Decreased Diversity of Gut Bacteria Linked With Colorectal Cancer Risk

Fusobacterium novum after being cultured in a thioglycollate medium.

A decreased diversity of gut microbiota was associated with an increased risk of colorectal cancer, according to the results of a study published recently in the Journal of the National Cancer Institute.

According to the study, prior research has shown a possible association between gut microbiota and colorectal cancer in humans, but “systematic epidemiologic comparisons between colorectal cancer patients and control subjects, considering comprehensive confounders and multiple comparisons, are lacking.”

To investigate this association further, Jiyoung Ahn, PhD, from the department of population health at New York University School of Medicine, and colleagues compared samples and data from participants in a case-control study that tested if fecal mutagens were linked to colorectal cancer. The participants all had adenocarcinoma of the colon and rectum and had been recruited from 1985 to 1989. The study included 47 patients with cancer and 94 matched control participants.

Ahn and colleagues extracted DNA from fecal samples and sequenced them to determine the gut microbial community structure of participants with cancer compared with controls.

The results showed that patients with colorectal cancer had a decreased overall microbial community diversity compared with control participants (P = .02). In addition, the researchers found that patients with cancer had a lower abundance of Clostridia (68.6% vs 77.8%), which include some bacterial family members that ferment dietary fiber, to butyrate, which is a major colonic metabolite that may inhibit inflammation and carcinogenesis in the colon. Patients with cancer also had an increased amount of Fusobacterium (OR = 4.11; 95% CI, 1.62-10.47) and Porphyromonas (OR = 5.17; 95% CI, 1.75-15.25), both bacteria related to inflammation in the mouth and gastrointestinal track.

In an accompanying editorial, Volker Mai, PhD, MPH, and J. Glenn Morris, Jr., MD, MPH, of the University of Florida, Gainesville, said that although the results of this analysis are exciting, caution must be used before promoting changes in microbiota as a method to curb colorectal cancer.

“Colorectal cancer occurrence is known to be influenced by host genetics, as well as factors such as obesity, nutrition, and exercise; given that these factors also influence microbiota, separation of cause and effect among all of these factors may become quite difficult,” they wrote. “Prospective microbiota studies designed to investigate temporal associations with a plethora of other disease outcomes are our best hope for developing the thorough understanding of the complexities involved that is required for harnessing the promise of improving health outcomes by modifying gut microbiota composition.”