BODY FUNCTIONSBRAINMICROBES Belly bacteria can shape … · Those microbes poop out waste products...

10/29/2018 Belly bacteria can shape mood and behavior | Science News for Students

https://www.sciencenewsforstudents.org/article/belly-bacteria-can-shape-mood-and-behavior 1/11

BY BETHANY BROOKSHIRE JUN 7, 2018 — 6:45 AM EST

BODY FUNCTIONS BRAIN MICROBES

Belly bacteria can shape mood and behaviorConversations between the brain and gut may influence stress, memory and more

You may not notice, but your guts and brain are in constant communication. And from what scientists have beenlearning from rodent studies, those chats between gut microbes and the brain can have a whole range ofimportant, hidden impacts.tommaso79, Dr_Microbe/iStockphoto; Adapted by L. Steenblik Hwang

When Margaret Morris goes to the grocery store, people ask if she’s throwing a party. Her cart isfilled with French fries, cheesecakes, meat pies and other tasty treats. “I snoop around looking forspecials,” she says. “I spend a lot of money on food.”

Morris is a neuroscientist — someone who studies the brain. She works at the University of NewSouth Wales in Sydney, Australia. And yes, she is throwing a party. But her guests aren’t people.This fastfood feast is destined for her lab rats. After a few weeks of all the junk food they caneat, Morris and her colleagues run the rodents through a series of tasks, testing the limits of theirlearning and memory.

By letting rats chow down on these foods, Morris is trying to find out how a junkfood diet(popular among people, too) impacts behaviors. She studies what’s called the gutbrain axis. It



Margaret Morris feeds her rats fun partyfoods. She’s shown that socalled “junk foods”alter their microbiome — and can impair theanimals’ memories.CATHYKEIFER/ISTOCKPHOTO

Explainer: What is theVagus?(https://www.sciencenewsforstudents.org/article/explain

what-vagus)

refers to the ongoing conversation taking placebetween the brain and gut. Because of thischatter, our innards — and the microbes living inthem — can affect how we think and behave. Ourbrain, in turn, can talk back to our stomach andintestines and their bacterial inhabitants.

By studying how residents of our gut influenceour brain, Morris and other scientists seek to findout just how much you are what you eat. Theirresults may one day enable us to change ourfeelings and behaviors — all with the right mix offoods and microbes.

From gut to brain

It’s no surprise that our brain sends signals toour gut to control digestion and other tasks. Thebrain sends its orders via the vagus (VAYgus)nerve. This long structure wanders from the verybase of the brain down to the gut. Along the way,it touches many other organs. The brain makes hormones — chemical signals that it drips into thebloodstream. These, too, flow to the gut. Both the vagus nerve and hormones can signal hungerand fullness. They can control, too, how quickly food moves through us.

But the gut doesn’t just listen. It also talks back.

Microbes inside our stomach and intestines help break down food. Those microbes poop out wasteproducts that can themselves serve as chemical messengers. These waste molecules can trigger acascade of signals throughout the rest of the body.

Some microbial crosstalk prompts stomachlining cells to send chemical textmessages to theimmune system. This can protect us from infection. Some microbes shoot molecular signals backup the vagus nerve. Others pump messages — hormones — into the bloodstream, from whichthey’ll travel to the brain. Those hormones can affect everything from memory skills to mood.

Scientists didn’t always know about the gut’s back chatter to the brain. Its role began to emergewhen they started digging into the microbes in, well, poop.

“When I was a student in the 1960s and 1970s, I thought everything was known,” says Mark Lyte.He’s a microbial endocrinologist — someone who studies microbes and the hormones they release— at Iowa State University in Ames. “Who considered that the poop you took in the morning was going to be such a hot topic? Poop was poop!”

Lyte was interested in microbes in the gut that might cause infection. But he soon realized thatsome gut bacteria were sending messages. Those signals consisted of chemicals that looked veryfamiliar. These germs were producing some of the same molecules used when one brain cell



The stomach, small and large intestines (andmore) get signals from the brain. But the gutisn’t just listening, it’s also talking back.CHOMBOSAN/ISTOCKPHOTO

Explainer: What isneurotransmission?(https://www.sciencenewsforstudents.org/article/explain

what-neurotransmission)

Scientists Say:Hippocampus(https://www.sciencenewsforstudents.org/blog/scientists-

say/scientists-say-hippocampus)

communicates with another — a process knownas neurotransmission.

“These are the same things we have in our brainsand [that our brain cells] use for communication,”he says. Soon, he wondered: “Could it be thatbacteria are communicating with us all the time?”

As scientists learned to listen in on that bacterialchatter, they discovered he was right. The brainand gut send constant cascades of notes backand forth, more than any social media. And thatpeaceful communication serves a critical purpose,Lyte says. “You have trillions of bugs in your gutand you rely on them for a lot of your nutrients.But they rely on you to sustain themselves,” hesays. “They need to communicate with you. Andyou need to communicate with them.”

Tummy trouble

Exactly what the messages say depends on who’ssending them. A gut filled with fruits andvegetables will house a different set of microbesthan one used to a diet of chips, soda and otherjunk foods. And the messages sent by thosedifferent sets of gut microbes may affect ourbrains differently.

This is where Morris’ rat parties come in.

After two weeks on a junkfood diet replete withcakes and fries, her lab rats take a memory test.Each rodent investigates a space filled withobjects. Then, after the rat leaves, Morris and hercolleagues move some of the objects around. Thenext day, they put the rat back into the space. Ifit notices a change in the furnishings, it willspend more time sniffing around the objects thathad moved.

Tests like this one rely on an area of the braincalled the hippocampus (there are two in eachbrain). These regions are very important forlearning and memory. But after a few weeks ofdowning junk food, a rat’s hippocampi no longerwork so well. The animals don’t seem to recognize which objects have been moved as well as dorats that noshed on healthy foods.

Could this be because of their gut bugs?



This is one method scientists use to test ananimal’s memory. The rat (or mouse) plays ina chamber with two objects (marked here bysmall boxes). Later, it returns to the samechamber, but one of the objects has moved.Does the rat notice?BARKER ET AL/J. OF NEUROSCIENCE 2007

Rats that dine on fast food have a less diversegroup of microbes in their guts, Morris and hergroup find. But their gut diversity returned whenthe scientists gave the junkfoodeating animals ahigh dose of a probiotic — a mix of beneficial gutbacteria. Their memory also improved.

Morris and her colleagues published their findingsin the March 2017 Molecular Psychiatry.

Gut feelings

What we put in our guts can affect our brains,but our brains also affect our guts. Indeed, Lyte’sgroup has shown that when mice go throughstressed times, their gut microbes feel the

pressure.

The scientists put mice into tight little tubes that restrained their movements. And the rodentsdidn’t like that very much. The stress they felt during this restraint test altered the share ofcertain bacteria in their gut. The new populations in the stressedout mice weren’t as diverse asthose in the relaxed mice. Guts in the stressed animals also were more susceptible to bacteria thatcould make the mice sick, the researchers found.

Social stress — such as being bullied — also can change bacteria living in animals’ guts, notes JiahPearsonLeary. He’s a neuroscientist in Pennsylvania at Children’s Hospital of Philadelphia. There,he bullies rats to see how it affects their guts.

Rats are social. They like to be around other rats. But they are not big fans of new roommates.So when a big rat “owns” its own cage, it doesn’t want some small newcomer rat moving in. If asmaller rat is foolish enough to approach him, the bigger rat will beat him up. When the same ratexperiences this bullying over and over, it can become very stressed. It’s known as social defeatstress. Some beaten rats may even show signs of depression. They might avoid socializing withothers. They may even lose interest in sweet rat treats.

Not all rats respond the same way, however. Some handle the stress better than others. They cantake longer before breaking under the strain. PearsonLeary has now shown that those who sufferthe most also tend to have very different gut bugs than do the rats who can better stand thepressure.

PearsonLeary took poop from rats that got stressed quickly and fed it to unstressed rats. (Ratsoften eat each other’s feces, he notes, so this isn’t too weird. “They enjoy it, I think.”) Poop willalways contain some of an animal’s gut microbes. So the unstressed rats also got a taste of thestressed rats’ gut microbes. “We just wanted to see how much of the behavior [might also] betransferred,” he explains.

After eating a poopshake made of a stressed rat’s feces, formerly chill rats now got stressedmuch faster when bullied. They showed depressed behavior more quickly, too.

PearsonLeary shared his team’s findings, last November, at the Society for Neuroscience meetingin Washington, D.C.

An intestinal ecosystem



They look quiet, but the bugs in your gut arepumping out a constant chemicalcommentary. Scientists have begun listeningin. And from those conversations, they’relearning a great deal about how the gut andthe brain coordinate.JULIE6301/WIKIMEDIA COMMONS (CC BY3.0

(https://creativecommons.org/licenses/by/3)

Scientists Say:Microbiome(https://www.sciencenewsforstudents.org/blog/scientists-

say/scientists-say-microbiome)

Clearly, our guts are not just a simple tube intowhich we send food. They are remarkablycomplex ecosystems. Their inhabitants are part ofwhat’s known as our microbiome (MykrohBYoam) — a community of microbes that live in andon us. In the gut, the star players are bacteriathat feed on the buffet flowing past them. Thesemicrobes don’t have an easy life, though. Theyare constantly fending off attackers.

That’s because our guts are home not just tobacteria, but also to viruses — tiny particlesmade of genes and coated in proteins. Thoseviruses hijack the cells they infect. They then turnthese cells into factories that pump out morevirus.

In the gut, most of this sinistersounding activityisn’t aimed at us. “Percentagewise, probably 95percent of the viruses are bacteriophages,”explains Veronica Peterson. She is aneuroscientist at University College Cork inIreland. Bacteriophages (BakTEEReeohfaajuhs) are viruses. They have no interest in us.They have come to infect the microbes hangingout within us.

Think of those viruses as predators hunting theirmicrobial prey in the savanna of your intestines.

Scientists had long known these viruses werepresent. They even have a name for thiscommunity: the virome (VYroam). But “we knewabsolutely nothing,” Peterson recalls, about whether the viruses were affected by our behavior —or might affect us.

She and her colleagues decided to probe the role of those viruses. They now report finding thatstress can shake up the bacteria in mousey guts.

And that’s not all. “After stress, there’s an increase [in] viral diversity, a lot of new types ofviruses,” Peterson reports.

Greater viral variety meant gut microbes are under more strain. No surprise, the types of bacteriain stressed mice went down. That indicates “that most of the viruses are bacteriophages, killing offbacteria,” Peterson says. Dead bacteria no longer send signals — positive or otherwise — to thebrain.

She, too, shared her findings at last November’s meeting of the Society for Neuroscience.

Gut bacteria and viruses have a third set of companions: fungi. “These are singlecelled fungi, likeyeasts,” notes PearsonLeary. (There are not mushrooms growing in your stomach. “Though thatwould make for a great image,” he says.) Those fungi might secrete signals that impact our



Some brands of yogurt boast that theycontain probiotics. But the benefits of thesebacterial boosts are not well established yet.Scientists have been testing the effects ofdifferent kinds of bacteria on mood. In thosetests, though, they usually sprinkle themicrobes in milk.YAKOBCHUKOLENA/ISTOCKPHOTO

immune systems — even our behavior. Right now, though, scientists know almost nothing aboutthem. Clearly, there still is a lot to learn about the ecosystems within us.

Psychology + microbiome = psychobiotics?

While all this research on mice and rats is intriguing, rodents aren’t people. Scientists can’t yetsay exactly whether and how gut inhabitants might impact our minds. But that hasn’t stoppedthem from working to tweak these microbes to our benefit.

The work has already spawned cool buzzwords. There’s probiotics — live microbes that can offerhealth benefits when ingested by people. John Cryan has coined a term for probiotics that affectthe brain: “psychobiotics” (SYkohbyOTTik). These would be live microbes able to changesomeone’s behavior.

Like Peterson, Cryan is a neuroscientist atUniversity College Cork. Psychobiotics, he says,“bring a rational way of offering a treatment.”Human cells are controlled by human genes,whose sets of instructions we can’t readilychange. But growing new bacteria or getting ridof old ones? That’s a (relative) cinch. “Unlike yourgenome, your microbiome can be changed. Andyou can change it through your diet,” he explains.

Cryan’s lab has been studying how themicrobiome affects behaviors — in mice and inpeople. His group is especially interested in onebug, Bifidobacterium longum (BIFFihdohbakTEEReeuhm LAWNGuhm). It is a rodshapedgut bacterium. In animals, it reduces signs ofstress.

What about in people?

To find out, Gerard Clarke handed out bacteria to22 healthy men. (Clarke works with Cryan atUniversity College Cork.) The microbes are “in asachet of powder,” he explains. “You tip it intoyour milk and you drink it.”

Later, the men performed a bunch of tough tasks. They plunged their hands in icy cold water foras long as they could. This is a painful test of enduring physical stress. The men also performedlearning and memory tests. Afterward, these volunteers answered questions about how stressedthey felt.

After four weeks of drinking bacterialaced milk, “they reported feeling less stressed,” Clarke says.The men also could now endure the icecold water for longer periods. They even did slightly betteron memory tests. Clarke and Cryan published their results 18 months ago in the journalTranslational Psychiatry.

We’re not there yet



Classroom questions(https://www.sciencenewsforstudents.org/questions/ques

belly-bacteria-can-shape-mood-and-behavior)

It might seem like the psychobiotic era is already here. “You can go into a healthfood store andbuy any sort of probiotic you want,” notes Cryan. “And you’ll even see some saying ‘good for yourbrain’ or ‘good for your stress.’” But don’t get too excited just yet, he warns. There’s not a singleshred of data to justify that those products on store shelves do anything at all, he notes.

But that could change. The first studies in people are currently underway.

Still, he cautions, people should never place all of their hopes on a single bacterial “silver bullet.”There are plenty of natural ways to help gut bugs flourish. A healthy diet early in life (with, as hesays, “lots of fruits and veg”) can help support a diverse microbiome. And those microbes can,hopefully, carry on cozy chats with your brain.

But keep in mind, not all beneficial bacteria mayaid everyone or be good for everything. Forinstance, the probiotics given rats to which Morrishad fed junk food saw a boost in one type ofmemory. In another task, however, the probioticshurt their performance. And that didn’t change nomatter what diet the rats had been eating. Sothere’s no one gut bug to rule them all.

Psychobiotics still need more science to back them up. But a healthy diet? That’s got plenty ofscience behind it.

“Am I taking a probiotic?” asks Morris. “No. Do I try to eat five servings of vegetables a day?Yes,” she says. “I think that’s the way to do it.”

Power Words(for more about Power Words, click here (https://www.sciencenewsforstudents.org/powerwordsaidstemliteracy) )

bacteria (singular: bacterium) Singlecelled organisms. These dwell nearly everywhere on Earth,from the bottom of the sea to inside other living organisms (such as plants and animals).

bacteriophage Also known simply as a phage. This is a type of virus that infects — and ultimatelykills — bacteria, but not before reproducing and spreading.

behavior The way something, often a person or other organism, acts towards others, or conductsitself.

bug The slang term for an insect. Sometimes it’s even used to refer to a germ.

cell The smallest structural and functional unit of an organism. Typically too small to see with theunaided 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.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion andstructure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom.Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials thatare the result of various reactions between different compounds.



chemical signal A message made up of molecules that get sent from one place to another.Bacteria and some animals use these signals to communicate.

colleague Someone who works with another; a coworker or team member.

constant Continuous or uninterrupted.

diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow andmaintain health.

diversity A broad spectrum of similar items, ideas or people. In a social context, it may refer to adiversity of experiences and cultural backgrounds. (in biology) A range of different life forms.

ecosystem A group of interacting living organisms — including microorganisms, plants and animals— and their physical environment within a particular climate. Examples include tropical reefs,rainforests, alpine meadows and polar tundra.

endocrinologist A doctor who specializes in conditions affecting the production of hormones or thebody’s response to hormones.

feces A body's solid waste, made up of undigested food, bacteria and water. The feces of largeranimals are sometimes also called dung.

function A relationship between two or more variables in which one variable (the dependent one)is exactly determined by the value of the other variables.

gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of aprotein. Offspring inherit genes from their parents. Genes influence how an organism looks andbehaves.

genome The complete set of genes or genetic material in a cell or an organism. The study of thisgenetic inheritance housed within cells is known as genomics.

gut An informal term for the gastrointestinal tract, especially the intestines.

gutbrain axis The ongoing chemical signaling system, or conversation, between the gut and brain.The brain sends signals to the gut via nerves and hormones. The gut then responds, sending signalsback up the nerves, releasing communicating chemicals or releasing hormones of its own. The gutbrainaxis is how our guts — and the organisms in them — can affect behaviors.

hippocampus (pl. hippocampi) A seahorseshaped region of the brain. It is thought to be thecenter of emotion, memory and the involuntary nervous system.

hormone (in zoology and medicine) A chemical produced in a gland and then carried in thebloodstream to another part of the body. Hormones control many important body activities, such asgrowth. Hormones act by triggering or regulating chemical reactions in the body.

immune system The collection of cells and their responses that help the body fight off infectionsand deal with foreign substances that may provoke allergies.

immunity The ability of an organism to resist a particular infection or poison by providing cells toremove, kill or disarm the dangerous substance or infectious germ. Or, when used colloquially, itmeans the ability to avoid some other type of adverse impact (such as firing from a job or beingbullied).

infection A disease that can spread from one organism to another. It’s usually caused by sometype of germ.



journal (in science) A publication in which scientists share their research findings with experts (andsometimes even the public). Some journals publish papers from all fields of science, technology,engineering and math, while others are specific to a single subject. The best journals are peerreviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here,is to prevent the publication of mistakes, fraud or sloppy work.

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 singlecell.

microbiome The scientific term for the entirety of the microorganisms — bacteria, viruses, fungiand more — that take up permanent residence within the body of a human or other animal.

nerve A long, delicate fiber that transmits signals across the body of an animal. An animal’sbackbone contains many nerves, some of which control the movement of its legs or fins, and some ofwhich convey sensations such as hot, cold or pain.

neuroscience The field of science that deals with the structure or function of the brain and otherparts of the nervous system. Researchers in this field are known as neuroscientists.

nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organismrequires as part of its food in order to survive.

organism Any living thing, from elephants and plants to bacteria and other types of singlecelledlife.

population (in biology) A group of individuals from the same species that lives in the same area.

predator (adjective: predatory) A creature that preys on other animals for most or all of its food.

pressure Force applied uniformly over a surface, measured as force per unit of area.

prey (n.) Animal species eaten by others. (v.) To attack and eat another species.

probiotic A beneficial bacterium that is found in food or can be added to the diet. It can fight badgerms in the body or perform functions, such as producing vitamins that support human health.

protein A compound made from one or more long chains of amino acids. Proteins are an essentialpart of all living organisms. They form the basis of living cells, muscle and tissues; they also do thework inside of cells. Among the betterknown, standalone proteins are the hemoglobin (in blood) andthe antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latchingonto proteins.

psychiatry (adj. psychiatric) A field of medicine where doctors study and treat diseases of thehuman mind. Treatments may consist of talking therapies, prescription drugs or both. People who workin this field are known as psychiatrists.

psychology (adj. psychological ) The study of the human mind, especially in relation to actions andbehavior. To do this, some perform research using animals. Scientists and mentalhealth professionalswho work in this field are known as psychologists.

psychobiotic A microbe or mix of microbes that can benefit mental health, such as someone’smood or memory.

rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs,hamsters and porcupines.

BODY FUNCTIONSBRAINMICROBES Belly bacteria can shape … · Those microbes poop out waste products...

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