Antibiotics may make fighting flu harder

Drugs hit beneficial bacteria that keep immune system alert

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, say researchers from Yale. Antibiotics quash the immune system’s infection-fighting power by killing friendly bacteria living in the intestines, the researchers report in an upcoming Proceedings of the National Academy of Sciences. 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, 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, 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,” says Alexander Chervonsky, an immunologist at the University of Chicago.

Lungs are normally sterile, 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, 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.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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