The human microbiome is the collection of microorganisms, their genetic material and their ecological interactions associated with the human body. In many scientific contexts, microbiome refers specifically to the microbial genomes and the microbial community inhabiting a particular environment, while microbiota refers more narrowly to the organisms themselves. The human microbiome includes bacteria, archaea, fungi, protists and viruses, although bacteria dominate many studied body sites.
Microbial communities occur on the skin, in the mouth, respiratory tract, gastrointestinal tract and reproductive tract, with particularly high microbial abundance and diversity in the large intestine. Different body sites have distinct physical and chemical environments and therefore support different microbial communities. The human microbiome is not a single uniform population distributed throughout the body. It is a collection of specialised microbial ecosystems that vary among individuals and change over time in response to age, diet, environment, medication, disease and many other factors.
The gastrointestinal tract contains one of the most densely populated microbial ecosystems in the human body. Large numbers of bacteria inhabit the colon, where they encounter dietary material that has escaped digestion by host enzymes. Microorganisms ferment some carbohydrates and other compounds and produce metabolites including short chain fatty acids. These metabolites can interact with intestinal cells and influence local and systemic physiology. Microbes also transform bile acids and other host derived molecules and participate in complex chemical exchanges.
The microbial community is influenced by intestinal pH, oxygen availability, transit time, diet and host secretions. Most of the colon is relatively oxygen poor compared with the upper digestive tract, favouring organisms adapted to anaerobic or low oxygen conditions. The gastrointestinal microbiome therefore demonstrates how microbial communities are shaped by the physical environment provided by the host.
The skin microbiome is adapted to highly variable conditions involving dryness, salt, lipids, temperature, oxygen exposure and different levels of moisture. Microbial communities differ between oily regions, moist regions and dry surfaces. Skin microorganisms interact with host tissues and with one another and can contribute to the ecological stability of the skin environment. The oral microbiome is similarly diverse and includes organisms living on teeth, the tongue, gums and other surfaces.
Dental plaque is a microbial biofilm containing bacteria embedded within an extracellular matrix. Under certain conditions, microbial metabolism can contribute to acid production and dental caries, demonstrating that resident microorganisms can have both beneficial and harmful effects depending on community composition and environmental circumstances. Microbiome research therefore focuses not simply on which organisms are present but also on what they are doing and how their activities affect host physiology.
The human microbiome develops dynamically throughout life. Infants acquire microorganisms from maternal and environmental sources, and early microbial colonisation is influenced by factors including mode of birth, feeding, environment and subsequent exposures. The microbial community changes during childhood as diet, physiology and immune development change. Adulthood generally involves more stable community patterns, although substantial variation exists among individuals and microbial composition can change in response to diet, illness, antibiotics, travel, stress and other factors. Ageing is also associated with changes in microbial communities. The microbiome is therefore not a fixed organ or permanent list of species. It is a dynamic ecosystem whose composition and function are continuously shaped by interactions among microorganisms, host physiology and the external environment.
The immune system and the microbiome influence one another extensively. Microbial molecules can stimulate host immune development and signalling, while immune mechanisms regulate microbial populations and help maintain boundaries between microorganisms and tissues. Healthy mucosal surfaces must tolerate enormous numbers of resident microbes while still responding appropriately to invasive pathogens. This requires complex regulatory mechanisms.
Disruption of microbial communities can alter the chemical signals encountered by immune cells, although the relationship between particular microbiome changes and human disease is often more complicated than simple cause and effect. Researchers increasingly distinguish between correlation and causation because finding different microbial abundances in people with a disease does not prove that the microbial difference caused the disease. Experimental studies, mechanistic investigations and controlled interventions are needed to establish causal relationships.
The microbiome is strongly influenced by diet and metabolism. Dietary fibre and other compounds that resist host digestion can become substrates for microbial fermentation. The resulting metabolites can affect intestinal cells and may influence energy balance, immune signalling and other physiological processes. Different dietary patterns can favour different microbial populations because microorganisms vary in their ability to use particular substrates. Medication also affects the microbiome.
Antibiotics can substantially alter bacterial communities, sometimes reducing susceptible organisms and changing ecological relationships among surviving organisms. Other medicines can influence microbial metabolism or composition as well. These effects illustrate that the microbiome is an ecological community rather than a static feature of the body. Environmental exposures, geography, household contacts and lifestyle can also contribute to variation between individuals.
The microbiome has important implications for medicine and biotechnology, but many claims about microbiome based health interventions require careful interpretation. Faecal microbiota transplantation can be highly effective for particular recurrent intestinal infections under appropriate clinical circumstances because it can restore aspects of a disrupted microbial community. Researchers are also investigating defined microbial consortia, engineered bacteria, targeted metabolites and other microbiome based therapies. Probiotics are more limited and strain specific than the general public use of the word microbiome sometimes suggests.
A product containing one bacterial strain cannot necessarily reproduce the functions of an entire complex microbial ecosystem. Similarly, commercial microbiome tests that report extensive lists of microbial species do not automatically provide clinically meaningful diagnoses. The field is developing rapidly, and robust interpretation requires distinguishing established clinical evidence from preliminary associations.
A common misconception is that the human microbiome is made exclusively of bacteria or that there is one ideal microbial composition that every healthy person should possess. In reality, healthy people can have substantially different microbial communities while sharing broad functional characteristics. Another misconception is that all microbiome organisms are beneficial. Some can become opportunistic pathogens or contribute to disease under particular conditions, while many others have effects that remain incompletely understood.
The microbiome is best understood as a dynamic ecosystem integrated with human physiology. Its members exchange metabolites, compete for resources, interact with immune and epithelial systems and respond to environmental change. Understanding the human microbiome therefore requires concepts from microbiology, ecology, genetics, immunology, nutrition, medicine and evolutionary biology rather than treating it as a simple list of bacteria living inside the body.
