Skin flora

Based on Wikipedia: Skin flora

You Are Never Alone

Right now, as you read this, approximately one trillion microorganisms are living on your skin. Not inside you—on you. Your body is a planet, and these bacteria, fungi, and other tiny creatures are its inhabitants.

This isn't a horror story. It's actually one of the most fascinating partnerships in biology.

For most of human history, we assumed our skin was either clean or dirty—a simple binary. Bacteria were the enemy, something to scrub away with soap and sanitize into oblivion. But modern genetic sequencing has revealed a far more interesting truth: your skin hosts roughly a thousand different species of bacteria alone, drawn from nineteen major groups called phyla. Most of them aren't just harmless—they're actively working to keep you healthy.

The Old Science Was Wrong

Here's where the story gets humbling for scientists. For decades, researchers believed that two species dominated human skin: Staphylococcus epidermidis and Staphylococcus aureus. This made sense based on what they could grow in their laboratories. The problem? Most skin bacteria refuse to grow in laboratory conditions.

It's like trying to take a census of a city by only counting people who show up at a single coffee shop.

Everything changed with a technology called 16S ribosomal ribonucleic acid sequencing, which identifies bacteria directly from their genetic material. No need to grow them in a petri dish—just read their genetic fingerprints. The results were startling: those supposedly dominant Staphylococcus species actually make up only about five percent of your skin's bacterial population. Scientists had been missing ninety-five percent of the picture.

The real bacterial breakdown looks quite different. The largest group, making up about fifty-two percent, belongs to Actinomycetota—a phylum that includes the bacteria responsible for that distinctive "earthy" smell after rain. Next comes Bacillota at twenty-four percent, followed by Pseudomonadota at sixteen percent, and Bacteroidota at six percent.

Three Different Worlds on One Body

Your skin isn't uniform. It's more like three distinct ecosystems crammed onto a single surface: oily regions, moist regions, and dry regions. Each attracts different microbial tenants.

The oily zones—your face, chest, and back—are dominated by bacteria that feast on sebum, the waxy substance your sebaceous glands produce. Propionibacteria and Staphylococci thrive here, breaking down fats and oils for energy.

In the moist territories—your armpits, the crook of your elbow, between your toes—Corynebacteria share the space with Staphylococci. These warm, humid environments create perfect conditions for bacterial growth, which is why these areas tend to be the most microbe-dense regions of your body.

The dry zones—your forearms, legs, and back of hands—host the most diverse bacterial communities. Here you'll find Betaproteobacteria and Flavobacteriales among many others, all adapted to survive with minimal moisture.

Interestingly, the oily regions actually support the richest variety of species. The popular assumption that moisture equals more life doesn't hold on human skin.

Your Microbial Fingerprint

Which parts of your body are most uniquely yours, microbiologically speaking? The spaces between your fingers and toes, your armpits, and—strangely—your belly button. Compare the bacteria living in these spots between two people, and you'll find remarkably different communities.

By contrast, the sides of your nostrils, the inside of your nose, and your back host bacterial populations that look fairly similar from person to person. Evolution has apparently settled on a standard microbial toolkit for these areas.

The Belly Button Biodiversity Project

In 2011, researchers at North Carolina State University launched one of the more charmingly named scientific initiatives in recent memory: the Belly Button Biodiversity Project. Their target was the navel—that small, often-ignored indentation in your abdomen.

Why belly buttons? They're nearly perfect microbial time capsules. The navel receives almost no ultraviolet light from the sun, which would kill bacteria. It's rarely scrubbed vigorously with soap. It doesn't produce any oils or secretions of its own. And it maintains a consistently warm, humid environment. If you want to study what your skin microbiome looks like when left relatively undisturbed, the belly button is the place to look.

Volunteers swabbed their navels with sterile cotton, twisted three times, and sent the samples back to the lab. What the researchers found defied easy prediction: while they could accurately guess which bacterial strains would be common and which would be rare, the specific rare strains varied wildly between individuals.

The common players were familiar—Staphylococcus, Corynebacterium, and representatives from Actinobacteria, Clostridiales, and Bacilli. But lurking alongside these usual suspects were exotic creatures. Two volunteers harbored Archaea—ancient microorganisms more closely related to the bacteria found in hot springs and deep-sea vents than to typical skin dwellers. One of these volunteers mentioned he hadn't bathed or showered in several years, which may explain why his navel had become hospitable to such unusual tenants.

Across ninety-five percent of participants, researchers identified approximately fourteen hundred different bacterial strains. Most were transient visitors that don't permanently colonize skin, but their sheer variety demonstrated something important: your belly button is a wilderness.

The Fungal Kingdom

Bacteria get most of the attention, but fungi also call your skin home. A study of the spaces between people's toes found fourteen different genera of fungi thriving in those dark, moist crevices. Some names you might recognize: Candida albicans, the yeast responsible for most yeast infections; Trichophyton rubrum, the fungus behind athlete's foot and ringworm; and Aspergillus flavus, a mold that can produce dangerous toxins in contaminated food.

The National Human Genome Research Institute conducted an even more comprehensive fungal census, sampling fourteen locations across the body. The results were surprising. Your heel—that calloused, seemingly inhospitable patch of skin—hosts the most diverse fungal community on your entire body, with about eighty different species. Toenail clippings harbor around sixty species. Between your toes, about forty.

Moving up the body, fungal diversity drops dramatically. Your palm, forearm, and inner elbow support eighteen to thirty-two species each. Your head and torso? Just two to ten.

This distribution makes a certain intuitive sense. Feet spend most of their lives trapped in the warm, dark, humid environment of shoes and socks—paradise for fungi. Your face and chest, exposed to air and light, offer fewer fungal opportunities.

Your Microscopic Bodyguards

Why does your body tolerate all these freeloaders? Because many of them aren't freeloading at all. They're paying rent.

Consider Pseudomonas aeruginosa. This bacterium has a complicated relationship with humans. Given the opportunity—say, if you have a weakened immune system or it gets into your bloodstream—it can cause serious infections in your bones, joints, digestive system, and lungs. It can trigger painful skin inflammation.

But while it's living peacefully on your skin's surface, P. aeruginosa produces a natural antibiotic called pseudomonic acid, which kills Staphylococcus and Streptococcus bacteria. (Doctors have isolated this compound and sell it as the antibiotic Mupirocin.) The same bacterium also secretes chemicals that inhibit the growth of several fungal species, including Candida and Aspergillus. It even suppresses Helicobacter pylori, the bacterium responsible for stomach ulcers.

This is why antibiotics can sometimes cause more problems than they solve. Kill off P. aeruginosa with oral or topical antibiotics, and you may inadvertently clear the way for harmful yeasts to colonize your skin. Your microbiome isn't just a collection of organisms—it's a balanced ecosystem where different species keep each other in check.

The Chemistry of Protection

Your skin has evolved several chemical weapons to manage its microbial population. The most important is simple acidity.

Healthy skin maintains a pH between four and four-point-five—roughly as acidic as tomato juice. This acidity comes from lactic acid in your sweat and additional acids produced by your resident bacteria. At this pH, beneficial bacteria like Staphylococci, Micrococci, Corynebacterium, and Propionibacteria flourish, while potentially harmful bacteria struggle.

The effect is dramatic. Gram-negative bacteria like Escherichia coli (the cause of many food poisoning cases) and harmful Gram-positive bacteria like Staphylococcus aureus (responsible for staph infections) can't establish themselves on properly acidic skin. The antimicrobial compounds your skin naturally produces work even better in acidic conditions, amplifying the effect.

When skin becomes alkaline—from certain soaps, for example—the defenses weaken. Bacteria stop adhering properly and shed more easily, but paradoxically, the skin also swells and opens pathways for bacteria to penetrate deeper. Your acidic skin barrier is a carefully calibrated defense system.

Your body also produces specialized antimicrobial proteins called cathelicidins. These molecules directly kill microbes, but they do much more than that. They trigger inflammation, promote the growth of new blood vessels, and stimulate skin repair. When cathelicidin production goes wrong, skin diseases follow. Atopic dermatitis, commonly called eczema, is associated with reduced cathelicidin production. Rosacea involves abnormal cathelicidin processing that causes excessive inflammation. Psoriasis has been linked to cathelicidin fragments that trigger the immune system to attack the body's own skin cells.

Vitamin D plays a crucial role in regulating cathelicidin production, which may partly explain why some skin conditions improve with sun exposure.

The Smell of Being Human

Sweat itself is virtually odorless. That distinctive human body odor? That's your bacteria talking.

Different bacterial species consume the components of sweat and produce different waste products. Propionibacteria, which thrive in the oily regions of adult skin, break down amino acids and release propionic acid—a compound with a sharp, vinegary smell. Staphylococcus epidermidis converts sweat into isovaleric acid, also known as 3-methyl butanoic acid, which contributes to the characteristic tang of body odor. Bacillus subtilis specializes in creating the pungent smell associated with feet.

What humans find offensive, other creatures find attractive. Flies, for instance, are drawn to many of the same compounds we associate with uncleanliness. Evolution has shaped both our bacteria's metabolism and our olfactory responses in an ongoing chemical conversation.

When the Balance Tips

The difference between a helpful microbe and a dangerous one often comes down to context. Skin microorganisms exist in a delicate equilibrium with your immune system—commensal, mutualistic, or pathogenic depending on circumstances. The same bacterium that protects you when you're healthy can turn deadly if your immune system becomes compromised.

Research on gut and lung microbiomes has demonstrated that resident bacteria play crucial roles in training and developing the immune system. Scientists are only beginning to explore whether the same holds true for skin, but early evidence suggests your skin microbiome may be teaching your immune system to distinguish friend from foe.

When skin is damaged, normally harmless bacteria can become dangerous. The diversity of species living on your skin appears to influence whether you develop conditions like dermatitis later in life—though scientists are still untangling whether reduced diversity is a cause or a consequence of skin disease.

Acne: A Bacterial Story

Acne vulgaris affects most people at some point in their lives. The condition involves excessive sebum production and inflammation, and areas with acne are typically colonized by Cutibacterium acnes (formerly called Propionibacterium acnes).

Here's the twist: C. acnes lives on everyone's skin, including people who never develop acne. The bacterium is a normal member of the skin community. So what determines whether it causes problems?

Strain matters. Certain strains of C. acnes are strongly associated with acne, while others appear on perfectly healthy skin. The total population of C. acnes doesn't differ much between people with acne and those without—it's which specific strains dominate that seems to make the difference.

Current treatments rely on antibiotics that reduce C. acnes numbers or activity. But researchers are exploring a more elegant solution: fighting bacteria with bacteria. Staphylococcus epidermidis, a common skin resident, produces succinic acid that inhibits C. acnes growth. Another bacterium called Lactobacillus plantarum, when applied to the skin, reduces inflammation and has been shown to shrink acne lesions. The future of acne treatment may involve carefully cultivated bacterial allies rather than indiscriminate antibiotics.

Eczema and the Staph Wars

Atopic dermatitis—the medical term for the most common form of eczema—presents a clearer bacterial villain. People with this condition show elevated populations of Staphylococcus aureus on both their affected skin and their apparently healthy skin. When eczema flares up, bacterial diversity plummets as S. aureus crowds out other species. When the flare subsides with treatment, diversity rebounds.

Current treatments combine antibiotics, corticosteroids, and sometimes diluted bleach baths to knock back S. aureus populations. But here again, probiotic approaches show promise. Staphylococcus epidermidis, the beneficial cousin of the problematic S. aureus, naturally increases during eczema flares—your body's own attempt to restore balance. Researchers are investigating whether boosting S. epidermidis populations could help control flares.

The gut may also play a role. Babies with low diversity of gut bacteria face higher risk of developing atopic dermatitis. Specifically, infants with low levels of Bacteroides bacteria and high levels of Bacillota seem more susceptible. Bacteroides produce anti-inflammatory compounds that may help prevent dermatitis from developing in the first place.

Psoriasis: The Diversity Paradox

Psoriasis typically strikes dry skin areas like elbows and knees. These regions normally host highly diverse bacterial communities in low overall numbers—the opposite pattern from oily areas. Studies have produced contradictory findings about exactly which bacteria are associated with psoriasis, with some research linking Bacillota and Actinomycetota to the condition and other studies associating these same groups with healthy skin.

One finding appears consistent: people with psoriasis have lower microbial diversity in their affected areas. But whether this reduced diversity causes psoriasis, results from it, or simply accompanies it remains unclear. Unlike acne and eczema, psoriasis hasn't yet yielded to probiotic treatment approaches.

Beyond Humans

Skin microbiomes matter beyond our species. The most dramatic example involves amphibians. A fungus called Batrachochytrium dendrobatidis—a chytrid fungus that produces swimming spores rather than the typical fungal threads—has caused catastrophic population declines in frogs and salamanders worldwide. The disease it causes, chytridiomycosis, has been implicated in the extinction of entire amphibian species.

This fungus represents skin flora gone horribly wrong on a global scale. Understanding how skin microbial communities function—and how they can be disrupted—has implications far beyond human medicine.

The Partnership Continues

The scientists who launched the Belly Button Biodiversity Project had a specific goal beyond cataloging navel bacteria: they wanted to change how people think about microbes. The default assumption—that bacteria are enemies to be eliminated—fundamentally misunderstands the relationship between humans and their microscopic partners.

You emerged from your mother's womb sterile, but within hours, bacteria began colonizing your skin. They've been with you ever since, evolving alongside you, dying and reproducing by the billions each day. They break down your sweat, fight off pathogens, train your immune system, and occasionally cause you problems when conditions shift in their favor.

You are not a single organism. You are an ecosystem. And the trillion creatures living on your surface are as much a part of you as your own cells.