There are subtle links between cancer cells (tinted green) and gut microbes (orange) (Image: Steve Gschmeissner/SPL)
Changes in our gut flora may encourage or battle cancer, so we need to get a few trillion of our foot soldiers on side, says a professor of medicine
THE human body is occupied by trillions of microorganisms, acquired at birth and maintained throughout our lifetime. Though we are mostly oblivious to thismicrobiome, it forms an intimate and essential part of our being. It is involved in many vital biological processes such as nutrition, the immune system and even mental health. Now evidence is mounting that the microbiome plays a role in cancer too.
In the past five years a comprehensive catalogue of the microorganisms living on and in the different surfaces and cavities of our bodies has been created – a sort of "getting to know your neighbours". This has revealed that the microbiome is a diverse community of more than 1500 species, with the vast majority residing in the intestine. We are just beginning to discover how these microbes influence the development and treatment of cancer.
The first evidence that "friendly" gut bacteria might have a darker side comes from studies in rodents, which developed fewer tumours when their microbiome had been wiped out, indicating the tumour-promoting potential of gut microbes.
In humans, the strongest evidence of the potential role of the microbiome in cancer comes from studies on colorectal cancer (CRC) – the third commonest form of cancer in the US and that country's second leading cause of cancer deaths. Recent research on people with CRC showed that their intestinal microbial communities become unbalanced. For example, compared with healthy people, the stools of people with CRC contain less of the bacterial groups Lachnospiraceae and Roseburia but an increased abundance of others such as Enterococcus and Streptococcus.
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How might a "dysbiotic" microbiome play a role in cancer? Roseburiamicrobes, for instance, produce high quantities of butyrate, so fewer of these organisms in the intestines could result in smaller amounts of protective butyrate. Studies in mice showed that low levels of colonic butyrate could foster the development of CRC.
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How else could gut microbes play a role in cancer? Recent evidence indicates that bacteria may produce noxious agents – gases, toxins, radical oxygen species – that can cause instabilities in the host's genetic material. For instance, the bacterium E. coli is found in abundance in the intestinal mucosa of people with CRC, compared with people without CRC. Interestingly, genomic analysis of these CRC-associated E. coli revealed the presence of a cluster of genes responsible for producing a substance called colibactin. This natural product triggers DNA breakage and genomic instability in different cells, including cells lining the intestine.
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How might a "dysbiotic" microbiome play a role in cancer? Roseburiamicrobes, for instance, produce high quantities of butyrate, so fewer of these organisms in the intestines could result in smaller amounts of protective butyrate. Studies in mice showed that low levels of colonic butyrate could foster the development of CRC.
...
How else could gut microbes play a role in cancer? Recent evidence indicates that bacteria may produce noxious agents – gases, toxins, radical oxygen species – that can cause instabilities in the host's genetic material. For instance, the bacterium E. coli is found in abundance in the intestinal mucosa of people with CRC, compared with people without CRC. Interestingly, genomic analysis of these CRC-associated E. coli revealed the presence of a cluster of genes responsible for producing a substance called colibactin. This natural product triggers DNA breakage and genomic instability in different cells, including cells lining the intestine.
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