A dense community of bacteria, fungi and mites lives on the human body full time, and most of it works in your favor.
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Biologists estimate that human skin carries on the order of a trillion bacterial cells at any given moment — a figure that a 2016 study published in PLOS Biology traces to an earlier body-wide bacterial census. That figure excludes the fungi, the microscopic mites and the viruses that infect the bacteria themselves. Estimates vary depending on how the sampling is done, since this is a genuinely difficult thing to measure, but the credible ranges all sit in the trillions.
That population is not contamination. Most of these organisms are not passing through but are adapted to human skin specifically, and some of them can live nowhere else. The more useful question is what they do once they are there.
Skin Is Several Habits That Support Several Organisms
An adult’s skin covers roughly 2 square meters, though that number understates the space actually available. Every hair follicle, sweat pore and sebaceous gland adds surface area, and follicles in particular provide sheltered interior space that the exposed exterior does not.
Microbiologists generally divide skin into three habitat types: sebaceous or oily sites such as the forehead, nose and upper back; moist sites such as the armpit, the inner elbow and the spaces between the toes; and dry sites such as the forearm and shin. Each supports a distinct community.
One of the more durable findings, from a 2009 study published in Science that sequenced bacterial communities at 20 skin sites in healthy volunteers, is that the community on one person’s forehead often resembles a stranger’s forehead more closely than it resembles the community on that same person’s forearm. Local conditions matter more than the identity of the host.
The dominant residents are specialists suited to those conditions. Cutibacterium acnes concentrates in oily follicles, where it metabolizes sebum — the skin’s natural oil — and releases fatty acids as a byproduct. Staphylococcus epidermidis is more common on moist skin. Malassezia yeasts, the most abundant fungi on the human body, have lost the ability to synthesize their own fatty acids and must obtain lipids from their host, which explains why they cluster where sebum is plentiful.
Larger organisms live there as well. Two species of Demodex mite spend their lives head-down inside human hair follicles, mostly on the face, feeding on sebum and shed skin cells. They measure a fraction of a millimeter and are present on the great majority of adults by middle age.
What These Resident Organisms Contribute
The best-established function is colonization resistance. An established community occupies the available space and consumes the available nutrients, leaving little for arriving organisms. Some skin bacteria go further and produce antimicrobial compounds that suppress competitors directly; a 2017 study published in Science Translational Medicine identified strains from healthy skin capable of directly inhibiting Staphylococcus aureus, a frequent cause of skin infection.
Resident microbes also help maintain the chemical conditions of the surface. Their metabolism of sebum and sweat contributes to what dermatologists call the acid mantle, a surface pH held roughly between 4 and 6 and therefore more acidic than the body’s interior. Many pathogens grow poorly in that range.
A third contribution is immunological. A 2012 study published in Science found that, in mice, resident skin bacteria shape the behavior of local immune cells, effectively teaching them which organisms to tolerate and which to treat as threats. This is a mouse-model finding, but it fits a broader idea, sometimes called the old friends or hygiene hypothesis, which holds that the immune system develops normally only in the presence of ordinary microbial exposure. The general direction of the evidence is consistent, though the human-specific mechanics remain under active investigation.
One contribution is noticeable daily. The secretions of the apocrine glands in the armpit are close to odorless when they reach the surface. A 2015 study published in Microbiome found that axillary bacterial composition tracks closely with body-odor intensity, consistent with the mechanism: skin bacteria break those precursor molecules into volatile sulfur compounds, and those compounds are what produce body odor. This is also why deodorants that act on bacteria are effective.
What We Still Don’t Know About These Organisms
In eczema, flares are strongly associated with S. aureus coming to dominate an otherwise balanced community, a state described as dysbiosis. Whether the imbalance triggers the flare or the flare creates the conditions that favor the imbalance is still debated, and the relationship is probably reciprocal. A similar question surrounds Demodex mites and rosacea, where high mite densities are associated with the condition but cause and effect have proven difficult to separate.
What is clear is that removing this community entirely is neither achievable nor desirable. Washing removes oils, debris and transient organisms, but the resident population re-establishes within hours from follicles and glands that soap does not reach.
The most accurate way to think about this is ecological. The human body functions as a habitat, one that a considerable number of other species have adapted to over a long period, and several of them perform maintenance work that would be noticed immediately if it stopped.
Turns out your skin is basically its own ecosystem with resident organisms. See how well you really know the rest of the human body with this science-backed test: Human Anatomy IQ Test
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