Is the microbiota involved in the development of colorectal cancer?
The colon is a site prone to tumor development and is also where most of the gut microbiota resides.
Several studies have shown that the bacterial communities present in people with colon cancer are different from those in healthy people, which can lead to dysbiosis. Both bacteria associated with the colon mucosa and bacteria found in stool have been studied in these studies. In addition, the composition of the microbiota in cancer patients differs between the tumor and the area surrounding the tumor (see our article to learn all about the gut microbiota).
Several studies have identified different bacteria, but there appears to be an overall enrichment of Fusobacterium and Campylobacter species associated with the tumor mucosa compared to non-tumor tissue in the same patients. An enrichment of these species has also been observed in cancer patients compared to healthy patients.
The study of bacteria has identified several as potential contributors to the development of colorectal cancer. Streptococcus gallolyticus, whose DNA has been found in 20-50% of cancerous tissues, may contribute to tumor growth in some individuals. However, it could also be an opportunistic bacterium that does not contribute to its development but grows easily in the tumor environment. Escherichia coli is found in higher abundance in tumor tissues and may contribute to colon carcinogenesis. Fusobacterium nucleatum may also be involved in the initiation of the disease through its modulation of inflammation.
However, rather than a single bacterium being responsible, it seems more likely that the entire bacterial community and the compounds it produces play a role in carcinogenesis (the process of cancer formation).
Fats and bile acids
Diets high in fat are associated with a higher incidence of colorectal cancer. Diets high in saturated fat (mainly from animal products) increase the production of bile acids. However, the bacteria in the gut microbiota are involved in the metabolism of bile acids and could therefore contribute to the link between cancer and saturated fat. In response to the ingestion of saturated fats, the liver produces bile acids known as conjugated bile acids. These are transformed (or deconjugated) by gut bacteria into secondary bile acids: lithocholic and deoxycholic acids. However, these two compounds are elevated in patients with colon cancer and promote inflammation.
In addition, the bacterium Bilophila wadsworthia is found in greater quantities in people who eat animal products (meat and dairy). It is pro-inflammatory and can be detected by Nahibu's gut microbiota analysis. Controlling the levels of this bacterium could reduce colon inflammation, which plays a role in IBD (inflammatory bowel disease) and cancer.
However, the metabolism of bile acids by microorganisms may also have positive effects. Ursodeoxycholic acid appears to have beneficial effects in humans and animals. It is even approved as a therapy for primary biliary cirrhosis. Certain intestinal bacteria, including strains of Clostridium, Ruminococcus, and Eubacterium, can produce it from chenodeoxycholic acid.
Fiber and short-chain fatty acids
Intestinal bacteria can produce short-chain fatty acids (SCFAs) from the plant fibers we ingest. These fibers, which are not digested by our bodies, are fermented by our intestinal flora. They are found in fruits, vegetables, legumes, and whole grains. The SCFAs produced have an anti-inflammatory effect and may therefore have a protective effect against cancer.
Proteins
On the other hand, a diet rich in protein can lead to the production of harmful compounds by the microbiota, such as polyamines. High levels of these molecules are found in certain diseases, including cancer. The oxidative stress resulting from the breakdown of polyamines is thought to be the cause of their toxicity. In addition, certain pathogenic bacteria use polyamines to increase their virulence.
Although the involvement of a high-protein diet in colon cancer remains a subject of debate, it is best to avoid consuming too much animal protein. On the other hand, dietary fiber found in fruits, vegetables, and legumes can limit protein fermentation in the colon and may therefore have a protective effect.
Alcohol
The product of alcohol breakdown, acetaldehyde, is carcinogenic and highly toxic. Bacteria in the oral microbiota are thought to be capable of forming this compound from alcohol, and it is possible that bacteria in the gut microbiota have the same property. It is therefore necessary to reduce alcohol consumption to limit the formation of this highly toxic compound.
