Mice and human intestines are cozy homes for vast communities of microorganisms. By now we know that gut microbes are vital to our well-being. In addition to protecting us from allergies and keeping us in sync with our daily rhythms, the microbiome regulates our immune system, preventing inflammation and fighting infection. Well now meet your virome! Researchers working with mice show that a viral infection in the gut may be beneficial to the host -- especially when antibiotics have wiped out the helpful intestinal bacteria. The findings were published in Nature this week.
Previous studies have found trace genetic evidence of the virome’s existence, but none have shown whether its presence is normal -- let alone harmful, neutral, or helpful.
To see if viruses can produce similar regulatory responses as those mediated by bacteria, a trio of researchers led by Ken Cadwell from New York University studied the murine norovirus (MNV), the mouse version of the stomach bug that causes us to throw up sometimes. Chronic MNV infection produces the same inflammatory damage to intestinal tissue as chronic bacterial exposure -- which means they have a similar, immune-triggering role.
The team fed the virus to mice who've been bred without normal gut bacteria and whose immune systems and intestines were underdeveloped as a result. They lacked immune cells, their nutrient-absorbing projections called villi were shrunken, and the valleys between their villi (called crypts) were smaller than normal. After two weeks of the viral infection, the mice had almost fully restored immune defenses, and their intestinal abnormalities due to the absence of gut bacteria were almost completely reversed.
Additionally, the norovirus also protected the guts of normal mice who were treated with antibiotics: Immune T cell levels in their blood doubled, and B cell antibodies were detectable in gut tissue and blood. The results from both germ-free and antibiotic-treated mice indicate that viruses can have similar roles as symbiotic gut bacteria.
“We have known for a long time that people get infected all the time with viruses and bacteria, and they don’t get sick,” Cadwell says in a news release. “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.”
In a series of experiments designed to mimic the impact of human antibiotic overuse, the researchers treated normal mice with antibiotics that depleted them of bacteria, and then they fed them a chemical that damages gut tissue. Mice who were pretreated, and then infected with MNV, lived longer and had longer villi and less gut tissue damage than those who hadn’t been infected. These MNV-infected mice showed several other signs of reduced damage in the gut: less water loss through diarrhea and less tissue overgrowth from ulcers and fluid buildup.
Next, the team plans to identify the effects of other gut viruses and how they interact with the microbiome.