Bullying alters ‘bugs’ in the gut, hamster data show

Feces provide link between social stress and certain changes in gut microbes


Gut microbes in hamsters changed after the animals experienced social stress, such as bullying.


Sometimes, a bully’s teasing is enough to make your stomach hurt. But a new study finds that bullying might change more than how a gut feels. It could alter which microbes live there — at least in hamsters.

Katherine Partrick is a graduate student at Georgia State University in Atlanta. Working at its Neuroscience Institute, she studies gut microbes. And she knew that studies have shown that social stress, such as bullying, can cause long-term health problems. Those problems often affect someone’s stomach or intestines.

She also knew that as in other mammals, the guts of people teem with microbes. More than 100 trillion bacteria, fungi and other microbes live as squatters in your gut. It might sound gross, but your body needs many of these microbes to stay healthy. Some help digest food or make vitamins. Others, though, may make you sick.

Gut microbes “can actually communicate with our brain and affect our health,” she notes. So she hypothesized that social stress might change which microbes would choose to colonize the digestive tract.

Studying this in people can be challenging. So Partrick and her team instead used Syrian hamsters. Yes, that’s the same animal many people keep as pets. But certain aspects of this rodent’s biology — and responses to stress — are enough like a human’s for it to “model” what might happen in people. Such animal models can help researchers answer questions about people without experimenting on them.

Syrian hamsters seemed well suited for this study because the males can get quite aggressive towards each other. When they meet, they tend to wrestle. The hamsters don’t seriously hurt each other. But the encounters have consequences. The winner gets to be the boss and can bully the other. Partrick thought that stress might change the bullied animal’s microbiome (My-kro-BY-oam), which is the sum of all the microbes living in its guts.

To test that, her group needed hamster feces. The reason: Roughly half of all cells in the fresh poop collected from the animals’ cages come from microbes in the rodents’ guts.

Next, the scientists analyzed the DNA in those cells. That would tell them what species the DNA came from.

Now the researchers introduced two males, putting one in the other’s cage. This created “a bullying situation,” Partrick explains. The animals “kind of rolled around, kind of tussled around with each other,” she notes. After wrestling, the loser — or “subordinate” hamster — ran around nervously. The winner chased him, so Partrick could easily see which hamster had lost the fight.

Partrick repeated this process with eight more pairs of males. And these hamster confrontations took place twice a day for five days. Five more hamsters lived in the same conditions as the paired hamsters but never faced off with another male. That made them a control group — one in which the conditions never changed. They would serve as a comparison to both the bullied and bullying hamsters.

Throughout the experiment, the scientists collected feces daily from the animals’ cages. And by comparing them, it quickly became clear that the face-offs had changed the microbiome in both the bullied hamsters and their bullies.

The researchers shared their findings in a paper that will appear in the June Behavioural Brain Research.

What they learned

The microbes shed in feces should reflect the general mix of organisms in the gut’s microbiome. Throughout the tests, gut microbes from hamsters in the control group didn’t change. After the other males began squaring off, however, their gut microbes did change — and in both the winners and the losers.

As a result of this social stress, some types of microbes became much more common in the winners. Others became more common in the losers or in both groups. Others decreased in one or both groups. 

These new hamster findings surprised Partrick. She had predicted that gut microbes in only the bullied hamsters would change. But now she thinks that the stress caused by fighting is behind changes in the bullies, too. “Social conflict in itself causes changes to the gut microbiota,” explains Partrick, “regardless of whether you win or lose.”

After male hamsters fought, the share of one type of bacteria dropped in the feces of both winners and losers. It’s Lactobacillales (LAK-toh-baa-sil-LAY-lees). Some bacteria in this genus are good for mammals. In fact, many people eat foods or take pills that contain these bacteria. Studies have hinted this might improve a person’s health. Such healthful bacteria are known as probiotics.

Social stress also boosted the share of some harmful bacteria. This included bacteria from the genus Clostridium (Klos-TRID-ee-um). These bacteria can cause food poisoning, diarrhea, fever, abdominal pain and inflammation. They increased in number for both winning and losing hamsters who had fought.

While social stress clearly affected gut microbes, Partrick found the opposite also to be true. She learned to predict which hamster would win a fight by “reading” its microbes. This suggests that some microbes in a hamster’s gut might influence how it behaves and interacts with others. Partrick would like to explore this idea in a future study.

Beyond hamsters

“It’s a great first study,” says Michael Bailey. He’s the lead investigator for microbial disease at Nationwide Children’s Hospital, in Columbus, Ohio. There he probes how microbes interact with our hormones, nervous systems and behaviors. He says of the new research, “I think it’s definitely provocative and should lead to more studies.”  

Anand Gururajan works in Ireland at the University College Cork. There he looks for links between gut microbiota and the brain. He was not involved in this study, but he is wrapping up a similar one in mice. He found it interesting that gut microbiota could predict how the hamsters would react to stress.

Studies in humans have seen similar gut-microbe differences in patients with depression, which is a stressful condition. But it’s not the same type of stress studied in the animals. Still, the study raises interesting questions about how widely across the mammalian world stress might affect the microbial squatters that can boost health or trigger disease.

Gururajan and Bailey both caution, however, that it’s too early to know if social stress affects gut microbes in people the same way it does in hamsters. Still, no one benefits from bullying. And if people respond as hamsters do, avoiding bullying may not only help you keep your friends — but also your gut-friendly microbes.

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