The first time I saw a Shannon diversity score in a skin study, I had to look up what it actually meant. The paper, in Cell Host & Microbe, mentioned facial samples scoring 2.3 on one cheek and 1.1 on the other in the same person. I assumed the numbers meant something self-explanatory. They did not.
So here is the version I wish someone had handed me.
What it actually is
Shannon and Simpson diversity indices are equations from ecology. Foresters use them to describe how many tree species live in a hectare of woodland and how evenly the population is split among them. Microbiome researchers borrowed both, applied them to bacteria identified through 16S rRNA gene sequencing, and the same logic carries over to the surface of your face.
A Shannon score of zero means one species owns everything. A score of 4.5 means dozens of species coexist in roughly equal numbers. Healthy facial skin in published studies tends to land between 1.5 and 3.0. Simpson is a sibling metric that emphasises evenness over richness. Two faces can have the same number of bacterial species but very different Simpson scores if one is dominated by a single overgrown genus and the other is balanced.
Why it matters
Lower diversity scores keep showing up next to skin trouble. A 2013 study in Genome Research found acne lesions had lower Shannon diversity than nearby healthy skin on the same face, with one or two Cutibacterium acnes strains dominating the community. Rosacea sites tend to follow the same pattern. So does eczematous skin in flare.
Higher diversity tracks with calmer, more resilient skin. The thinking, supported by research from Julia Segre’s group at the NIH, is that a varied community holds opportunistic pathogens in check and produces a richer mix of metabolites that calibrate the skin’s immune response. A balanced microbiome does barrier reinforcement, pH maintenance, and inflammation regulation all at once.
What you can do
You cannot get a clinical Shannon score for your face today. Direct-to-consumer skin microbiome tests exist but their reproducibility is uneven. So work upstream. The behaviours that raise diversity in published studies are the boring, slow-skincare ones: pH-balanced cleansing, fewer total products, less aggressive exfoliation, ceramide and squalane support rather than stripping, and ingredients that feed the resident community.
Microbiome Glow Serum is built around fermented postbiotics that support diversity rather than override it. Postbiotic ingredients are doing most of the meaningful work in this category right now. You are not aiming for a specific number. You are aiming for the conditions in which a varied community can stabilise.
The contrarian view: stop chasing your diversity number
The temptation, the moment consumer skin microbiome tests improve, will be to treat the Shannon score the way fitness culture treats VO2 max. A number to optimise, post about, retest every six weeks. I would push back on that hard. Diversity scores are sensitive to sampling site, time of day, what you cleansed with that morning, and whether you wore makeup. A single test is a snapshot of a fluctuating system. Treating it as a grade encourages exactly the over-correction it warns against.
The real numbers
The Cell Host & Microbe 2016 paper by Oh et al., which mapped the skin microbiome across 18 body sites in healthy adults, reported facial Shannon scores ranging from roughly 1.2 to 3.1, with forehead and cheeks averaging around 2.0. Studies on acne-affected versus clear skin published in the Journal of Investigative Dermatology have shown diversity reductions of 30 to 50 percent in lesional sites compared with adjacent healthy skin on the same individual. After a course of topical antibiotics, Shannon scores can take 8 to 12 weeks to return to baseline.
FAQ
Can I test my skin microbiome at home? Several companies offer it, but the science of consumer testing is still maturing. Useful for curiosity, not for medical decisions yet.
What kills diversity fastest? Broad-spectrum topical antibiotics, harsh high-pH cleansers used twice daily, daily aggressive exfoliation, and antibacterial face washes are the most consistent disruptors in the literature.
Do postbiotic products actually raise diversity? Small studies suggest fermented postbiotics support diversity over weeks of use, though the effect size is modest. The clearer benefit is calming inflammation, which creates conditions where diversity can rebuild on its own.
Is higher diversity always better? Not always. Some pathogenic species add to a diversity count without adding to health. Evenness and species identity both matter.
How long until a microbiome-friendly routine shows up visibly? Most readers see meaningful changes in barrier feel and reactivity within four to eight weeks. Visible tone evens out around the three-month mark.
Sources
- Oh J et al. Temporal stability of the human skin microbiome. Cell Host & Microbe, 2016.
- Fitz-Gibbon S et al. Propionibacterium acnes strain populations in the human skin microbiome. Journal of Investigative Dermatology, 2013.
- Grice EA, Segre JA. The skin microbiome. Nature Reviews Microbiology, 2011.
- Byrd AL et al. The human skin microbiome. Nature Reviews Microbiology, 2018.
Related: microbiome skincare guides.