The human microbiome is not something we are simply born with in a finished form. It develops over time as microorganisms encounter a newborn’s body, compete and cooperate with one another, respond to the infant’s changing environment, and interact with the developing immune system and organs.
Microorganisms begin establishing themselves around birth, but the process continues rapidly throughout infancy and childhood. The microbes found in the gut, mouth, skin, and other body sites are shaped by different influences, including delivery, feeding, contact with caregivers, the surrounding environment, medications, illness, and the gradual introduction of solid foods.
Understanding this development helps explain why the microbiome changes so dramatically during early life—and why researchers are interested in its possible connections to immune development, metabolism, and health.
What the microbiome is at birth
The microbiome refers to the communities of microorganisms that live on and in the human body. These communities include bacteria, but also fungi, viruses, archaea, and other microscopic organisms. The collection of microbial genes is sometimes called the microbiome as well, while the organisms themselves are more precisely described as the microbiota.
A newborn does not have the diverse, relatively stable microbial communities found in an older child or adult. Instead, microbial colonization begins around birth and proceeds through a series of ecological changes.
The gut is especially important because it eventually becomes home to a dense and complex microbial community. But the microbiome is not a single system. The microbes living in the intestine differ from those on the skin, in the mouth, and in the reproductive and respiratory tracts. Each body site provides a different environment, so its microbial community develops along a different trajectory.
The earliest microbes also do not necessarily remain dominant. Some organisms are well suited to the conditions of the newborn gut but become less competitive as oxygen levels, available nutrients, intestinal chemistry, and diet change.
The first microbes arrive during a period of rapid change
For much of the early twentieth and twenty-first centuries, scientists debated how much microbial exposure occurs before birth. The clearest and most important microbial transition occurs around birth and afterward, when a newborn encounters abundant microorganisms from people and the surrounding environment.
During delivery, an infant is exposed to microbes associated with the mother and the birth environment. The precise exposure differs depending on the circumstances of birth, including whether delivery is vaginal or by cesarean section. These differences can influence the early composition of the infant microbiota, although they are only one part of a much larger developmental process.
After birth, microbial exposure continues constantly. Skin-to-skin contact, feeding, handling by caregivers, household surfaces, and the broader environment all provide opportunities for microorganisms to reach the infant.
This does not mean that every early exposure produces a permanent member of the microbiome. Colonization is selective. A microorganism must be able to survive in a particular environment, obtain nutrients, interact with other microbes, and tolerate the host’s defenses.
Feeding reshapes the developing gut microbiome
Nutrition is one of the strongest influences on the infant gut microbiome.
Human milk does more than provide calories, proteins, fats, vitamins, minerals, and other nutrients. It also contains complex carbohydrates called human milk oligosaccharides, or HMOs. Infants digest many of these carbohydrates poorly themselves. Certain gut bacteria, however, can use them as an energy source.
This creates a selective environment in which particular microbial groups can thrive. Some of these bacteria produce short-chain fatty acids and other metabolites that can affect the intestinal environment and interact with the host.
Human milk also contains immune-related components and microorganisms, adding further complexity to the relationship between feeding and microbial development.
Formula feeding creates a different nutritional environment. Modern infant formulas can support normal infant growth, but their composition differs from human milk, including in the types and amounts of carbohydrates available to gut microbes. Consequently, microbiome development in breastfed and formula-fed infants can follow somewhat different patterns.
These differences should not be interpreted as a simple ranking of one feeding method over another. Infant nutrition involves many considerations, and microbiome composition is only one part of infant health.
The gut changes as the infant grows
The newborn intestine is not a static habitat. Its physical and chemical conditions change substantially during the first months of life.
Early in colonization, oxygen can be relatively more available in parts of the gut than it is later. This favors microorganisms that tolerate or use oxygen. As microbial activity and the intestinal environment change, oxygen becomes less available, creating conditions that favor many anaerobic organisms—microbes that grow without oxygen.
The developing community also alters its own environment. Microbes consume nutrients, produce metabolic byproducts, change acidity, and interact with the intestinal lining. These changes influence which organisms can establish themselves next.
The result is a process of ecological succession: early microbial communities help create conditions that permit different communities to become established later.
Meanwhile, the infant’s immune system is learning to distinguish between harmless organisms, beneficial microbial signals, and genuine threats. The intestinal lining and immune system respond to microbial molecules and metabolites, helping shape the relationship between host and microbiota.
Solid foods trigger another major transition
The introduction of solid foods produces one of the most consequential shifts in the developing gut microbiome.
Before complementary foods are introduced, the infant diet is relatively limited. Once foods containing different carbohydrates, proteins, fats, and fibers become part of the diet, microbes encounter a much broader range of potential nutrients.
Dietary fiber is particularly important because many human digestive enzymes cannot break it down completely. Microbes can ferment some of these carbohydrates and produce metabolites such as acetate, propionate, and butyrate. These short-chain fatty acids can influence the gut environment and have effects on intestinal cells and immune processes.
As the diet becomes more varied, the gut microbiome generally becomes more diverse and increasingly resembles the functional organization seen later in childhood and adulthood.
This is not simply a matter of adding more bacterial species. The functions that microbes perform—and the metabolic interactions among them—also change as the available nutrients change.
The immune system and microbiome develop together
Microbiome development and immune development are closely intertwined.
The immune system does not merely defend against microbes; it also has to learn how to coexist with the enormous microbial population that lives in the intestine and on other body surfaces. Signals from microorganisms can influence immune-cell development and the behavior of immune pathways. In turn, immune defenses help determine which microbes can persist.
The intestinal barrier is another important part of this relationship. The gut lining separates the microbial community from deeper tissues while allowing controlled interactions between microbial products and host cells.
A healthy relationship is therefore not characterized by the absence of immune activity. Rather, the immune system must maintain an appropriate balance: controlling potentially harmful organisms while tolerating and interacting with the resident microbial community.
This early-life relationship is one reason researchers are studying whether disruptions in microbiome development can influence later susceptibility to certain diseases. Associations have been reported between early microbial patterns and various immune, metabolic, and gastrointestinal conditions, but association does not establish that a particular microbiome pattern causes a disease.
Antibiotics can alter the trajectory
Antibiotics are another important influence on the developing microbiome because they can kill or suppress susceptible bacteria along with the organisms responsible for an infection.
The consequences depend on factors such as the antibiotic used, the duration of treatment, the timing of exposure, and the microbial community present beforehand. Some microbial populations can recover after treatment, while others may take longer to return or may be replaced by different organisms.
This does not make antibiotics inherently harmful. When a bacterial infection requires treatment, antibiotics can be essential and lifesaving. The important distinction is between using antibiotics when medically indicated and assuming that changes to the microbiome are harmless or irrelevant.
Other medications, illness, hospitalization, diet, and environmental exposures can also influence microbial communities.
Skin and mouth microbiomes develop differently
The gut receives much of the scientific attention, but the microbiome develops across the entire body.
The skin is exposed to air, changing moisture levels, temperature, clothing, bathing, and contact with other people. Different regions of skin therefore support different microbial communities. The organisms found in oily areas, moist areas, and relatively dry areas are not identical.
The mouth provides another distinct ecosystem. Saliva, the developing teeth, food, and local chemical conditions create habitats for microbial communities that change as the mouth develops. Tooth eruption introduces new surfaces on which microorganisms can establish themselves and can alter the oral ecosystem.
These communities do not develop independently. Microbes can move between body sites, and behavior, diet, hygiene, and the surrounding environment can affect several microbial habitats at once.
The microbiome becomes more complex rather than simply “maturing”
It is tempting to picture microbiome development as a straight line from a simple newborn microbiome to a perfect adult microbiome. Biology is more complicated.
Microbial communities fluctuate. Some organisms appear temporarily and then decline. Others become established and persist. The relative abundance of particular organisms can change substantially without meaning that the entire community has become unhealthy.
There is also no single microbial composition that defines a healthy person. Two healthy individuals can have noticeably different microbial communities while maintaining similar biological functions.
For this reason, researchers increasingly examine what microbial communities do—not only which species are present. Microbes can carry genes for producing metabolites, processing nutrients, modifying bile acids, interacting with the intestinal lining, and communicating with host immune pathways. Different organisms may perform overlapping functions, meaning that a change in species composition does not necessarily produce an equivalent change in function.
When does the microbiome become “adult”?
There is no exact birthday at which the human microbiome becomes adult.
The largest changes occur during early life, particularly as feeding patterns, intestinal conditions, and microbial interactions change. By later childhood, the gut microbiome has generally become more complex and comparatively stable than it was during infancy. Yet it remains responsive to diet, illness, medications, lifestyle, and other environmental factors throughout life.
Even an adult microbiome is not fixed. A major dietary change, infection, antibiotic course, or other physiological disturbance can shift microbial populations.
What early life establishes is therefore better understood as a developing ecological foundation rather than a permanent microbial blueprint.
What shapes microbiome development most strongly?
No single factor determines a child’s microbiome. Development reflects the interaction of many influences:
- Birth circumstances: Delivery affects the microorganisms encountered around birth and can influence early microbial succession.
- Feeding: Human milk, formula, and later complementary foods provide different nutrients for microbes.
- Dietary transition: The introduction and increasing variety of solid foods greatly expand the nutrients available to gut microorganisms.
- Antibiotics and other medications: These can selectively suppress or alter microbial populations.
- Environment: Household members, pets, geography, sanitation, and everyday environmental contact all contribute microbial exposures.
- Host biology: Genetics, immune responses, intestinal physiology, and development influence which microbes can persist.
- Illness and other exposures: Infections and changes in health can temporarily or persistently reshape microbial communities.
These factors also interact. For example, the effect of an antibiotic depends partly on the microbial community present when treatment begins, while the effects of diet depend on which microbes are available to use the nutrients.
What early microbiome research can—and cannot—tell us
Scientists have learned that early microbiome development is dynamic and closely connected with human development. But important questions remain.
Finding that children with a particular health condition tend to have different microbiomes from healthy children does not by itself show that the microbial difference caused the condition. The reverse may be possible: illness, diet, medication, or other factors associated with the condition may have changed the microbiome.
Likewise, there is no scientifically justified recipe for creating a single “ideal” infant microbiome. Commercial probiotics and other microbiome-directed products may have specific uses, but their effects depend on the organism, dose, age, clinical situation, and outcome being considered. More bacteria is not automatically better, and greater microbial diversity is not universally synonymous with better health.
The central lesson is more nuanced. From birth onward, the microbiome is an evolving ecosystem that develops alongside the infant. Nutrition, microbial exposure, host defenses, and the physical environment continually reshape it. Early microbial communities help shape the conditions in which later communities emerge, while the developing body simultaneously determines which microorganisms can survive.
By childhood, this ongoing interaction has produced a much more established microbial ecosystem—but the relationship between humans and their microbes remains dynamic throughout life.
