What changes occur in the gut microbiome after SCI?

Authors: Sharon Jang , Vanessa Mok , Dominik Zbogar | Reviewer: Phillip Popovich | Published: 5 May 2020 | Updated: ~

Movement of the gut slows as a consequence of neurogenic bowel, which is a common in SCI, especially with higher levels of injury.⁷

After SCI, one of the main group of nerves that innervates the gut, the sympathetic nervous system, becomes impaired. This can impact the gut microbiota in 3 ways: through slowing gut movement, through modifying the ability for microbiota to thrive, and through modifying the immune system in the gut. The slower movement of the gut contents, one of the results of impaired bowel function (known as neurogenic bowel dysfunction) after SCI, can impact the microbiota at the far end of the intestines by delaying the delivery of important nutrients. This is of concern because bacteria in the gut thrive by fermenting or breaking down foods such as starches and fibre, and produce metabolites such as butyrate. One study with weak evidence found that people with chronic (at least 12 months post-injury) complete SCI had significant decreases in butyrate-producing bacteria compared to the non-SCI population. As butyrate has anti-inflammatory effects on the nervous system, researchers believe that low butyrate levels can negatively affect long-term recovery following SCI due to increased inflammation.

Refer to our chapter on Bowel Changes After Spinal Cord Injury for more information about how the bowel changes after SCI.

Millions of bacteria grow on the mucus in the gut, forming a biofilm.⁸

Secondly, the sympathetic nervous system is responsible for stimulating mucus secretion within the gut. With a lack of input from the sympathetic nervous system, the production of mucus is decreased. This has important implications for the microbiota in the intestines, as mucus acts as a surface that allows for bacteria to bind to it, thus creating a biofilm. A biofilm is a group of bacteria that has formed a structured community on a surface. With a reduced amount of area for bacteria to thrive, the types of bacteria living in the gut may be altered.

Thirdly, the gut has a protective barrier to prevent bad bacteria from entering the body through the intestinal walls. This immune system within the gut is known as the gastrointestinal-associated lymphoid tissue (GALT) and is controlled by the sympathetic nervous system. With a lack of signaling from the sympathetic nervous system, functioning of the GALT may become compromised. In addition, chronic stress or trauma (which can be brought on by SCI) may change the permeability of the wall of the intestine, allowing harmful bacteria into the body. This may be a potential source of inflammation following SCI and may partly explain why people with SCI are more prone to long-term complications. While research with SCI rat models shows this compromise of the protective barrier and movement of bacteria into places it does not usually reside, (like the blood), it is not yet known if the same is true for humans.

Individuals with SCI are found to have different bacteria comprising their microbiota, compared to able-bodied individuals. For example, one study (weak evidence) found that the diversity of gut bacteria and number of overall bacteria of individuals with chronic tetraplegia were less than in able-bodied individuals. Conversely, another study found individuals with SCI to harbour greater bacterial diversity in their gut relative to people without SCI. However, the greater variation of bacteria in people with SCI consist of bacteria less commonly found in able-bodied individuals. The implications of these differences are unknown and have yet to be researched.