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Everyone in this category uses both words. Almost nobody uses the second one correctly, and the definition is far stricter than the marketing suggests. This is my favourite subject, so bear with me.
The short answer
A probiotic is alive. A postbiotic is a preparation of deliberately inactivated microorganisms, and it has to actually contain those cells or their components. Here is the part almost nobody mentions: a filtrate with the cells removed is not a postbiotic either. Which means a great deal of what is sold under that word does not meet its definition any more than the probiotic products do.
I have written about the probiotic definition already. This article is about the other one, because it is newer, more precise, and almost universally misquoted.
Probiotic
“Live microorganisms which when administered in adequate amounts confer a health benefit on the host.”
ISAPP consensus, 20141
Postbiotic
“A preparation of inanimate microorganisms and/or their components that confers a health benefit on the host.”
ISAPP consensus, 20212
Read quickly, the second looks like a loophole. Anything derived from a microorganism, apparently. That is how the word is used in cosmetics, and it is wrong.
The consensus statement sets conditions. A preparation has to clear all of them.
1. It must contain the cells or their components. Inactivated microbial biomass has to be present. Not merely something the organism once touched.
2. The inactivation must be deliberate. ISAPP specifies heat, UV, sonication “or any deliberate treatment that inactivates the microbe”.4 Cells that happened to die along the way have not been through that step.
3. The source organism must be characterised. Same principle as the strain question. You cannot describe a preparation properly without knowing what it came from.
4. There must be a demonstrated benefit. The same requirement the probiotic definition carries, and the same one most products skip.
This is the part that matters for reading a cosmetic label. ISAPP was explicit that some things are excluded.2,3
Not postbiotics
Filtrates with the cells removed. If the material has been processed so that no cell biomass or cell components remain, it ceases to qualify, regardless of any benefit it may have.
Purified single molecules. Isolated proteins, peptides, exopolysaccharides or short-chain fatty acids on their own.
Chemically synthesised compounds, even if identical to something a microorganism makes.
Vaccines. Explicitly named and set aside.
Now look at a cosmetic ingredient list. The most common microbial entries in this category are ferment filtrates and ferment lysates. A lysate contains broken-open cells, so it has the components and can qualify. A filtrate, by name and by process, is the liquid left after the solids have been filtered out.
So the category has two problems, not one. Most products called probiotic are not alive. And a good share of the ones called postbiotic do not meet that definition either.
I would rather say this than have someone find it.
The lotion base in both of my products contains a sea kelp bioferment, which appears on the label as a ferment filtrate. By the standard I have just described, that ingredient is probably not a postbiotic. It is a ferment filtrate, which is a perfectly respectable cosmetic ingredient with its own literature, and it should be called that.
The live organisms in my products are in a separate chamber and are added at the moment of use. That part is the probiotic. The rest of the formula is a lotion, and some of what is in it came from microorganisms without being either of these two things.
The point
A standard is only worth anything if it applies to the person holding it. If I am going to ask you to check labels this carefully, my own has to survive the same reading.
Live probiotic
Contains: viable cells, countable in CFU.
Needs: protection from water, preservatives, heat and oxygen. Usually special packaging.
Ask for: a strain, a count, and a time point.
True postbiotic
Contains: deliberately inactivated cells or their components. Often a lysate.
Needs: very little. Stable, safe, no cold chain. A real advantage.
Ask for: the source organism, the inactivation method, and the evidence.
Ferment filtrate
Contains: the liquid from a fermentation, cells filtered out.
Needs: nothing unusual. It is a conventional cosmetic ingredient.
Ask for: nothing special. Just do not let it be sold to you as either of the other two.
Wrong question, and I say that as someone who has built a company around the harder option.
Postbiotics have genuine advantages. They are stable. They do not need to be protected from the formula they sit in. They carry no viability risk, which makes them safer for immunocompromised users and simpler to manufacture at scale. There is real evidence behind inactivated preparations, and I covered that literature in the last article.
Live organisms offer something a fixed preparation cannot: metabolic activity that continues and responds after application. That is worth the engineering, in my view. But it is a choice with costs, and anyone who tells you otherwise is selling something.
✗ The myth
“Postbiotic is the general term for anything derived from a microorganism.”
✓ The fact
It is a specific term requiring inactivated cells or their components to be present, from a characterised organism, with demonstrated benefit.
✓ Which of the three words is being used, and whether the ingredient list supports it.
✓ Lysate or filtrate. One contains cell components, the other has had them removed.
✓ Whether the source organism is named at all.
✓ For anything claiming to be live, a count and the time point it applies to.
Key takeaways
✓ Probiotic means live. Postbiotic means deliberately inactivated. They are not two words for the same idea.
✓ A postbiotic must actually contain inactivated cells or their components.
✓ Filtrates with the cells removed, purified single molecules, and synthesised compounds are explicitly excluded.
✓ Which means a good share of cosmetic ingredients marketed as postbiotics do not meet that definition either.
✓ That includes the sea kelp bioferment in my own lotion base, which is a ferment filtrate and should be described as one.
✓ Neither is better. Postbiotics are stable and safe. Live organisms keep working after application. Different tools, different costs.
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. doi:10.1038/s41575-021-00440-6
3. Frequently asked questions about the ISAPP postbiotic definition. Front Microbiol. 2023;14:1324565. Clarifies that preparations processed so that no cell biomass or components remain do not qualify as postbiotics. doi:10.3389/fmicb.2023.1324565
4. International Scientific Association for Probiotics and Prebiotics. What is a postbiotic? “The inactivation step may be heat, UV light, sonication, or any deliberate treatment that inactivates the microbe… Cells or cell fragments must be present as well.” isappscience.org
5. Piqué N, Berlanga M, Miñana-Galbis D. Health benefits of heat-killed (tyndallized) probiotics: an overview. Int J Mol Sci. 2019;20(10):2534. PubMed 31126033
6. 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.
