Picture three people seated side-by-side on the airplane. The person in the middle has come down with the flu and spends the two-hour flight coughing, shivering, and drifting in and out of a feverish sleep. The others have good reason to be vexed—indeed, the person on the right ends up with the exact same symptoms the next day.

The person seated on the left, however, gets away scot-free. Despite her certain exposure to the influenza (flu) virus, she has no symptoms. The coughs and chills don’t materialize the next day or any other day.

A metabolite produced by microbes acts as a go-between for gut microbes and the immune system

Different people having the same exposure to sickness-causing germs with completely different effects is a scenario that continues to puzzle scientists. So recently, researchers led by Thaddeus S. Stappenbeck, MD, PhD, from the Department of Pathology & Immunology at Washington University School of Medicine, St. Louis (USA), turned to mice to further investigate the factors that affect how the body responds to influenza virus infection in the respiratory tract. They found a factor related to both diet and gut microbiota that led to less severe symptoms in flu-exposed mice.

The researchers reported, in a recent issue of Science, that they worked with a mouse genetically modified to produce a high number of type I interferons that they thought would affect how flu harmed the animals’ lungs. They honed in on a molecule called “desaminotyrosine” (or DAT). This molecule is normally produced by the gut microbiota from dietary polyphenols called flavonoids—compounds with anti-oxidant activities, that humans consume in common items like dark chocolate, berries, tea, and red wine.

The authors noted that two factors were needed to ensure these effects on immune response: a diet rich in flavonoids plus the right gut microbes to use said flavonoids

When mice were given DAT in their drinking water a week before exposure to the potentially lethal influenza virus, they experienced less weight loss and had a better rate of survival than those where were not given DAT. Somehow, DAT was reducing the lung damage inflicted by the viral infection.

Yet, researchers found the levels of viral infection were equal in both groups of mice: those that received the treatment and those that didn’t receive the treatment.

Indeed, DAT seemed to work on the immune system’s response to the virus rather than the virus itself. Stappenbeck and colleagues also describe in the study how DAT managed to do this: it was serving to boost the body’s immune response—thereby reducing the lung damage.

DAT is produced by gut microbes, but which ones? This was the next question the researchers set out to address. They tested the effects of several different bacterial species and found the human-associated gut bacterium Clostridium orbiscindens was the best producer of DAT.

Putting all the pieces together, it seems the gut microbe Clostridium orbiscindens may break down compounds in the diet to produce DAT—and when a mouse host is exposed to the flu virus, DAT serves to modulate the immune system and prevent severe damage to the lungs. Essentially, a metabolite produced by microbes is acting as a go-between for gut microbes and the immune system.

The authors noted that the effects on immune response required two factors: a diet rich in flavonoids plus the right gut microbes to use those flavonoids.

It’s not yet certain whether DAT has the same effect in humans. But let’s say this: if you were to consume dark chocolate every day during flu season—exclusively as a preventative measure, of course—no one could say it was entirely without scientific support.



Steed AL, Christophi GP, Kaiko GE, et al. The microbial metabolite desaminotyrosine protects from influenza through type I interferon. Science. 2017; 357(6350):498-502. doi: 10.1126/science.aam5336.