Taking antibiotics when they aren’t necessary could make the flu or other viral infections worse, a new study suggests.

Mice on antibiotics can’t fight the flu as well as mice that haven’t taken the drugs,
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, say researchers from Yale. Antibiotics quash the immune system’s infection-fighting power by killing friendly bacteria living in the intestines,
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, the researchers report in an upcoming Proceedings of the National Academy of Sciences,
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. These friendly, or “commensal,” bacteria help defend against viruses by keeping the immune system on alert for viral invaders, the team discovered.

“There’s a lot of beneficial effects of having commensal bacteria,” says Akiko Iwasaki,
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, a Yale immunologist who led the study. “This is one that was unexpected, but makes sense.”

Scientists knew that friendly bacteria in the intestines could help stop disease-causing bacteria from setting up shop in the gut. And previous experiments hinted that gut microbes could influence how well the immune system works,
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, but researchers thought the effect was mainly confined to the digestive system. “What’s fascinating about [the new study] is that there’s a distant regulation of resistance to viruses by gut microbiota,
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,” says Alexander Chervonsky, an immunologist at the University of Chicago.

Lungs are normally sterile,
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, so it was a bit of a surprise that killing bacteria as far away as the colon would have any effect on how well the lungs could fight viruses.

Iwasaki and her colleagues treated mice for a month with four antibiotics commonly given to people with bacterial infections, then infected the rodents with the flu. Antibiotic treatment impaired the mice’s ability to make an important flu-fighting molecule called interleukin-1 beta or IL-1 beta, the researchers found. IL-1 beta is necessary to combat influenza and other viruses.

Antibiotic-treated mice didn’t have generally weakened immune systems, though. The antibiotic-treated mice were still able to fight herpes, because the immune system fights off herpes and some other viruses using a different molecular weapon.

Gut bacteria are constantly priming the immune system to make IL-1 beta, keeping the immune system vigilant against the flu and other viruses. The researchers aren’t sure yet which bacteria in the gut are responsible for the virus-defense mechanism. “We know for sure that there are certain bacteria that can’t do it,” Iwasaki says. Sphingomonas bacteria, for instance, don’t stimulate the virus-fighting response.

Some Lactobacillus bacteria,
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, on the other hand, are known as “friendly” gut bacteria and may play a role in virus defense. Mice treated with an antibiotic called neomycin, which wipes out most types of Lactobacillus bacteria while leaving Sphingomonas bacteria alone, have a hard time fighting the flu. If researchers can figure out which bacteria are responsible, it may be possible to make probiotics that will boost virus-fighting capabilities.


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