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The word “probiotic” has a formal definition, agreed by the scientists who study them. Most products using the word on a skincare label do not meet it. Here is the definition, and what it actually rules in and out.
The short answer
Probiotics are living microorganisms. That is not a marketing position, it is the definition used by the international scientific body that governs the term. If the organisms in a product are not alive when you use it, the word does not apply, however the label is written.
In my last article I wrote about the community of microorganisms living on your skin. This one is about the word that gets attached to products claiming to work with it.
Probiotic skincare is discussed constantly and defined almost never. So let’s start with the definition, because it settles most of the argument.
In 2014 an expert panel convened by the International Scientific Association for Probiotics and Prebiotics reaffirmed the definition originally set out by the FAO and WHO.1 Probiotics are:
“live microorganisms which when administered in adequate amounts confer a health benefit on the host”
Short sentence, three separate requirements. Each one is a place a product can fail.
1. Live
Alive at the point of use, not at some earlier stage of manufacturing. This is the requirement most skincare products fail.
2. Adequate amounts
A countable quantity, not a trace. Which means a number should appear somewhere. It rarely does.
3. A demonstrated benefit
Shown, not assumed. And in cosmetics this word carries complications, which I come back to at the end.
Here is the part that matters most when you’re reading a label. The same scientific body defined a second term in 2021, for preparations that are not alive.2 Put the two definitions next to each other and the distinction is unmissable.
Probiotic
“Live microorganisms which when administered in adequate amounts confer a health benefit on the host.”
ISAPP consensus, 2014
Postbiotic
“A preparation of inanimate microorganisms and/or their components that confers a health benefit on the host.”
ISAPP consensus, 2021
Live in one, inanimate in the other. The field went to the trouble of creating a separate word precisely so the two would not be confused.
This is not a criticism of postbiotics. Lysates, ferment filtrates and cell components are legitimate cosmetic ingredients with their own literature. The problem is only that they are frequently sold under the other word.
✗ What the label suggests
“Contains probiotics” implies living organisms are in the jar and will reach your skin.
✓ What is often inside
Fragments of organisms that were killed during manufacturing, or a ferment they were grown in. Neither is alive.
This is the fair question, and it’s the one I’d ask if I were reading this rather than writing it. If postbiotics are legitimate ingredients, why go to the considerable trouble of keeping cells alive?
Because a living cell can act in three different ways, and only one of those is available to a preparation that arrives dead.
Mode one
As a living cell
A metabolically active organism responds to the environment it lands in. It is a dynamic ingredient rather than a fixed dose of a molecule, which is a genuinely different thing from anything else in a formulation.
Mode two
As a producer
The organisms used in this category are lactic acid bacteria, so named because that is what they make. Lactic acid is not an exotic ingredient, it has been in cosmetics for decades. The difference is that here it is being produced in place rather than added from a drum.
Mode three
As what remains
Cells do not live forever. When they break down, their components remain, which is to say they become a lysate. Everything a postbiotic offers is still there at the end.
That is the argument in one line: a live preparation gives you all three modes, a non-living one gives you the third from the outset. You are not choosing between live and postbiotic. You are choosing between all three and one.
Worth saying plainly
I have described what these organisms are and what they do. I have deliberately not told you what that does for your skin. The formal definition of a probiotic includes a health benefit, and health is not a claim skincare gets to make. That boundary is a real one and I would rather name it than write around it.
One more thing worth knowing before you judge the category too harshly. Living cells in a cosmetic are recent. The techniques for keeping them viable through manufacturing, storage and use are still being worked out, largely inside companies rather than in the open literature.3
Which means two things are true at once. Scepticism about the label is warranted, because most products using the word do not meet the definition. And the field is early enough that the answers are still being written, including by people like me. Both of those can be true, and I think you should hold both.
I want to be fair here, because this is the part I know from twenty years of doing it. Keeping microorganisms alive inside a cosmetic is genuinely hard, and most products that fail the definition are not being dishonest. They are running into physics.
A cosmetic formula is a hostile environment for a living cell.3 Four things in particular work against viability:
Water. Most creams are largely water. Water lets microorganisms become metabolically active, which sounds useful but means they consume their reserves and die long before you open the jar.
Preservatives. Cosmetics need preservation to be safe. Preservatives work by killing microorganisms. They cannot tell the difference between contamination and the organisms you deliberately added.
Temperature. Shipping, warehouses, a bathroom shelf in August. Cells experience all of it.
Oxygen. Depending on the organism, exposure during filling and storage can be lethal.
In plain terms
A standard cream is designed to stop microorganisms surviving in it. That is what makes it safe to keep in your bathroom for a year. Asking that same formula to also keep microorganisms alive is asking it to do two opposite jobs at once.
Solving it means designing around the problem rather than adding organisms to a conventional base and hoping. That usually involves keeping the formula dry until the moment of use, since without water the cells stay dormant rather than active.
You do not need a laboratory to assess this. You need three answers, and their absence is itself informative.
1. Is the organism identified, and to a level that matches what is being claimed? A genus on its own tells you very little. A species tells you a great deal. A strain designation becomes necessary when a specific effect is being claimed. That one gets its own article.
2. Is viability stated, with a number? The definition requires adequate amounts. A brand that has counted its organisms can tell you the count. A brand that hasn’t, can’t.
3. Is it explained how they stay alive until you use them? Given everything above, a product that is alive at point of use has done something deliberate to achieve it. Ask what.
If a brand can’t answer these, it isn’t necessarily selling you something bad. But it is probably selling you a postbiotic under the wrong name.
I said I would come back to the third part of the definition, and this is the section where I have to be careful about my own house as well as everyone else’s.
The definition requires a demonstrated health benefit. That phrase comes from food and medicine, where health claims are permitted and expected. Cosmetics operate under different rules and do not make health claims.
So the honest position is this: the word probiotic was defined for a context that isn’t cosmetics. When it’s used on a skincare label, the parts about being alive and being present in countable amounts translate directly and can be verified. The third part does not translate cleanly, and anyone telling you otherwise is skipping over something. That applies to me too.
Key takeaways
✓ Probiotic has a formal definition and it requires the organisms to be alive.
✓ Postbiotic is the correct term for preparations that are not alive. It was defined separately, on purpose.
✓ Keeping cells alive in a cosmetic is a real technical problem, not an excuse.
✓ A living cell can act three ways: while alive, through what it produces, and through what remains after it breaks down. A non-living preparation offers only the third.
✓ Ask how the organism is identified, ask for a viability number, and ask how viability is maintained.
✓ The “health benefit” part of the definition doesn’t transfer neatly into cosmetics. Be wary of anyone who pretends it does.
References
Links go to the peer-reviewed record. You are welcome to check my work.
1. Hill C, Guarner F, Reid G, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506–514. PubMed 24912386
2. Salminen S, Collado MC, Endo A, et al. The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics. Nat Rev Gastroenterol Hepatol. 2021;18(9):649–667. PubMed 34326533
3. Rezaei F, Rollin JA. A Microbial Formulation Perspective on Probiotic Skincare: Viability, Challenges, and Current Approaches to Maintain Probiotic Viability. Biotechnol Bioeng. 2026. doi:10.1002/bit.70268
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About the author
Dr. Farzaneh Rezaei, PhD, Microbial Formulation Scientist
Founder of Fafabiotic, with two decades developing microbial technologies. She writes Ask the Scientist to explain the science behind probiotic skincare without the marketing layer.
Educational content only. This article is not medical advice and is not intended to diagnose, treat, cure or prevent any disease. Last reviewed 5 August 2026.
