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# 3.7 Infection and disease

One of the important functions of the gut microbiome is to reduce the risk of harmful pathogenic bacteria from colonising the gut. A reduction in healthy bacteria can lead to infection caused by the pathogenic bacteria, such as Clostridium difficile (C. difficile) and Helicobacter pylori (H. pylori). Both of these infections can have significant impact on health, and H. pylori is an important cause of stomach ulcers.

Changes in the gut microbiome has also been shown to be associated with the severity of the response to infection with the SARS-CoV-2 virus – the virus which causes COVID-19 (Figure 22).

Figure 22 (a) A transmission electron micrograph shows four red-orange coloured SARS-CoV-2 coronaviruses, with the typical ‘corona’ appearance around the virus; (b) A 3D representation of the SARS-CoV-2 virus with the spike proteins, which have a key role in enabling the virus to enter host cells and cause infection.

Yeoh et al. (2021) found that in patients with COVID-19, there was a reduction in the gut bacterial groups which would usually promote a good immune response. The more severe the disease, the more significant the changes in the gut microbiome. The researchers also found that these changes persisted for more than 30 days after the symptoms had resolved and proposed that dysbiosis may play a role in the development of post-COVID syndrome (more commonly known as long COVID). In some individuals, symptoms such as breathlessness, fatigue and pain persist for a prolonged period – in some instances even years – following initial infection.

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