Microbes are present throughout the alimentary tract, from mouth to anus. They play a key role in processing food that is consumed and breaking it down in order to be used or stored by the body. In the intestine, nutrients are absorbed following digestion.

However, there are some dietary components, such as nondigestible fibres found in fruits, vegetables and grains, that are not broken down. And so we rely on our gut bacteria to digest these for us, which they do by a process called fermentation. This process generates energy and small chemicals called short-chain fatty acids, predominantly acetate, butyrate, and propionate. These play an important role in providing energy to cells, and their effects are exerted around the whole body. Microbes can also digest dietary proteins and fats, producing breakdown products that feed into a number of host physiological processes. Indigestible food matter is excreted from the body as waste. The human body is constantly undergoing a series of chemical reactions through dynamic metabolic processes called metabolism.

The gut contains the most metabolically active microbial community in the whole body. We’ve just heard about short-chain fatty acids, which microbes produce, that have a key role in host energy metabolism. But there are other intermediates, such as succinate and lactate, and end products of metabolic processes, such as creosols and endols, which are extremely important to the host too. Other functions of microbes include production of vitamins, biogenic amines, and bile acids, which play important roles in host physiological processes. A growing body of research has found associations between an altered gut microbiota and disease states such as metabolic syndrome, inflammatory bowel disease, and liver disease.

This demonstrates that host metabolic activities that are intertwined with the gut can be affected by diet and lifestyle. For example, published work has shown that there are specific microbes in obese people that have the ability to harvest more energy from the diet. Hence, it is possible that the gut microbiota is a contributing factor in the pathophysiology of obesity. Did you know that 70% of the immune system is located in the gut? This figure from a published review paper nicely summarises what the gut environment looks like in health and disease. A single layer of epithelium separates the gut lumen from underlying tissues. In a healthy gut environment, commensal gut bacteria outnumber pathogens by competing for nutrients and producing antibacterial compounds.

Both are separated from the epithelium by a dense mucus layer. The short-chain fatty acid butyrate, produced by gut bacteria, plays a critical role in protecting this barrier, as its absorption into the gut strengthens the epithelial wall by creating tight junctions between cells and preventing microorganisms from crossing into the lamina propria. Furthermore, short-chain fatty acid stimulation of dendrites can impact on regulatory T-cells, increasing anti-inflammatory cytokines, and via macrophages, down regulate pro-inflammatory cytokine release. Should the gut environment be disrupted, however, damage can occur to the barrier, enabling pathogens to penetrate and inflammatory responses occur. An example of this is bacterial sepsis, where the integrity of the epithelium is compromised and bacteria enter into circulation, resulting in systemic inflammation.

Some studies have demonstrated that probiotics can be an effective dietary strategy to prevent this by increasing levels of commensal bacteria in the gut to compete with pathogens and fortifying the gut barrier through production of short-chain fatty acids. So throughout our bodies, the many complex processes that occur can often involve the community of microorganisms living in our gut. As more information is uncovered, the importance of the gut microbiota and their activities and interaction with the human host is being increasingly recognised.

These include important functions which, one, balance commensal and pathogenic bacteria in the gut and reduce the opportunity for pathogens to flourish; two, produce short-chain fatty acids and microbial metabolites, which have systemic effects in the body; three, improve tight junctions, resulting in better maintenance of the gut barrier, reducing inflammation, and improving immune function. Having a diet that considers our microbial communities could help us to better shape our health, as increasing levels of positive microorganisms in the gut can impact on a range of mechanisms around the whole body that are a benefit to the host.