New research points to viruses inhabiting the gut as possible culprits in inflammatory bowel disease (IBD). IBD, which includes Crohn’s disease and ulcerative colitis, is a group of debilitating conditions caused by inflammation in the gut, leading to cramps, diarrhea, and bleeding. There is ample evidence that this inflammation could be caused by a combination of genetic factors and an improper immune reaction to the community of bacteria that reside in the gut. Many studies have explored the connection between gut bacteria and IBD; in fact, bacterial diversity is lower in patients with IBD, although it is not clear what causes this change. In addition to trillions of bacteria, the gut is also home to a diverse population of viruses; both bacteria and viruses are members of the community of microbes known as the “microbiome.” Most of these viruses are bacteriophages, which are viruses that infect bacteria, including those found in the gut, and insert genes into the bacterial DNA. The close relationship between gut bacteriophages and bacteria raises the possibility that there could also be a relationship between these resident viruses and IBD, although this connection is only beginning to be explored.
To examine the possible link between gut viruses and IBD, fecal samples from men and women with IBD in Chicago, Los Angeles, and the United Kingdom were examined for viral genetic material. Importantly, to control for potential environmental effects, the IBD samples were compared to samples from healthy volunteers living in the same area—sometimes even the same household. The most abundant viruses identified in all samples were members of two groups of bacteriophages called Microviridae and Caudovirales. The healthy participants had similar numbers of members from these two viral groups. However, in the participants with Crohn’s disease or ulcerative colitis, the Caudovirales viruses were not only more abundant than the Microviridae, but there were also more types of Caudovirales viruses. In other words, the Caudovirales group of viruses appeared to have expanded and diversified in the volunteers with IBD. Even though these bacteriophages have a close relationship with bacteria and rely on them to reproduce, the Caudovirales diversification did not appear to be simply due to an increase in bacterial diversity, because bacterial diversity was lower in the IBD participants than in healthy individuals.
These results introduce a new twist to the complicated understanding of IBD. The exact role gut viruses may play in this disease—or in any other diseases and conditions in which gut bacteria have been found to play a role, such as diabetes, obesity, metabolic diseases, and cancer—remains to be determined. Nonetheless, the discovery of this link between gut viruses and IBD could open the door to designing better treatments or preventative measures in the future.