Your Skin’s Most Abundant Bacteria Isn’t the Most Active One, Study Finds

If you've ever wondered why your skin acts up in specific spots — a stubborn patch of eczema behind the knee, breakouts along the jawline, an itch that won't quit — the answer may have less to do with which microbes are camped out on your skin than with which ones are actually switched on. A study published in Nature Biotechnology used a technique called metatranscriptomics (measuring which genes microbes are actively expressing as RNA, rather than just counting their DNA) to map microbial activity at five body sites in 27 healthy adults. The results overturn a basic assumption: the bacteria and fungi with the most DNA on your skin are often not the ones doing the most metabolic work.

Key takeaways:

• The skin bacterium Cutibacterium acnes dominates skin DNA (up to 90% of the microbial genome pool at some sites) but contributes only a fraction of the active RNA output, while Staphylococcus bacteria and Malassezia fungi punch far above their genomic weight.
• Which microbes are metabolically "awake" differs by body site — cheeks, scalp and toe webs each favor different active species and pathways, hinting at why conditions like eczema, acne and dandruff show up where they do.
• The workflow is new and the cohort was healthy adults only, so the next real test is whether the same RNA signatures shift in people with skin disease.

Staphylococcus and Malassezia Outactivate Skin's Most Abundant Bacterium

The study's central finding is that a microbe's genomic abundance (how much of its DNA is present) does not predict how metabolically active it is. Across the five sites the researchers sampled — scalp, cheek, volar forearm, antecubital fossa (inner elbow) and toe web — C. acnes made up between 46% and 90% of the metagenome (the pooled DNA of all microbes present) at every site except the toe web, yet its share of the metatranscriptome (the pooled RNA, a proxy for which genes are actually being used) dropped to just 2% to 31%. Meanwhile the fungi Malassezia restricta and Malassezia globosa, which made up only 3–8% and 0.1–12% of skin DNA respectively, generated a disproportionate share of RNA: M. restricta transcripts made up 23–30% of activity on cheeks and scalp, while M. globosa transcripts ran as high as 21–81% on scalp, inner elbows and forearms. The researchers attribute this partly to fungal cells simply being larger and more metabolically busy per cell than bacteria, even though C. acnes carries a bigger population.

What This Divergence Means for Eczema, Acne and Dandruff

For anyone managing a chronic skin condition, the takeaway is that counting microbes is not the same as understanding what they're doing to your skin. Malassezia species, the fungi that dominated RNA activity at several sites in this study, are the same organisms long implicated in dandruff, seborrheic dermatitis and atopic dermatitis (eczema), and the paper found their gene expression shifted by location — for example, secreted fungal phospholipase C (an enzyme that breaks down skin lipids) was higher on cheeks than on the scalp, suggesting the fungus adapts its behavior to local oil availability rather than acting identically everywhere. On the bacterial side, C. acnes activity is already tied to acne: separate research has shown that vitamin B12 supplementation can suppress the bacterium's own B12-making genes and push it toward producing inflammatory byproducts, and prior research cited by the same paper found that reduced expression of that vitamin B12 pathway in C. acnes is itself associated with increased inflammation in acne lesions. For people who suspect their eczema, acne or itch is a "bacteria problem," this work is a reminder that it may really be an "active gene expression problem" involving both bacteria and fungi.

Mapping RNA From 27 Adults Across Five Skin Sites

To get here, the team first had to solve a technical problem: skin carries far less microbial material than the gut (roughly 1,000 to 10,000 bacterial cells per square centimeter, several orders of magnitude below gut density), and RNA degrades quickly, making it hard to sequence reliably. The researchers first validated a dedicated skin metatranscriptomics workflow in a pilot cohort, including a longitudinal group resampled across three consecutive days, and found gene-expression patterns were far more stable within the same person over time (median correlation ≥0.897) than between different people — confirming the method captures a real, individual-specific signal rather than noise. They then applied the validated workflow to the main study of 27 healthy adults sampled at scalp, cheek, volar forearm, antecubital fossa and toe web. Of 135 attempted samples, 102 metatranscriptome libraries and 130 metagenome libraries passed quality control, with roughly 98% of metatranscriptomic sequencing reads confirmed as microbial rather than human in origin. A related Nature Biotechnology study published around the same time, comparing RNA- and DNA-based skin profiling, independently reached a similar conclusion: the two sequencing approaches capture genuinely different, complementary information about the skin microbiome.

What Active Skin Microbes Could Mean for Managing Your Own Skin

None of this changes what you should do at the sink tomorrow morning, and the researchers themselves stop well short of recommending any product or intervention based on these results. What it does offer is a more precise vocabulary for a conversation with a dermatologist: if a topical treatment or oral medication seems to reduce visible bacteria or fungi without improving symptoms, this study helps explain why — DNA-based counts (the kind most microbiome tests report) may simply be measuring the wrong thing. The paper also found that skin commensals (microbes that normally live on skin without causing harm) transcribe a range of antimicrobial genes in situ, including several previously uncharacterized bacteriocins (small proteins bacteria use to kill or inhibit rival microbes), and identified more than 20 genes whose expression correlates with shifts in other species' abundance — early evidence of a microbial "conversation" happening on your skin in real time. None of these genes have been turned into a treatment or test, so there is nothing here to buy or try yet.

What This Study Doesn't Yet Answer

The obvious limitation is that all 27 participants were healthy adults, so this is a map of normal skin activity, not a comparison against eczema, acne, rosacea or psoriasis. The study also cannot say whether the active-but-low-abundance microbes are cause or consequence of the local skin environment — Malassezia might dominate transcription on oily cheek skin because it thrives there, or its activity might itself be shaping that environment, and this snapshot can't distinguish the two. Sample sizes at some individual sites were also modest once quality filtering was applied, and the study measured only 27 people's skin at one point in time (with a smaller three-day repeat), not the months or years over which real skin conditions develop and resolve.

Where Skin Microbiome Research Goes Next

The paper itself sets no timeline for what comes next, and it studied only healthy skin. But the logical follow-up experiment is obvious: apply the same RNA-based workflow to people with diagnosed skin conditions — eczema, acne, psoriasis and chronic wounds are the natural candidates — to see whether disease states carry a distinct "active microbiome" signature rather than just a different microbial census. The authors describe their workflow as clinically tractable, meaning it is designed to work with practical clinic-collected samples rather than only in a specialized lab, which is a prerequisite for any future diagnostic use — but such a test does not exist yet, and the current paper does not claim to be building one.

Common Questions

Does this mean skin bacteria like C. acnes don't matter? No — C. acnes still dominates the skin's microbial DNA and is well established as relevant to acne; the study shows its RNA activity is lower than its population size would suggest, not that it's irrelevant.

Is Malassezia dangerous to have on your skin? Not inherently — Malassezia lives on almost everyone's skin without causing problems, but this study adds to evidence that its activity level, not just its presence, may matter for conditions like dandruff and eczema in some people.

Could this lead to a new skin microbiome test? Potentially, but not yet — the workflow described here is a research tool validated in healthy adults, and the authors have not tested it as a diagnostic in people with skin disease.

Why does gene activity differ so much between skin sites? Each site (oily cheek and scalp, dry forearm, moist toe web) offers different amounts of lipids, moisture and nutrients, and the study found microbes tune their gene expression, such as lipid-digesting or antioxidant genes, to match those local conditions.

References

1. Minghao Chia, Amanda Hui Qi Ng, Aarthi Ravikrishnan, et al. (2025). Skin metatranscriptomics reveals a landscape of variation in microbial activity and gene expression across the human body. Nature Biotechnology. https://doi.org/10.1038/s41587-025-02797-4
2. Minghao Chia (2025). Skin metatranscriptomics reveals a landscape of variation in microbial activity and gene expression across the human body. https://doi.org/10.52843/cassyni.k5p8vm
3. Martin Glatz, Philipp Bosshard, Wolfram Hoetzenecker, Peter Schmid-Grendelmeier (2015). The Role of Malassezia spp. in Atopic Dermatitis. Journal of Clinical Medicine. https://doi.org/10.3390/jcm4061217
4. Dezhi Kang, Baochen Shi, Marie C. Erfe, Noah Craft, Huiying Li (2015). Vitamin B ₁₂ modulates the transcriptome of the skin microbiota in acne pathogenesis. Science Translational Medicine. https://doi.org/10.1126/scitranslmed.aab2009
5. (2025). RNA and DNA sequencing of skin microbiomes yield complementary yet distinct profiles. Nature Biotechnology. https://doi.org/10.1038/s41587-025-02801-x