The human body is home to trillions of microorganisms, including bacteria, fungi, viruses, and other microscopic life forms. Many live on the skin, in the mouth, and throughout the digestive tract. Collectively, these microbial communities are known as the human microbiome.
Far from being accidental passengers, these organisms can influence digestion, immune function, metabolism, and the chemical environment of the body. At the same time, the microbiome is not simply a collection of “good” bacteria. It is a complex ecosystem whose effects depend on which organisms are present, what they are doing, and the conditions around them.
Understanding the microbiome helps explain why human health is shaped not only by our own cells and genes, but also by the microbial communities that live with us.
What exactly is the human microbiome?
The term microbiome generally refers to the community of microorganisms living in a particular environment, along with the genetic material and biological activities associated with that community. In discussions of human health, it usually means the microorganisms and their collective genetic information that inhabit the human body.
The organisms themselves are sometimes called the microbiota. The distinction is useful: microbiota refers primarily to the organisms, while microbiome is often used more broadly to include their genes and the ecosystem they form. In everyday scientific writing, however, the two terms are frequently used somewhat interchangeably.
The human microbiome includes several major groups:
- Bacteria, which make up much of the microbial population studied in the human body
- Archaea, single-celled organisms distinct from bacteria
- Fungi, including yeasts and other microscopic fungi
- Viruses, including viruses that infect human cells and bacteriophages, which infect bacteria
- Other microorganisms and microbial genetic material
These communities are not distributed evenly throughout the body. The microorganisms living in the large intestine are very different from those found on the skin or in the mouth because each site provides a different environment.
Where are these microorganisms found?
Microbes live on virtually all body surfaces that come into contact with the outside environment. Some of the most extensively studied communities are found in the gut, mouth, skin, and reproductive and urinary tracts.
The gut, particularly the colon, supports an especially dense and diverse microbial ecosystem. Food residues that escape digestion by human enzymes can become substrates for microbial metabolism. The resulting compounds can affect the cells lining the intestine and, indirectly, other parts of the body.
The skin presents a very different environment. Areas that are oily, moist, or relatively dry support different microbial communities. These organisms interact with one another and with the skin’s physical and chemical defenses.
The mouth also contains distinct microbial habitats, including the teeth, tongue, gums, and other surfaces. Oral microbes can contribute to both normal health and diseases such as dental caries and periodontal disease.
Importantly, there is no single microbiome that every healthy person should have. Microbial communities vary among individuals and change over time.
How does the microbiome affect the body?
The microbiome matters because microorganisms perform biological functions that can interact with human physiology. Some of the clearest examples involve digestion, metabolism, protection against harmful microbes, and immune development and regulation.
Microbes help process substances the body cannot digest on its own
Human digestive enzymes cannot break down every component of food. In the colon, microbes can ferment certain carbohydrates and other compounds that reach them undigested.
One important result is the production of short-chain fatty acids, including acetate, propionate, and butyrate. These molecules can serve as energy sources for cells in the intestine and can influence intestinal barrier function, immune signaling, and metabolism.
Microbial metabolism also produces many other compounds. Some are potentially beneficial, some are neutral, and some can become harmful under particular circumstances. What matters is not simply the presence of a particular bacterium, but the activities of the microbial community and the environment in which those activities occur.
The microbiome helps defend against invading microbes
Microorganisms already occupying a body site can make it harder for potentially harmful organisms to establish themselves. They may compete for nutrients and attachment sites, alter local conditions, or produce substances that inhibit other microbes.
This phenomenon is often called colonization resistance. It is one reason disruption of a microbial community can sometimes create opportunities for organisms that were previously kept under control.
Antibiotics provide a familiar example. While they can be essential for treating bacterial infections, they may also eliminate or suppress beneficial and harmless bacteria. Changes in the microbial community can temporarily alter the ecological balance of the gut and other body sites.
The microbiome interacts with the immune system
The immune system does not develop and operate in isolation from microorganisms. Microbial exposure helps shape immune responses, particularly at body surfaces such as the intestine.
The relationship is complicated. The immune system must tolerate large numbers of harmless or beneficial microbes while remaining capable of responding to pathogens. Microbes and immune cells therefore participate in an ongoing process of communication.
The intestinal microbiome can influence immune signaling through microbial molecules and metabolic products. In turn, the immune system helps determine which organisms can persist by creating physical, chemical, and immunological conditions within the gut.
This is better understood as a two-way relationship than as microbes simply “boosting” immunity.
What does a healthy microbiome look like?
There is no universally defined microbial profile that can be labeled the ideal human microbiome.
Researchers often discuss diversity, community stability, and functional capacity, but these concepts require context. A microbiome containing many different organisms is not automatically healthier than one containing fewer. Some microbial species can be beneficial in one setting and problematic in another, and a species’ effects can depend on what other organisms are present.
What matters biologically is partly what the community is capable of doing. Two people can have substantially different collections of microbial species while carrying out similar functions.
The microbiome also changes naturally. Age, diet, medications, illness, hormones, geography, lifestyle, and other environmental factors can alter microbial communities. Temporary changes are not necessarily signs of disease.
What happens when the microbiome is disrupted?
A substantial change in a microbial community is sometimes described as dysbiosis. The term is useful for describing an altered microbial state, but it should not be treated as a diagnosis or as a simple explanation for disease.
Microbial disruption can occur for many reasons. Antibiotics are an obvious example, but illness, changes in diet, inflammation, and other environmental conditions can also reshape microbial communities.
The relationship between dysbiosis and disease is particularly important—and easy to misunderstand. Researchers have found associations between altered microbiomes and numerous conditions, including certain gastrointestinal disorders and metabolic and immune-related diseases. But an association does not establish that the microbial change caused the disease.
Disease itself can change the microbiome. Diet, medication, inflammation, and changes in behavior can do so as well. In many cases, scientists are still determining whether a microbial difference is a cause, a consequence, or simply a marker of another biological process.
How is the microbiome established?
Microbial colonization begins around birth and develops rapidly during early life. The microorganisms a person encounters are influenced by their environment, feeding, medications, infections, and other factors.
The early microbiome is not fixed. It changes as the body develops and as a child’s diet and environment change. The immune system and microbial community also influence one another during this period.
By adulthood, microbial communities are generally more established, but they remain dynamic. A person’s microbiome can shift in response to changes in diet, medication, illness, travel, and other circumstances.
The idea that everyone has had the same microbiome throughout life is therefore misleading. A person’s microbial ecosystem has a history, but it is not static.
What role does diet play?
Diet is one of the major factors that can shape the gut microbiome because microbes depend on the substances that reach them.
Dietary fiber is particularly important because many human enzymes cannot fully digest it. Certain gut microbes can ferment fiber and produce short-chain fatty acids and other metabolites. Different types of fiber can support different microbial activities.
This does not mean that a single food will reliably “feed your good bacteria” in a simple, universal way. Individual microbiomes differ, and the effects of a food depend on the overall diet and the existing microbial community.
For most people, the practical implication is broader: a varied diet containing a range of plant foods and adequate fiber provides the gut microbiome with diverse substrates. Nutritional recommendations should still be based on the person’s overall health and dietary needs rather than on the goal of manipulating a particular microbe.
What about probiotics and fermented foods?
Probiotics are live microorganisms that, when consumed in adequate amounts, provide a health benefit in a specific context. The effects are strain-specific and condition-specific. A probiotic product that helps with one problem cannot automatically be assumed to improve general health or permanently change someone’s microbiome.
Fermented foods are also not synonymous with probiotics. Some fermented foods contain live microorganisms, while others may not contain substantial numbers of living microbes by the time they are eaten. Their nutritional effects can extend beyond their microbial content.
This is one reason claims that a particular supplement or food will “reset” or “balance” the microbiome should be viewed cautiously. The microbiome is an ecosystem, not a single switch that can be turned from unhealthy to healthy by adding one organism.
Can the microbiome influence medicines?
Yes. Microorganisms can chemically modify some compounds, including certain medications. In principle, these microbial transformations can affect how a drug behaves in the body.
The relationship can work in both directions. Drugs can alter microbial communities, while microbial enzymes can change the compounds to which the body is exposed.
This field, sometimes called pharmacomicrobiomics, is still developing. It may eventually help explain some differences in how individuals respond to medications, but microbiome testing is not yet a universal way to predict drug response in routine medical care.
Can a microbiome test tell you whether you are healthy?
Usually not by itself.
Commercial microbiome tests commonly analyze microbial DNA from a stool or other sample and report which microorganisms or microbial genes were detected. Such testing can be useful for research, but interpreting an individual’s result is much harder than identifying microbes in a sample.
A stool sample, for example, does not provide a complete picture of every microbial community throughout the body. More importantly, scientists do not have a definitive reference microbiome that can reliably classify every individual as healthy or unhealthy.
A report showing that one bacterium is unusually abundant therefore does not necessarily mean that something is wrong. Clinical interpretation requires evidence linking a microbial pattern to a specific condition and showing that acting on that information improves health.
Why the microbiome is important to medical research
The microbiome has changed how researchers think about human biology. Instead of viewing microorganisms primarily as sources of infection, scientists now recognize them as participants in complex biological systems.
Research is investigating microbial contributions to gastrointestinal disease, immune disorders, metabolism, cancer, neurological processes, and responses to medicines. Some approaches aim to alter microbial communities through diet, targeted probiotics, prebiotics—substances that selectively support certain microbes—or microbial transplantation.
One established medical application is fecal microbiota transplantation, in which processed stool from a screened donor is used to restore aspects of the intestinal microbial community. It has an important role in treating recurrent Clostridioides difficile infection in appropriate clinical settings.
Many other microbiome-based treatments remain experimental. Promising biological mechanisms do not automatically translate into effective therapies, and clinical trials are needed to determine whether deliberately changing a microbial community actually improves outcomes.
The key idea: the microbiome is an ecosystem, not an organ of “good” and “bad” bacteria
The most useful way to understand the human microbiome is as a collection of changing ecosystems that interact continuously with the human body.
Microbes compete and cooperate with one another. They consume nutrients, produce metabolites, interact with human cells, respond to environmental changes, and influence—and are influenced by—the immune system. The same organism may have different effects depending on its abundance, location, surrounding microbes, and the condition of its host.
That complexity is precisely why the microbiome matters. Human health is not determined solely by human cells and human genes. It also depends, in part, on the microbial communities living alongside us and on the chemical and biological relationships they maintain with our bodies.
