In a study reported in the Journal of the National Cancer Institute, Ahn and colleagues found that decreased diversity of gut bacteria and differences in levels of specific bacteria were associated with significantly increased risk of colorectal cancer.
The study involved measurement of 16S ribosomal RNA genes in fecal bacterial DNA from 47 colorectal cancer patients and 94 control subjects matched for sex and body mass index. Analysis of 794,217 gene sequences showed that patients with colorectal cancer had significantly reduced overall microbial community diversity compared with controls (P = .02).
Patients had higher relative abundance of Bacteroidetes (16.2% vs 9.9%) and depletion of Firmicutes (74.0% vs 80.3%), with a significant depletion of the Firmicutes gram-positive Clostridia (68.6% vs 77.8%, P = .005). Carriage of the gram-negative Fusobacterium was significantly greater in cancer patients (31.9% vs 11.7%, P = .004) and was associated with significantly increased risk of colorectal cancer (odds ratio [OR] = 4.11, P = .004) on multivariate analysis adjusting for age, sex, body mass index, race, smoking, and sequencing batch. Increased carriage of the gram-positive Atopobium (OR = 14.36, P < .001) and the gram-negative Porphyromonas (OR = 5.17, P = .001) were also associated with significantly increased risk of colorectal cancer.
Clostridia ferments dietary fiber and other complex carbohydrates to butyrate, a major colonic metabolite that may inhibit colonic inflammation and carcinogenesis. Fusobacterium contributes to colitis and periodontal disease. Atopobium is associated with Crohn’s disease and has been reported to inhibit colon cancer apoptosis in vitro. Porphyromonas is commonly found in the mouth and gastrointestinal tract and is associated with periodontal disease.
The investigators concluded, “Because of the potentially modifiable nature of the gut bacteria, our findings may have implications for [colorectal cancer] prevention.” ■
Ahn J, et al: J Natl Cancer Inst 105:1907-1911, 2013.