For as long as humans have walked the earth, we’ve been accompanied by microscopic beings that are part of our bodies but not ourselves. The viruses and bacteria that coexist with humans have shaped who we are, but that doesn’t mean they are all benevolent.

As research presented at the 2017 American Society of Preventive Oncology meeting in Seattle showed, viewing our microbes as wholly good or bad might be an oversimplification.

Dr. Meredith Hullar is a researcher at Fred Hutchinson Cancer Research Center who studies the gut microbiome and its complex interplay with diet and cancer risk. She spoke at the meeting Monday about her research and that of others who are looking at how gut bacteria can increase or decrease colorectal cancer risk.

“Gone are the days of one organism, one disease. We’re really looking at how a whole community of microbes influences disease risk,” Hullar said. “For example, scientists are looking at how the metabolism of the microbiome — all of the members of the microbial community — influences its host,” she said. Researchers are exploring questions such as whether the microbiome produces metabolites that promote tumor growth or may prevent cancer.

There are several different types of human bacteria that can directly fuel colorectal cancer risk, Hullar said. Those include the bacterial species Streptococcus gallolyticus and Fusobacterium nucleatum, as well as certain types of E. coli and conglomerate communities of bacteria that form layers in the gut that are called biofilms. All of these microbes act in different ways on the cells that line our gut to increase risk of tumor formation, she said.

But interestingly, many other types of gut bacteria can alter cancer risk — either up or down — through indirect routes that involve what we eat.

Because these bacteria help digest our food, what we eat influences what they excrete in our gut. Some microbial byproducts — such as those produced when processed meat or high–protein foods are digested – increase cancer risk. Compounds formed in the gut by bacteria during the digestion of a high–fiber food – for example, cruciferous vegetables such as broccoli – seem to be linked to a lower risk of colorectal cancer, Hullar said. She and her colleagues are studying the different microbial communities in people with high or low levels of one of those compounds linked to a lower risk of cancer, known as enterolignans. They are now researching whether changing people’s diets changes the amount of enterolignans they produce and how the human cells that line the gut may respond to those compounds in cancer prevention.