Your skin is covered in bacteria, and that is the good news

Skin is not a clean surface with things landing on it. It is an inhabited one.

Estimates vary with the method, but the figure usually quoted is around a million bacteria per square centimetre, alongside fungi, viruses and mites, spread across more than a thousand species when you count the whole body. They are not passengers waiting to be washed off. On the current evidence they are part of how skin works.

This is now the most oversold idea in skincare, which is a shame, because the underlying science is genuinely interesting and the honest version is more useful than the marketing one.

What actually lives up there

The community is not the same everywhere. Sequencing work through the early 2010s established that skin sites differ from each other more than most people expect, and that the pattern follows the terrain.

Oily areas like the forehead, the nose and the upper back are dominated by species that metabolise sebum, Cutibacterium acnes chief among them. Damp sites such as the crook of the elbow and behind the knee favour staphylococci and corynebacteria. Dry, exposed areas like the forearm carry the most varied mix of all. Your forehead has more in common with a stranger's forehead than with your own forearm.

The other finding worth knowing is that these communities are reasonably stable. Sample the same person a year apart and much of the population is still there, which is not what you would expect from something that gets soaped daily.

Why they seem to matter

Two lines of research make the case that these microbes do a job rather than just occupying space.

The first is competition. Staphylococcus epidermidis, one of the commonest residents, produces compounds that inhibit less welcome organisms, and the research credits the resident community with holding potential troublemakers in check simply by being established and using up the resources.

The second is more direct. Work published in 2021 found that commensal bacteria influence skin barrier function and repair through a signalling pathway in skin cells, which is a mechanism rather than a correlation. Mice raised without those microbes had measurably different barriers.

Then there is the association side, which is the strongest evidence in the field and also the most easily misused. Disruption of the community, which researchers call dysbiosis, has been repeatedly associated with skin conditions including eczema, acne and rosacea. In atopic dermatitis in particular, the proportion of Staphylococcus aureus rises during flares and falls afterwards.

An association is not a treatment. Nobody has shown that a moisturiser corrects any of this, and a cosmetic that claimed to would be making a medicinal claim it could not support. What the association does establish is that the community is doing something worth not disturbing.

Where the evidence stops

Here is the part the category tends to skip.

There is no agreed test behind the phrase "microbiome-friendly". No standard defines it, no independent body certifies it, and two products carrying the words may have been assessed in completely different ways or not assessed at all. Some brands run sequencing before and after use. Some mean their formula avoids harsh surfactants. Some mean nothing in particular.

Nor has it been established that adding a prebiotic or a postbiotic to a cream reliably changes the community living on a person's face in a way that matters to them. The ingredient research is real and often promising. The gap between an ingredient behaving well in a dish and a measurable improvement on human skin over months is wide, and it has not been closed.

The honest way to describe microbiome-conscious formulating is as an approach rather than an outcome. It says something about how a product was designed. It does not, on current evidence, promise a result.

What the research does support in practice

Strip out the ingredient argument and a fairly clear set of principles remains, and they apply whatever you use.

Skin sits at a slightly acidic pH, usually somewhere around 4.5 to 5.5, and the resident microbes appear to prefer it there. Traditional soap is strongly alkaline, and the research indicates that high-pH cleansing raises skin pH for a period afterwards. Cleansers formulated nearer skin pH are the easier choice.

Harsh surfactants strip lipids along with dirt. The squeaky-clean feeling that some cleansers leave behind is the sound of a barrier that has lost more than it should have. Skin should feel comfortable after washing, not tight.

Washing less aggressively, and less often, is generally the safer direction. Antibacterial products have their uses and daily facial cleansing is not one of them.

And be sceptical of anything promising to rebuild, restore or rebalance a microbiome. The claim runs well ahead of what has been shown, and a brand willing to make it about bacteria will probably make it about other things too.

The short version

Your skin carries a large, varied and mostly stable population of microbes. The evidence that they contribute to barrier function is good and improving. The evidence that any particular cosmetic ingredient reliably improves them is not there yet.

Which leaves the sensible position roughly where it has always been. Wash gently, stay near skin pH, leave things alone where you can, and treat the more dramatic microbiome claims as the marketing they mostly are.

If you want the detail on how we approach this in the formulations, it is set out on the skin microbiome page.