Researchers generally group gut bacteria into six different categories, with varying amounts of these microbes residing in different parts of the digestive tract.

There are up to 100 trillion microbes in the gut, and maintaining the relative balance in this ecosystem is important for our health.

Trillions of bacteria, viruses, fungi, protists, and archaea live in our guts, producing a wide range of molecules, hormones, and more—underscoring the immune system's pivotal role in the microbiota-gut-brain axis.

In 2026, University of Cambridge researchers reported that a group of gut bacteria called CAG-170 may be a key marker of good health, occurring far more often in healthy individuals and less commonly in people with chronic diseases.

What is the gut microbiome?

The gut microbiome is a microscopic ecosystem that runs off the work of microbes in the digestive tract. These workers make getting energy from food possible. What and how that energy manifests depends on what building blocks these microbes have to work with, meaning their composition and functions are strongly influenced by diet. Gut microbes have their own genes, which affect how a person's genes act.

The gut microbiome—a complex community of up to 100 trillion microbes—has a changing composition, influenced by our life stage, diet, medications, environment, and other factors.

Some animals, including certain tropical ants, have little or no microbial populations in their gut, upending the idea that all animals rely on this ecosystem to help maintain their health and proper functioning.

A technical review of what researchers know about the gut microbiome

This overview from the National Institutes of Health provides a deeper (but still accessible) look at the different bacteria in the gut, how the microbiome develops through life, its purpose, and more.

Gut bacteria are evolving and can acquire as many as 100 new genes per year, on average.

How the microbiome affects your health

The gut microbiome is so influential to overall human health that some researchers consider it a separate organ. Its conditions, influenced by environmental and lifestyle factors such as medication and exercise, contribute to immunity, metabolism, inflammation, medical treatment efficacy, brain development and function, and the development of disorders and diseases, including asthma, depression, inflammatory bowel disease, and Alzheimer's.

What happens when the gut microbiome is out of whack?

An imbalance in gut microbiome flora can lead to a condition known as dysbiosis. This condition is typically characterized by a drop in microbiota diversity and an increase in harmful unicellular and acellular organisms, such as viruses, that can negatively impact the host's immune system and overall health. Genetics, infections, dietary choices, antibiotics, food additives, artificial sweeteners, and other lifestyle and environmental factors can cause dysbiosis.

Farts are tiny status reports from our gut microbiome, with American adults typically passing gas more than 30 times daily.

Gut microbiome experiments involving transfers of gut microbes from depressed people to rodents indicate that the rodents may then display depression-like symptoms.

Bacteria in our intestine influence the brain via the gut-brain axis.

What is a microbiome (and why are they important)?

Jump to: What is the microbiome? How microbiota benefit the body The role of probiotics Can diet affect one’s microbiota? Future areas of research What is the microbiome? Picture a bustling city on…

What's next for gut microbiome science?

A Stanford microbiologist explores what we don't yet know about the gut microbiome—such as whether we can engineer the bacterial species within it—and the current scientific advances in the field.

The gut microbiome's community of viruses is collectively known as the virome, but most of its viruses remain unidentified.

Corynebacterium species are particularly responsible for our funky smell

Myriad bacteria contribute to our body odor, but research suggests that Corynebacterium species are strongly associated with the bad smells emanating from adults. Staphylococcus and Cutibacterium genera may also play leading roles.

Listen to how the gut microbiome influences your emotions

The gut-microbiome connection may involve the enteric nervous system, a collection of hundreds of millions of neurons throughout the gut that transmit signals up the vagus nerve to the brain.

Scientists have found that the Great Barrier Reef's microbiome includes an estimated 362,802 distinct viruses and 876 distinct bacterial and archaeal species—584 of which had never been documented before 2026.

Microbes are essential to creating the many varieties of cheese, influencing texture and taste

Microbes break down proteins and fats and produce compounds with distinct tastes. These microbes can include bacteria (like the one used to give Swiss its trademark holes), molds (like the ones that give blue cheese its distinctive veining), and countless other tiny organisms.

What constitutes a 'healthy' diet?

A natural diet, typically high in plant-based foods and low in processed foods, appears to be the main contributor to a healthy, diverse gut microbiome.

Visualize the different parts of the digestive tract, a roughly 30-foot tube which runs from our mouths to our rear.

Probiotics, which are living bacteria and yeasts, are not recommended for most patients.

Exercise causes diet-independent changes in the gut microbiome.

Bacteria use chemical signals to communicate and coordinate with one another

Through a system called quorum sensing, bacteria produce and absorb compounds from their environment to identify allies or competitors and adjust their behavior accordingly. Some bacteria produce food-digesting enzymes only in the presence of similar bacteria, thereby preventing competitors from consuming their meal.

Specific gut microbiome 'fingerprints' are associated with cancer recurrence, with up to 94% accuracy.

Fecal transplants can help treat infections by transplanting healthy gut bacteria from a donor into a patient's gastrointestinal tract using samples of human waste, which are turned into a liquid mixture and often delivered via a colonoscopy procedure.

Some dementia cases may be linked to treatable microorganism infections in the brain.

Western babies often lack a gut microbe common elsewhere.

See how the gut and the brain communicate through a bundle of nerves running from the brain to the digestive tract, serving as both a physical and chemical link.

Lining up every microbe on Earth would stretch to about 105,000 light-years

There are about 10²⁷ such organisms on the planet, many of which are adapted to produce energy through uncommon reactions in unconventional environments, such as within rocks or near volcanoes on the ocean floor. Studies of these organisms have reframed astrobiologists' search for life beyond Earth by broadening the parameters under which life can exist.

Researchers are investigating how altering the gut microbiome may influence the development of rheumatoid arthritis, which is an autoimmune disease.

Can the gut microbiome drive addiction?

New research suggests an imbalance of gut microorganisms may influence the likelihood of addiction and addictive behaviors by increasing the severity of withdrawal symptoms, triggering brain inflammation in regions of impulse control, and more.

Scientists seeking gut insights are attaching flatulence sensors to volunteers' underwear

This fart-tracking device detects gas generated in the gut, providing insights about gut microbial activity. The underwear sensors measure a component of what is technically called flatus—digestive system gas.

Fungi can cause disease, but they’re also our planet’s key decomposers

Fungi are their own taxonomic kingdom separate from flora and fauna, acting as the key decomposers and nutrient recyclers for our planet. Watch this video to understand the diverse types of fungi and the stages of the fungal life cycle. This video illustrates how fungi species may spread, eat nematode worms, damage nearby plants, and much more.

Gut microbiome rewrites may help reduce allergies

Researchers are investigating different strategies to determine if one day they could give people a combination of gut microbes to set them up for fewer allergies. In animal research, scientists have already prevented severe allergic responses in allergy-prone mice—supplying them with gut microbes from healthy, non-allergic human babies.

Vitamin B1 is linked to gut motility—helping fight constipation—in 2026 research

A study published in early 2026 found a link between vitamin B1 and the successful movement of food through your GI tract, known as gut motility. The work incorporated genetic and survey data about poop frequency to identify genes associated with gut motility and then separately linked poop frequency data with how much vitamin B1 people ingested, theorizing that the vitamin supports the production of acetylcholine, a neurotransmitter that stimulates GI system contractions.

View a breakdown of the identified species on Earth, led by about one million insect species, while mammals such as humans make up one of the smallest groups.

The reason some people love exercise may be because of their microbiome

Certain gut microbes in mice trigger dopamine release and drive motivation to exercise more. Scientists are investigating whether humans carry similar microbial patterns that influence our natural inclination towards movement, exploring how unseen microbial ecosystems may shape mammalian fitness behavior and reinforce innate reward pathways. (Some users may experience a paywall.)

'Gram-negative' bacteria: Bugs with extra defenses along their cell walls

The term 'Gram-negative' bacteria refers to bugs which react a certain way under a Gram stain test. These bacteria are harder to kill due to their double-layer cell walls with a thin peptidoglycan layer in between. This structure makes the delivery of drugs into the bacterial cell substantially more difficult.

The gut microbiome may play a major role in autoimmune disease and symptom flare-ups

Certain unhealthy foods can promote gut bacterial growth and inflammation in the lining of the gut, potentially disrupting its lining—leading to a leaky gut that may allow pathogens to dribble out into the bloodstream, fueling further inflammation that can both cause disease flare-ups and set the stage for future disease development.

A macrophage can eat 100 bacteria, on average, before it's exhausted

These specialized immune cells are first responders during inflammation, responding to a threat or injury and then performing other inflammatory response tasks, including summoning neutrophils to help attack invaders.

How humans digest food

Every day, the average human eats close to six pounds of food—all of which must be processed by the digestive system. Ten organs collaborate to transform food that the mouth chews into the energy needed to stay alive. (Stretched out, these organs...

Cells are categorized by the presence of contained genetic material and other organelles

While prokaryotes contain one or more cells with free-floating genetic material in a region called the nucleoid, eukaryotes possess a nucleus specialized for DNA storage. Plant cells possess cell walls—a reinforced structure surrounding their membrane—made of cellulose, while fungal cell walls contain chitin. Animal cells have lysosomes housing enzymes that break down biomolecules. (Some readers may experience a paywall.)

Model organisms like yeast, mice, and flies help scientists decode the human body

Some species are used to quickly study the effects of human diseases and treatments because they are short-lived and have similar systems to those found in humans. Bacteria, like E. coli, have been used as human proxies given their rapid growth and ease of genetic manipulability.

Every virus is classified into 7 groups based on genetic differences

Viruses infecting humans have been named based on symptoms ("chickenpox") or the location or animal in which they were first discovered ("Bolivian Haemorrhagic Fever" and "Monkeypox"). For clarity among scientists and to reduce stigma, virologists classify the 15,000 known viruses (270 of which infect humans) by their basic genetic material.

Lengthy incubation periods are why it's tough to link foodborne illness to specific foods.

Bacteriophages—viruses that infect bacteria before multiplying within them—exist on Earth in numbers greater than all other living organisms combined.

One in six Americans contracts food poisoning each year.

Some organelles are believed to have once been independent cells

Mitochondria and chloroplasts, which are both responsible for energy production in cells, have DNA distinct from that of their host cells and are surrounded by cellular membranes resembling those of bacteria. Through symbiogenesis, these and other specialized structures were absorbed into larger cells, forming the modern-day biological units.