The Link Between Gut Microbes and Your Pet's Brain

It might seem like your pet is led by their stomach: begging at the table for bits of food and always eager for treats. Researchers are discovering that the idea might not be that far off for certain behavior.

Animals are swarming with good bacteria, viruses, and other microorganisms that keep them healthy. Microbes, particularly those in the gut microbiome, help digest nutrients and protect against pathogens. Studies suggest that gut microbes can influence allergies, intestinal disease, or even cancer. And now links between the brain and the gut are coming to light, hinting that the bacterial contents of your pet's intestines could influence their aggression and anxiety.

Gut Microbes & the Brain

The brain has an intricate network designed to control and communicate with an animal’s body. Signals travel from the brain to initiate functions such as movement or emotion. And they also travel to the brain to simulate sensations, such as touch, sight or hearing.

Evidence has surfaced that suggests signals to the brain can originate from the gut¹.

In general, bacteria consume nutrients, and they release chemicals into their environment. They might release things like a by-product of a processed nutrient — such as short-chain fatty acids from fiber² — or a toxin designed to kill competing microbes. Some of these chemicals, however, are compounds that can have effects on the brain.

Dopamine, serotonin, and GABA, for instance, are chemical messengers called neurotransmitters that control animal brains³. Dopamine is often referred to as the “feel-good hormone,” because of its well-studied role in reward-motivated behavior. GABA, on the other hand, blocks signals between neurons and represses the activity of other neurotransmitters.

Scientists have long known that bacteria produced some of . But researchers did not understand what that meant in the context of the brain until when a study was published showing that bacteria might be related to stress and anxiety in mice⁵.

The team looked at germ-free mice — which are completely free of microbes — and found that they had stronger responses to stress than mice that still had bacteria in their guts. But these differences disappeared when the researchers put certain bacteria in the intestines of germ-free mice.

Could this mean that the bacteria were somehow linked to stress in the mice? Could gut bacteria affect mental health?

From Mice to Humans to Pets

In the years since, researchers have been exploring the potential link between gut microbes and the brain, called the gut-brain axis. Anxiety, depression, Alzheimer’s disease, autism, behavior, fatigue, and mood are just some of the myriad disorders and mental states that appear to be connected to bacterial companions in the gut^1,6.

Animal models provide the bulk of the evidence. Germ-free mice, for instance, have been instrumental in tying anxiety-like and depressive-like behaviors to gut microbes⁷. One study found that germ-free mice were less anxious than those that had a gut microbiome⁸. Others have shown that neonatal stress in rats has a long-term impact on the gut microbiome, and this changed their response to stress later in life^9,10.

Studies have now been done in humans as well and found similar links among gut microbes and neurological disorders¹¹. For example, people who had their vagus nerve — a key nerve in the central nervous system’s control over the digestive tract — cut at a young age had a lower risk of Parkinson’s disease¹². But scientists still don’t fully understand how the gut microbiome interacts with the brain. Microbes appear to communicate with the brain through various metabolites, components of the immune system and the vagus nerve¹¹. Even gut cells themselves have been identified that have a direct line to the brain via the vagus nerve¹³. And stimulating the vagus nerve is rewarding for the brain, which can treat depression¹⁴. Researchers don’t yet know whether gut bacteria have a direct impact on the brain — and cross the blood-brain barrier — or if they are missing another key piece to the puzzle.

Recently, some evidence for the same thing occurring has emerged in pets. A study published in PeerJ found that seized from a dogfighting organization were more likely to be aggressive based on their gut microbiomes¹⁵. The researchers looked at the gut bacteria from 31 dogs, 21 of which displayed aggression toward other dogs and 10 which did not. They discovered some bacteria were lower in aggressive dogs, such as Bacteroides and Dorea, and some were elevated, such as Lactobacillus. The genus bacterium was higher in non-aggressive dogs.

While the sample size is somewhat small, the data suggest that gut microbes are linked to canine aggression and could help to diagnose canine aggression before it becomes a problem. Exactly how the bacteria might contribute to aggression is unclear, since the study relied on 16S sequencing. Future studies can help corroborate the findings and begin to explore specific roles for various species.

Manipulating the Gut Microbiome

Just as probiotics could help with digestive health, scientists are exploring whether something similar could be used for diseases linked to the gut-brain axis¹⁶. Introducing bacteria, called , into the gut to help alleviate symptoms might help treat people (or pets) with anxiety or depression. For instance, studies have shown that giving Lactobacillus or Bacteroides to mice can decrease anxiety-like behavior^17,18. Human research, too, has found that probiotics enhance mood¹⁹. Those who drank milk with Lactobacillus reported that their mood had improved at the end of the trial. While is not a medical condition, such studies provide a foundation for researchers to ask new questions about moving forward with possible treatments for depression and anxiety.

When combined with the knowledge that bacteria produce chemicals that can have effects on the brain¹⁶, it’s possible that could be a novel way to improve mental health. And as scientists learn more about the links between gut microbes and the brain, are sure to improve.

Closing Remarks

Many of the links between the brain and gut microbiomes are preliminary, but the idea still presents a fascinating avenue of research. Could we manipulate the gut microbiome to treat depression and anxiety? Can your pet’s behavior improve with the use of targeted ? These are some of the questions microbiologists are striving to answer.

References
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