What Is the Skin Microbiome? How It Works, Why It Matters & How to Protect It — Complete Guide

Scientifically grounded  ·  Dermatology-informed  ·  No marketing claims

At a Glance

For most of skincare history, bacteria on the skin were treated as a problem to be eliminated. Antibacterial soaps, alcohol toners, and harsh cleansers were considered thorough and effective. The goal was a "clean" skin surface.

We now know this was precisely wrong. The skin surface is not sterile — and it should not be. It is home to trillions of microorganisms that perform functions no skincare product can replicate: defending against pathogens, supporting the immune system, maintaining pH balance, and actively reinforcing the skin barrier.

The science of the skin microbiome has transformed dermatological thinking over the past two decades. Understanding it changes not just what you put on your skin — but what you should stop putting on it.

What Is the Skin Microbiome?

The word "microbiome" refers to the complete community of microorganisms living in a particular environment — their collective genes, metabolic activity, and interactions with the host. The skin microbiome is this community as it exists on and within the skin's surface.

The skin is the human body's most microbially diverse organ. Research estimates that one square centimetre of skin can host up to one million microorganisms. Across the entire skin surface, the total number is estimated in the trillions. These microorganisms exist in distinct communities that vary based on body site — the face, scalp, armpits, and forearms each host different microbial populations, shaped by local temperature, humidity, sebum levels, and pH.

Most of these microorganisms are permanent residents — called commensals — that have co-evolved with human skin over millions of years. They are not passing through. They are part of the skin's normal functioning. The minority that cause problems — called pathogens or opportunistic pathogens — typically only cause harm when the balance of the community shifts in their favour.

Who Lives There — The Key Players

The skin microbiome is dominated by bacteria, but also includes fungi, viruses, and microscopic mites. Understanding the key players — and what they do — explains why balance matters more than total elimination.

What Does the Skin Microbiome Do?

The skin microbiome performs several functions that are difficult — or impossible — to replicate with skincare ingredients alone. These are not passive effects. They are active, ongoing biological processes happening on your skin right now.

1. Competitive exclusion — crowding out pathogens

Beneficial bacteria occupy the physical space on skin and consume available nutrients, leaving less room and fewer resources for harmful pathogens to establish themselves. This is called competitive exclusion — and it is one of the most fundamental mechanisms of microbiome protection. Remove the beneficial bacteria through over-cleansing or antibiotics, and the space they occupied becomes available for whatever comes next.

2. Antimicrobial production

S. epidermidis and other commensal bacteria produce natural antimicrobial compounds — including bacteriocins and serine proteases — that directly inhibit the growth of pathogens like S. aureus and certain fungal species. This is a biological defence system that no synthetic preservative system can fully replicate.

3. Immune regulation

The skin microbiome communicates directly with keratinocytes and immune cells in the epidermis, helping to calibrate the skin's immune response. A diverse, balanced microbiome supports immune tolerance — the ability to not react to harmless substances. A disrupted microbiome is associated with heightened immune reactivity — the skin overreacting to things it would normally ignore. This is the microbiological basis of sensitisation and the "suddenly reactive skin" phenomenon that many people experience. For more on how this affects sensitive skin, see our full guide.

4. Acid mantle maintenance

Several commensal bacteria produce organic acids as metabolic byproducts — including lactic acid and short-chain fatty acids. These contributions help maintain the skin surface's slightly acidic pH (4.5–5.5). When the microbiome is disrupted, this contribution is lost, and the acid mantle pH can drift upward — impairing barrier repair enzymes and creating conditions where S. aureus thrives more easily.

5. Barrier support

The microbiome and the skin barrier are in constant chemical communication. Commensal bacteria produce compounds that stimulate ceramide synthesis, support keratinocyte function, and reinforce tight junction integrity. The relationship is bidirectional — a healthy barrier supports a healthy microbiome, and a healthy microbiome supports a healthy barrier. Read our guide to ceramides in skincare for more on how to support this from the barrier side.

The Microbiome–Barrier Connection

The skin microbiome and the skin barrier are not separate systems — they are deeply interdependent. Understanding their relationship explains why so many skin conditions are so difficult to resolve when only one is addressed.

This is the cycle seen in eczema: barrier disruption allows S. aureus to colonise, which releases toxins that further damage the barrier, which allows more colonisation. Research confirms that S. aureus can be detected on the skin of eczema patients even in clinically clear, non-flaring skin — meaning the microbiome imbalance persists even when symptoms appear resolved.

The practical implication: supporting only the barrier without addressing the microbiome — or vice versa — is an incomplete approach for chronically reactive or eczema-prone skin. Both systems need to be supported simultaneously. For a deeper look at TEWL measurement and barrier function, see our TEWL explainer.

The Skin Microbiome and Common Skin Conditions

The research connecting microbiome balance to visible skin conditions has grown substantially in the past decade. The relationships are not simple cause-and-effect — they are bidirectional, with each condition both influencing and being influenced by the microbiome.

Eczema (atopic dermatitis)

Evidence: Strong

S. aureus colonises the skin of approximately 90% of people with moderate-to-severe atopic dermatitis, compared to around 5–30% in people without the condition. This colonisation is not a coincidence — it is a driver. S. aureus produces serine proteases and toxins that directly break down ceramides, degrade tight junctions, and trigger immune pathways that sustain the inflammatory cycle. Reducing S. aureus colonisation — through microbiome-supportive care rather than broad-spectrum antibiotics — is now recognised as a meaningful component of eczema management.

Acne (acne vulgaris)

Evidence: Moderate-to-strong

The relationship between Cutibacterium acnes (C. acnes) and acne is more nuanced than previously understood. It is not simply that C. acnes causes acne — the organism is present on all skin types, including clear skin. What matters is strain diversity and relative dominance. Acne-prone skin shows reduced diversity within C. acnes strains and overrepresentation of certain inflammatory strains. Approaches that preserve overall microbiome diversity — rather than eliminating C. acnes entirely — are increasingly favoured in dermatological research.

Rosacea

Evidence: Emerging

Rosacea is associated with elevated levels of Demodex mites — which, at higher densities, may trigger immune responses in susceptible individuals — and with altered bacterial populations on facial skin. The microbiome differences seen in rosacea are consistently observed but causality is not yet fully established. What is clear is that microbiome-disruptive products — high-alcohol toners, harsh exfoliants, fragrance — consistently worsen rosacea symptoms.

Psoriasis

Evidence: Emerging-to-moderate

Psoriatic skin shows reduced microbiome diversity compared to healthy controls, with shifts in bacterial communities both at plaques and in uninvolved skin. Research into the gut-skin axis suggests that gut microbiome imbalance may also contribute to systemic inflammation that manifests in psoriasis — though this remains an area of active investigation.

What Disrupts the Skin Microbiome?

The skin microbiome is remarkably resilient — it can recover from short-term disruption. But repeated or sustained disruption shifts its composition in ways that take significantly longer to resolve. Most common disruptors are in the bathroom cabinet.

Signs Your Skin Microbiome May Be Out of Balance

Microbiome imbalance — called dysbiosis — does not always announce itself clearly. Many of its signs overlap with other skin concerns, which is why it is frequently missed or misattributed.

How to Support Your Skin Microbiome

The most important insight from microbiome research is counterintuitive: the best way to support your skin microbiome is mostly about what you stop doing, not what you add. The microbiome has been managing itself successfully for the entirety of human evolution — it mainly needs you to stop disrupting it.

Prebiotics, Probiotics, and Postbiotics — What the Evidence Actually Says

The terms "probiotic," "prebiotic," and "postbiotic" have migrated from gut health into skincare — often with more marketing ambition than scientific rigour. Here is what the current evidence supports.

Prebiotics — the most consistently supported

Prebiotics are ingredients that selectively feed beneficial bacteria without being consumed themselves. In skincare, this typically means complex sugars, fermented extracts, and certain plant-derived compounds that S. epidermidis and other beneficial bacteria can metabolise, while S. aureus is less able to utilise.

Examples with evidence of skin microbiome support include inulin, beta-glucan, oat-derived compounds, and certain fermented botanical extracts. These can be incorporated into standard cosmetic formulations at effective concentrations. They do not require live bacteria to be present — they work by selectively shifting the nutrient environment.

Probiotics (live) — limited by formulation stability

The appeal of putting live beneficial bacteria directly on the skin is intuitive. The challenge is that most live bacteria cannot survive in cosmetic formulations — they require specific temperatures, pH conditions, and absence of preservatives that are incompatible with shelf-stable products. Some encapsulation technologies are emerging to address this, but the evidence for topical live probiotics in standard skincare formulations remains limited and inconsistent.

Postbiotics — the emerging category with real promise

Postbiotics are the beneficial metabolic byproducts produced when bacteria ferment substrates — including antimicrobial peptides, organic acids, enzymes, and cell wall fragments. These are biologically active compounds that retain functional properties without requiring live organisms to be present. Fermented ingredients — including fermented lactobacillus filtrate, fermented saccharomyces extracts, and others — are increasingly used in skincare formulations with growing evidence that they support barrier function, skin comfort, and microbial balance. This is the category that Bold Purity's AquaBlur™ and SkinReset™ PDRN Serum draw on in their formulation approach.

Best Skincare for Supporting the Skin Microbiome

Microbiome-supportive skincare is not a single product — it is a layered approach that addresses each stage of your routine. The sequence matters: a microbiome-disruptive cleanser undoes every subsequent step. Below are the three product categories with the highest evidence base, and how Bold Purity's formulations address each one.

Frequently Asked Questions

The Microbiome Is Not a Problem to Solve. It Is a System to Support.

The shift in understanding from "bacteria on skin are bad" to "bacteria on skin are essential" is one of the most significant changes in dermatological thinking of the past two decades. And it has direct, practical implications for what you do — and stop doing — every day.

Most skincare routines that cause long-term problems do so not because of what they fail to add, but because of what they continuously strip away. The commensal community that has lived on human skin for millions of years is extraordinarily good at its job — it just needs a surface that is not being disrupted faster than the microbiome can recover.

Less disruption, pH-compatible products, no fragrance, a functioning skin barrier. These are not complex interventions. But they are the ones the science most consistently supports — and they are the foundation on which everything else in a skincare routine should be built.