Researchers believe they have found the first strong indication that the gut is a natural home to viruses that are as helpful as “friendly bacteria” in maintaining health and keeping infection at bay.
The human body carries thousands of species of bacteria – collectively known as the “microbiome” – that we now know are essential to health. The highest diversity and concentration of these is found in the gastrointestinal tract or the gut.
Now, in the journal Nature, microbiologists at NYU Langone Medical Center, New York, NY, describe how they discovered evidence in mice of what they call the “virome” – and how it plays a role in gut health similar to that of the microbiome.
Over the course of two years, the team undertook several experiments in mice. They found infection with the common murine norovirus (MNV) helped the mice heal inflammation-damaged gut tissue and restore their gut’s immune system after its microbiome had been destroyed by antibiotics.
Furthermore, the researchers also discovered that MNV strengthened the ability of the mice’s immune system to defend against tissue damage.
Previous studies have found genetic traces in the gut to suggest there might be such a thing as a virome. But none has gone so far as to show that a virome might be a natural thing, and whether it might harm, help or have no particular effect on the host.
Senior study investigator and assistant professor Ken Cadwell says their study provides “compelling” evidence about how viruses and bacteria naturally work together in the mouse gut.
Scientists have long been puzzled by how people get infected all the time with viruses and bacteria without falling ill.
“Now we have scientific evidence that not every viral infection is bad, but may actually be beneficial to health, just as we know that many bacterial infections are good for maintaining health,” Prof. Cadwell explains.
The finding “lays the groundwork for further research on precisely how the virome supports the immune system, which likely applies to humans, as well,” he adds.
For their study, the team used mice genetically engineered to be susceptible to inflammatory bowel disease (IBD) – a collective term for diseases like ulcerative colitis and Crohn’s disease that involve inflammation of the gut.
While the exact cause of IBD is unknown, scientists are coming to the view that it results from altered interactions between the gut’s microbiome and its immune system – causing the immune system to attack healthy tissue.
In previous work, the team had also found inducing chronic infection with MNV in the mice led to the same inflammatory damage as chronic exposure to bacteria – suggesting both types of infection have a similar effect on the immune system.
In the new study, they raised IBD mice in sterile conditions and lacking a normal microbiome – which meant their intestines and immune systems did not develop properly – and fed them MNV.
The IBD mice’s underdeveloped immune systems lacked white blood cells known as T-cells and B-cells, and their underdeveloped guts had shrunken and thinner villi with less tissue than normal between them. Villi are long, thin, fleshy fingers that project from the gut wall and give it the large surface area it needs to absorb a maximum amount of nutrients from digested food.
After feeding MNV to the IBD mice, the researchers kept them in the sterile conditions and did not expose them any other germs. After two weeks, the mice showed much improved – nearly restored to normal – immune systems and almost complete restoration of the gut wall.
Further tests showed that it was MNV driving the restoration. The researchers found evidence of increased immune system signaling by antiviral type 1 interferon proteins.
The team repeated a similar experiment with normal mice: they fed them MNV after wiping out their microbiome with antibiotics. They found the mice’s blood T-cell count doubled, and samples of gut wall and blood showed presence of B-cell antibodies.
The team now plans to repeat these tests with other types of gut viruses and to find out whether their ability to harm or benefit the gut varies from person to person.
Prof. Cadwell urges people not to infect themselves with viruses on the basis of these results – they could do themselves serious harm.
The National Institutes of Health and several other granting bodies funded the study.
Recently in another article, Medical News Today reported how metabolic syndrome may be prevented by healthy gut bacteria. Researchers writing in the journal Gastroenterology explained how an altered microbiome promotes the inflammation that leads to metabolic syndrome.