A new study has shown that the composition of viruses in the gut may play an important role in inflammatory bowel diseases (IBD). If you’ve been reading the blog for a while, you’ve seen us write about something called the virome. The virome is the collection of viruses in the body and similarly to the microbiome, it may have profound affects on human health. This study led by scientists at Washington University in St. Louis and published in Cell is the first to correlate a disease with changes in a person’s virome.

IBD, specifically Crohn’s disease and ulcerative colitis (UC), are diseases that have been characterized by decreased bacterial diversity in the gut. However in this study, the scientists found that patients with Crohn’s and UC showed greater diversity of viruses than healthy individuals. This suggested that viruses played a role in the disease.

The team of scientists studied individuals in Boston, Chicago, and the United Kingdom with the disease. They took stool samples from patients with UC and Crohn’s and sequenced their viral DNA. They compared this to the viruses in stool samples from healthy individuals living in the same areas and households. Patients with the disease had a higher number of viruses than those without IBD. Specifically, they found that Crohn’s and UC patients had higher levels of Caudovirales bacteriophages (viruses that infect bacteria) that were specific to each disease.

Further research is needed to better understand the relationship between the virome and the microbiome but as we see from the increase in bacteriophages, there is certainly a relationship between these two systems. While the authors state that it does not look as if changes in the virome were secondary to changes in bacterial populations, it is not yet clear if changes in the virome are the result of bacterial alterations in the gut or if it may lead to microbiome changes - or a combination of the two. This study is the first of its kind to show a connection between disease and the virome and I think we are going to see several more studies in the coming years showing this type of correlation with disease. While we generally think of viruses as causing infections like influenza, their impact on chronic disease may be vast.

Bacteriophages (bacterial viruses) attached to a bacterial cell wall.

A review in the journal Cell written by Herbert Virgin describes the exploration of the virome, a part of the overall microbiome which includes viruses, as an emerging study of importance in relation to human health. The size of the mammalian virome –viruses that infect mammals- is still unknown, but it is believed by scientists that, in the human body, viruses may outnumber the bacterial microbiome by at least 10x, making the virome out number human cells by 100 times or more.

The human virome includes all the viruses that affect human cells, along with those that affect our bacterial, archaeal, and fungal microbiome, in addition to other viral derived genetic material. Viruses have already been studied as important pathogens for each of these hosts, but their mutualistic and beneficial interactions are only beginning to be explored. This is similar to the story of bacteria, which are only now being recognized for their importance in humans. In addition, it appears there are many important, complex interactions within the microbiome that include the virome. For example, it is likely that viruses that attack bacteria are embedded in the gut mucous and attack and destroy bacteria before the bacteria can reach the gut lining, providing host immunity.

Investigation into the viral microbiome is still at its beginning. Discovery of new viruses and their interactions with humans is likely to continue and perhaps increase with the increase in availability of viral genome information. We encourage everyone to read the linked article. The virome is a fascinating subject which we hope to explore in future blog posts.