A microbiome is the entire community of microorganisms living in a particular environment, plus all the molecules they produce and the genetic material they carry. The term was originally coined to describe not just the microbes themselves but their whole “theatre of activity,” including their metabolic byproducts, structural components, and even free-floating DNA and viruses in the mix. People most often hear the word in the context of the human gut, but microbiomes exist everywhere life does: on your skin, in your lungs, in the soil beneath a wheat field, and in the open ocean. Understanding what a microbiome actually encompasses, and what it does for the organisms and ecosystems it inhabits, turns out to be far more interesting than a simple dictionary definition.
Not Just the Microbes Themselves
A common source of confusion is the difference between “microbiota” and “microbiome,” and plenty of scientists use the two interchangeably even though they technically mean different things. The microbiota refers narrowly to the living microorganisms present in a given environment. The microbiome is a bigger concept: it includes those living organisms and the entire spectrum of molecules they produce, such as proteins, lipids, signaling molecules, and toxins, along with viruses, phages, and extracellular DNA that are present in that environment but would not technically count as “alive.”1PubMed Central. Microbiome definition re-visited: old concepts and new challenges This matters because a lot of what a microbiome does for its host comes not from the microbes being physically present but from what they secrete and leave behind. When researchers measure a microbiome, they are cataloging genes, chemical products, and ecological interactions, not just counting bacteria.
More Than Bacteria
When most people picture the microbiome, they picture bacteria. Bacteria do dominate, especially in the human gut. But a complete microbiome also includes fungi, viruses, archaea (an ancient group of single-celled organisms), and sometimes single-celled parasites called protists.2PubMed Central. Gut Microbiota beyond Bacteria-Mycobiome, Virome, Archaeome, and Eukaryotic Parasites in IBD These non-bacterial members have been studied far less than their bacterial neighbors, but research is catching up. Viruses and phages (viruses that infect bacteria rather than human cells) are now recognized as key players that shape which bacterial species thrive and which get outcompeted. Gut fungi and archaea are profoundly altered in conditions like obesity and appear to influence how the body handles fat storage and energy balance.3PubMed Central. The role of the gut non-bacterial microbiome (virome, mycobiome, archaeome) and its impact on obesity
Thinking of the microbiome as “a bunch of bacteria” therefore misses a large part of the picture. The fungal community (sometimes called the mycobiome), the viral community (the virome), and the archaeal community (the archaeome) each have their own dynamics and their own effects on the host. Research into these components is still in relatively early stages compared to the decades of bacterial research, but the trend is clear: all of these microbial kingdoms interact with each other and with human cells in ways that matter for health.
What the Gut Microbiome Actually Does
Your gut harbors the densest microbial community in your body, and its functions go well beyond digestion. One of its most studied roles is the fermentation of dietary fiber into short-chain fatty acids, which are small molecules that serve as fuel for the cells lining the intestine, help regulate inflammation, and influence everything from appetite hormones to blood sugar control.4PubMed Central. Health Benefits and Side Effects of Short-Chain Fatty Acids The three main short-chain fatty acids, acetate, propionate, and butyrate, are produced almost entirely by microbial fermentation. Without gut microbes, your body simply could not make them.
The gut microbiome also trains and calibrates the immune system. Roughly 70 to 80 percent of your immune cells reside in the gut, and they are in constant dialogue with the microbes living alongside them.5PubMed Central. The Interplay between the Gut Microbiome and the Immune System in the Context of Infectious Diseases throughout Life and the Role of Nutrition in Optimizing Treatment Strategies This interaction does not stay local. The immune education that happens in the gut shapes how your body responds to threats throughout the rest of the body, from your lungs to your bloodstream. Researchers have also identified a gut-brain axis, a communication pathway in which microbial metabolites act as signaling molecules between the intestinal ecosystem and the central nervous system, potentially influencing mood, stress responses, and neurodevelopment.6PubMed Central. Microbial metabolites at the nexus of gut-brain communication and neurodevelopmental disorders
Microbiomes Elsewhere on Your Body
The gut gets the headlines, but your skin and respiratory tract support their own distinct microbial communities with their own important functions. The skin microbiome provides a living layer of defense: colonizing microbes protect against invading pathogens, fine-tune local immune responses, and help maintain the physical barrier that keeps the outside world out.7PubMed. Microbiota and maintenance of skin barrier function The specific mix of microbes varies dramatically across body sites. Your forehead, your armpit, and the crease behind your knee each host microbial populations as different from one another as the communities in a desert and a rainforest.
The respiratory tract microbiome was long overlooked because healthy lungs were assumed to be sterile. That assumption turned out to be wrong. The airways harbor communities of bacteria, and certain species appear to protect against infections. Anaerobic bacteria commonly found in the mouth and lungs, such as Prevotella and Veillonella, are associated with better outcomes during pneumonia and appear to activate immune defense pathways in the lower airway. Meanwhile, nasal bacteria like Corynebacterium and Dolosigranulum can directly inhibit the growth and virulence of lung pathogens, and their abundance in the upper airway correlates with a lower risk of respiratory infections.8PubMed Central. Insights into the role of the respiratory tract microbiome in defense against bacterial pneumonia The respiratory microbiome also plays a role in immune training, especially during early life.9PubMed Central. The Respiratory Tract Microbiome and Human Health
How Your Microbiome Takes Shape
You are not born with a fully formed microbiome. It assembles rapidly in the first weeks and months of life, and several factors shape what that founding community looks like. Birth method is one of the biggest early influences. Infants born by cesarean section show substantially reduced levels of Bacteroidetes (a major group of beneficial gut bacteria) and higher levels of Firmicutes compared to vaginally born infants. In some comparisons, the difference in Bacteroidetes abundance was 200- to 500-fold.10Scientific Reports. Fucosylated oligosaccharides in mother’s milk alleviate the effects of caesarean birth on infant gut microbiota These are not trivial gaps; they reflect entirely different starting communities that take time to converge toward a more typical adult pattern.
Breastfeeding, antibiotic exposure, and the presence of older siblings or pets all independently influence the diversity and composition of an infant’s gut microbiome during the first month of life.11PubMed Central. Human milk oligosaccharides, antimicrobial drugs, and the gut microbiota of term neonates: observations from the KOALA birth cohort study Antibiotic exposure and cesarean delivery were both associated with decreased microbial diversity, while factors like contact with animals and having siblings in the household were associated with shifts in composition. Human milk contains sugars called oligosaccharides that selectively feed certain beneficial bacteria, essentially giving breastfed infants a head start in cultivating a healthy microbial community. These early-life patterns matter because a less diverse founding microbiome has been linked to higher rates of allergies, asthma, and metabolic issues later in childhood, though the exact causal chain is still being worked out.
What Happens When the Microbiome Goes Off Balance
The term “dysbiosis” describes a state where the microbial community has shifted away from its healthy composition, often with a loss of diversity and an overgrowth of potentially harmful species. Dysbiosis in the gut can weaken the intestinal barrier, allowing bacterial molecules to leak into surrounding tissues and trigger inflammation in distant organs, including the liver and fat tissue.12PubMed Central. Diet-Induced Gut Dysbiosis and Leaky Gut Syndrome This “leaky gut” process has been implicated in conditions ranging from obesity and metabolic syndrome to inflammatory bowel disease, irritable bowel syndrome, nonalcoholic fatty liver disease, and gastrointestinal cancers.13PubMed Central. The Influence of the Gut Microbiome on Obesity, Metabolic Syndrome and Gastrointestinal Disease
Antibiotics are one of the most common triggers of dysbiosis. While they are sometimes lifesaving, they do not discriminate well between harmful and beneficial microbes. The resulting collateral damage can reduce microbial diversity and leave the gut vulnerable to opportunistic infections, most famously Clostridioides difficile, a bacterium that causes severe diarrhea and can be life-threatening. Antibiotic-induced disruptions can persist for weeks or even months after the course of antibiotics ends, which helps explain why repeated antibiotic use is associated with longer-lasting changes in gut health.14PubMed Central. The Lasting Imprint of Antibiotics on Gut Microbiota: Exploring Long-Term Consequences and Therapeutic Interventions Diet also plays a major role: the evidence linking low-fiber, high-fat diets to gut dysbiosis and intestinal inflammation has been growing for years.15PubMed Central. The Microbiome in Visceral Medicine: Inflammatory Bowel Disease, Obesity and Beyond
Trying to Fix It With Probiotics and Transplants
Given how much the microbiome influences health, the question of whether you can deliberately improve it is a natural one. Probiotics, prebiotics, and postbiotics all aim to do this in different ways. Probiotics are live microorganisms (usually bacteria or yeast) taken in supplements or fermented foods. Prebiotics are specific fibers and sugars that feed the beneficial microbes already in your gut. Postbiotics are the useful metabolic byproducts that gut microbes produce, extracted or manufactured and taken directly.16PubMed Central. Probiotics, prebiotics, and postbiotics in health and disease The evidence for these interventions varies widely depending on the specific product, dose, and medical condition in question. Some probiotic strains have solid data behind them for particular conditions like antibiotic-associated diarrhea, while the broad “take probiotics for gut health” advice that fills store shelves is often running ahead of the science.
The most dramatic microbiome intervention is fecal microbiota transplantation, in which stool from a healthy donor is transferred into a patient’s gut to restore a functional microbial community. This sounds unappealing, but for recurrent C. difficile infections that do not respond to standard antibiotics, it works remarkably well. Early case reports and small series suggested cure rates of about 90 percent.17PubMed Central. Treating Clostridium difficile infection with fecal microbiota transplantation Systematic reviews have since confirmed that fecal transplant appears to be a highly effective therapy for recurrent C. difficile infection.18PubMed. Systematic review with meta-analysis: long-term outcomes of faecal microbiota transplantation for Clostridium difficile infection Researchers are now exploring whether similar transplants might help with other conditions linked to dysbiosis, though the results so far have been far less clear-cut than they are for C. difficile.
Microbiomes Beyond the Human Body
Microbiomes are not a human phenomenon. Every animal, plant, body of water, and patch of soil on Earth supports its own microbial community, and many of these are essential to how the planet functions at a basic level. In agriculture, the root zone of a plant, known as the rhizosphere, supports a microbiome that plants actively shape. Plants secrete specific compounds from their roots to selectively recruit beneficial microbes, and these recruited microbes help the plant acquire nutrients, cope with drought or poor soil, and fight off pathogens.19PubMed Central. The rhizosphere microbiome: Plant-microbial interactions for resource acquisition Some researchers are now investigating whether manipulating soil microbiomes could help crops grow with fewer chemical fertilizers.
In the ocean, microbial communities drive the chemical cycles that keep the planet habitable. Despite living in a vast, dilute environment where nutrients are sparse and surfaces are temporary, marine microbes power essential transformations in global carbon, nitrogen, and oxygen cycling.20PubMed. The global ocean microbiome Soil microbiomes play a similar planetary-scale role, particularly in carbon cycling. Soil microbes control the rate at which organic carbon is processed and stabilized, influence the formation of soil aggregates that protect carbon from being released as COâ‚‚, and in this way help determine how much carbon stays locked in the ground versus entering the atmosphere.21PLoS Biology. Can we manipulate the soil microbiome to promote carbon sequestration in croplands? This has made the soil microbiome a subject of serious interest in climate research.
The Indoor Microbiome
Since most people in industrialized societies spend the vast majority of their time indoors, the microbiome of the built environment matters more than you might expect. Homes, offices, hospitals, and schools each host complex microbial ecosystems shaped by the people who move through them, the ventilation systems in place, building materials, and local climate. The indoor microbiome plays a role in modulating infectious disease risk and in shaping healthy immune development, especially in young children.22PubMed. The indoors microbiome and human health The microbial profile of a home with pets, for instance, differs markedly from a pet-free home, and those differences may partly explain why children raised with animals tend to have lower rates of certain allergies. Hospital microbiomes are a particular concern because the microbial communities on surfaces and in the air of clinical settings can harbor antibiotic-resistant organisms.
Your Microbiome Follows a Clock
One of the more surprising discoveries in microbiome research is that your gut microbes operate on a daily rhythm. Up to about 20 percent of intestinal bacteria fluctuate in abundance and activity over a 24-hour cycle, rising and falling in concert with your body’s own circadian clock.23PubMed Central. Circadian Rhythms, the Gut Microbiome, and Metabolic Disorders These rhythms are not just a passive response to when you eat: experiments in mice kept in constant darkness showed that microbial oscillations persist even without light cues, meaning the host’s internal clock is actively driving the pattern. When researchers knocked out a core clock gene in intestinal cells, the microbiome’s daily rhythms broke down, and so did the normal daily fluctuations in microbial metabolites like branched-chain fatty acids and secondary bile acids.24Nature Communications. The intestinal clock drives the microbiome to maintain gastrointestinal homeostasis
The relationship runs both ways. Gut microbes produce molecules like butyrate that appear to feed back and influence the rhythmic activity of the host’s own clock genes. And the timing of meals matters: feeding patterns alter both the community structure of the microbiome and its metabolic output, which in turn affects host immune and metabolic function.25PubMed. Circadian Rhythm and the Gut Microbiome This bidirectional loop helps explain why chronic disruptions to circadian rhythms, like those experienced by shift workers and frequent travelers crossing time zones, are associated with metabolic problems and gut inflammation. The microbiome is not just along for the ride; it is part of the timekeeping machinery.
How Scientists Read a Microbiome
When researchers want to know what lives in a microbiome and what those organisms are doing, they have two main genomic approaches. The older and cheaper method targets a single gene, the 16S ribosomal RNA gene, which exists in all bacteria and archaea but varies enough between species to serve as a kind of barcode. This approach is fast and widely used, but it has limitations: it captures only part of the microbial community revealed by more comprehensive methods, and it struggles to identify rarer or less abundant species.26Scientific Reports. Comparison between 16S rRNA and shotgun sequencing data for the taxonomic characterization of the gut microbiota
Shotgun metagenomic sequencing is the more powerful alternative. Instead of targeting one gene, it sequences all the DNA in a sample at once, capturing bacteria, viruses, fungi, and archaea simultaneously. When enough sequencing reads are available, shotgun sequencing detects organisms that 16S misses entirely, especially at finer taxonomic levels where the two methods can disagree substantially.27PubMed Central. Comparison between 16S rRNA and shotgun sequencing in colorectal cancer, advanced colorectal lesions, and healthy human gut microbiota Shotgun sequencing also reveals functional information: not just which species are present but what genes they carry and, by inference, what metabolic activities they are capable of. The tradeoff is cost and computational complexity, which is why 16S remains popular for large-scale surveys and initial screening.
Coevolution and the Holobiont Idea
The depth of integration between hosts and their microbiomes has led some biologists to rethink how they define an individual organism. The “holobiont” concept treats a host plus its associated microbiome as a single evolutionary unit, rather than viewing the host and its microbes as entirely separate entities.28PubMed Central. The study of host-microbiome (co)evolution across levels of selection The logic is straightforward: if the host’s fitness depends on its microbial partners, and if the microbes’ fitness depends on the host environment, then natural selection acts on the combined system, not just on one partner in isolation. This framing remains debated, with some researchers arguing it overstates the degree of true coevolution and others finding it a productive way to think about the tight dependencies that clearly exist. But the underlying observation is hard to argue with: the line between “you” and “your microbes” is blurrier than it once seemed. Your genome encodes roughly 20,000 genes; the collective genomes of the microbes living on and inside you encode millions more. How much of what you experience as your own biology, from your digestion to your mood to your susceptibility to illness, is really a joint venture with organisms you cannot see remains one of the more fascinating open questions in biology.