Bacteria in the human gastrointestinal tract vastly outnumber our own cells, and an increasing body of evidence suggests that these and other microbes can modulate our risk of developing–and just maybe, successfully treating–cancer.1

“We know for certain that pathogenic bacteria cause cancer,” said Susan E Erdman, DVM, MPH, who studies microbe-triggered inflammation and cancer at the Massachusetts Institute of Technology (MIT) in Cambridge. “Helicobacter pylori is an example of that. It directly and indirectly causes stomach cancer in humans.”

Dr Erdman and colleagues induced colon and mammary tumors in lab mice by infecting them with a related bacteria species, H. hepaticus.2

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The list of carcinogenic viruses and bacteria likely overlaps with other cancer risk factors, like tobacco smoking. There is evidence that tobacco smoke can activate human papillomavirus type 16 (HPV-16)’s oncogenes.3 It is even possible that immunotoxic or immunosuppressing environmental exposures leave people vulnerable to carcinogenic infections, in a manner similar to human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS).

“How many cancers are caused by viruses in a central way?” asked Paul Ewald, PhD, an evolutionary biologist who specializes in the study of infectious disease. Dr Ewald directs the evolutionary medicine program at the University of Louisville in Kentucky. “We don’t know. It’s 15 to 20 percent right now, but of the remaining 80%, we just don’t know. It’s not that the virus benefits from inducing the cancer; it benefits from nudging a cell toward a cancer, reducing its exposure to the host immune system.”

“There’s mounting scientific evidence that direct or indirect effects of bacteria influence nearly everything about how we function, think, and feel,” Dr Erdman said, alluding to recent evidence that gut bacteria might release hormones into circulation that affect appetite and other facets of brain function. “It’s likely that microbes influence either directly or indirectly the origins and progression of tobacco and non-tobacco cancers.”

The molecular mechanisms whereby microbes can trigger or hasten tumor development appear to vary, involving both microbes’ direct carcinogenic effects and host cells’ reactions against infections.

Some commensal gut bacteria and opportunistic pathogens appear to cause cancer or other diseases “only under certain permissive conditions,” she said.