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.
“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.
aggressive (n. aggressiveness) Quick to fight or argue, or forceful in making efforts to succeed or win.
animal model A nonhuman animal used to stand in for people in research testing. Which animal a lab uses will depend on how closely parts of its body or chemical-signaling systems match those in people.
bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).
behavior The way something, often a person or other organism, acts towards others, or conducts itself.
bug The slang term for an insect. Sometimes it’s even used to refer to a germ.
bullying (v. to bully) A group of repeated behaviors that are mean-spirited. They can include teasing, spreading rumors about someone, saying hurtful things to someone and intentionally leaving someone out of groups or activities. Sometimes bullying can include attacks using violence (such as hitting), threats of violence, yelling at someone or abusing someone with violent language. Much bullying takes place in person. But it also may occur online, through emails or via text messages. Newer examples including making fake profiles of people on websites or posting embarrassing photos or videos on social media.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
control A part of an experiment where there is no change from normal conditions. The control is essential to scientific experiments. It shows that any new effect is likely due only to the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of it to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that gives scientists something against which they can compare their experimental data.
depression (in medicine) A mental illness characterized by persistent sadness and apathy. Although these feelings can be triggered by events, such as the death of a loved one or the move to a new city, that isn’t typically considered an “illness” — unless the symptoms are prolonged and harm an individual’s ability to perform normal daily tasks (such as working, sleeping or interacting with others). People suffering from depression often feel they lack the energy needed to get anything done. They may have difficulty concentrating on things or showing an interest in normal events. Many times, these feelings seem to be triggered by nothing; they can appear out of nowhere.
digest (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.
digestive tract The tissues and organs through which foods enter and move through the body. In people, these organs include the esophagus, stomach, intestines, rectum and anus. Foods are digested — broken down — and absorbed along the way. Any materials not used will exit as wastes (feces and urine).
DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
feces A body's solid waste, made up of undigested food, bacteria and water. The feces of larger animals are sometimes also called dung.
graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).
gut An informal term for the gastrointestinal tract, especially the intestines.
hormone (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body.
hypothesis (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
microbiome The scientific term for the entirety of the microorganisms — bacteria, viruses, fungi and more — that take up permanent residence within the body of a human or other animal.
microbiota The microorganisms that live in a particular place or geological period. Scientists call the entirety of the microorganisms in a human or other animal its microbiome .
microscopic An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.
nervous system The network of nerve cells and fibers that transmits signals between parts of the body.
neuroscience The field of science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.
probiotic A beneficial bacterium that is found in food or can be added to the diet. It can fight bad germs in the body or perform functions, such as producing vitamins, that support human health.
rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.
social (adj.) Relating to gatherings of people; a term for animals (or people) that prefer to exist in groups. (noun) A gathering of people, for instance those who belong to a club or other organization, for the purpose of enjoying each other’s company.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
stress (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative.
subordinate Someone or something that is lower in rank, based on power, importance, value or some other characteristic.
tract A particular, well-defined area. It can be a patch of land, such as the area on which a house is located. Or it can be a bit of real estate in the body. For instance, important parts of an animal’s body will include its respiratory tract (lungs and airways), reproductive tract (gonads and hormone systems important to reproduction) and gastro-intestinal tract (the stomach and intestines — or organs responsible for moving food, digesting it, absorbing it and eliminating wastes).
trillion A number representing a million million — or 1,000,000,000,000 — of something.
Journal: K.A. Partrick et al. Acute and repeated exposure to social stress reduces gut microbiota diversity in Syrian hamsters. Behavioural Brain Research. Vol. 345, June 1, 2018, p. 39-48. doi: 10.1016/j.bbr.2018.02.005.