For many years now it has been widely known that smoking, certain dietary factors, obesity, and many other factors are major contributors to cancer incidence. However, a growing body of research has shown that specific changes in the intestinal flora, or dysbiosis, can be found in the stools or on the colonic walls of patients with diseases such as colon cancer.

This has led to hypotheses suggesting that cancers such as colon cancer are in fact bacteria-related diseases, with some species being referred to as “drivers,” and others “promoters.”

“Thus, it appears that microbiota may be considered a platform offering host and environment interactions for studying CRCs. The hypothesis that colon cancer might be a bacteria-related disease is suggested and perspectives are discussed.”

Although most of the aforementioned studies did not state whether the dysbiosis found in colon cancer patients was a cause or consequence of the disease, quantification of major bacterial groups allowed an identification of the Bacteroidetes family as predominant (present as the most influential main factor) in colon cancer.

In fact, the common intestinal commensal known as Bacteroidetes fragilis has been found to induce spontaneous colon cancer in mice bred to mimic familial polyposis in humans. Generally, lower numbers of tumors are seen in mice that are “germ-free” than in those with “conventional” (but not necessarily healthy) gut flora, regardless of whatever carcinogens or mutations are used to induce cancers.

In addition to this, Fusobacterium nucleatum is associated with both gum disease, as it is a key component of periodontal plaque, and colorectal cancer; however it may only be a promoting cofactor. The phyla known as Firmicutes (62%), Bacteroidetes (26%) and Proteobacteria (11%) are the most dominant of bacteria found adhered to precancerous polyps.

Mechanisms of carcinogenesis caused by colonic bacteria include bacteria-induced DNA alteration, which may be caused by E. coli and Enterococcus faecalis; bacterial enzyme activity, which can activate carcinogens or make them more potent; and chronic inflammation.

However, Lactobacillus and Bifidobacterium species of bacteria, found in the vast majority of commercial probiotics, can reduce this inflammation, protect against DNA damage, and inhibit carcinogen activating enzymes. On top of this, fermentation of dietary fibre by beneficial species of bacteria produces short chain fatty acids, which protect against tumor formation.

A Link Between Internal Bacterial Imbalance and Breast Cancer

Another study has also found a possible link between dysbiosis and breast cancer. For the research, the breast microbiota from tumor tissue and paired normal adjacent tissue was analysed from 20 patients. Half of the taxonomic units that were more abundant in normal tissue belonged to the genus Sphingomonas, while two of the three more abundant in tumor tissue belonged to the Methylobacterium genus. This was because Sphingomonas was not present in tumors.

However, dysbiosis found in this study may be due to a downregulation of antibacterial genes in tumors, but the decrease in expression of these genes due to other environmental factors may still be a cause of tumor formation. Dysbiosis found in areas outside of the intestine is also likely to indicate insufficient immune function or even leaky gut syndrome, as it is entirely possible that certain bacteria are able to influence the expression of a cell’s antibacterial genes in order to ensure their own survival.

Another Gut Microbiome-Cancer Connection

In addition to breast and colon cancer, chronic Salmonella enterica infection has been associated with gallbladder cancer; while Chlamydia pneumonia, Haemophilus influenza, and Candida albicans have been linked with lung cancer.

It is also widely known that untreated Helicobacter pylori can result in stomach cancer.

Yet another mechanism behind dysbiosis-driven cancers is the activation of Pathogen Recognition Receptors, which is caused by dysbiosis. In fact, mice lacking the PRR known as TLR-4 have a reduced risk of colon, liver, skin, and pancreatic cancers. This is because of an increase in free radicals and subsequent inflammation and DNA damage.

However, all of these findings demonstrate the importance of not only anti-cancer species of bacteria, but also a strong immune system in general, just as cancers “caused” by a virus such as HPV are prevented by the immune system’s ability to quickly clear the infection.

Once again, gut health and internal flora prove to be absolute key factors in determining our overall health and chance of developing disease.

This article originally appeared at Natural Society.

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