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This is the fairest challenge to everything I have written so far. If preparations of dead cells also do things, why go to the trouble of keeping them alive? The honest answer has two halves, and only one of them is about biology.
The short answer
It depends what you are asking the organism to do. Some effects have been demonstrated with cells that were deliberately killed. Others require a cell that is metabolically active. But the word itself is not negotiable: probiotic means live, so a product that is not alive is something else, whatever that something else may be worth.
In the last two articles I set out what a probiotic is and why naming the organism matters. This one takes the obvious objection seriously.
I have a commercial interest in the answer, so I want to give the opposing case its strongest form first. It is stronger than most people in my industry admit.
There is a body of literature showing that deliberately inactivated microorganisms produce measurable biological responses. It is substantial enough to have earned a name.
“The probiotic paradox”
A 2010 review in Nutrition Research Reviews collected the evidence that both live and dead cells act as biological response modifiers, in many cases producing similar responses.1
A 2019 overview in the International Journal of Molecular Sciences reached a similar conclusion for heat-killed preparations, describing immunomodulatory effects and effects on barrier integrity, and noting a real advantage that live organisms cannot match: a safety profile and a shelf life that require no special handling at all.2
So the honest position is not that non-living preparations do nothing. Some of them clearly do something, and the reason is not mysterious. A bacterial cell is made of molecules that a host can detect whether or not the cell is alive.
Present either way
Cell wall material such as peptidoglycan and lipoteichoic acid. Surface proteins. Nucleic acids. Exopolysaccharides. Whatever the organism produced before it was inactivated, if it was not washed away in processing.
These are structural. Killing the cell does not remove them. That is the entire basis of the postbiotic field, and it is a legitimate one. Worth noting that present and released are not the same thing: unless a cell has actually broken open, its contents are still inside it. More on that distinction in a later article.
Now the other side. Several mechanisms attributed to probiotics are not properties of the material a cell is made of. They are properties of the cell doing something, and they end when it dies.
It keeps producing
A dead cell carries whatever it made before it died. A living one carries on making it. Lactic acid bacteria are named for a compound they generate continuously while active.
It responds
Metabolism shifts with the conditions the organism finds. A fixed preparation of molecules cannot do that. It is the same dose regardless of where it lands.
It occupies space
Competitive exclusion, meaning competition for attachment sites and nutrients, requires an organism capable of competing. This is the mechanism most clearly dependent on viability.
And you keep the rest regardless
Cells do not last forever. Whatever the organism is made of stays in the formula either way. So choosing live is not a trade against the alternative. It is the three above, added to it.
The difference is not a longer list of ingredients. It is the difference between a fixed dose of molecules and an ongoing process.
Two separate questions have been hiding inside one, which is why this argument never resolves.
Question one. Does every documented effect require a living cell? No. Some have been shown with deliberately inactivated preparations, and pretending otherwise would mean ignoring a real literature.
Question two. Does something have to be alive to be called a probiotic? Yes. That is not a matter of opinion or of how strong the evidence is. It is the definition, agreed by the field, and it is the first word in it.3
Which is why the field created a second term. If a preparation is not alive and it has a demonstrated benefit, it has a name already, and that name is postbiotic.4 Nothing is being taken away from it. It is being called what it is.
Two things are worth separating, because the second one is the actual problem.
The first is a genuine scientific question about which mechanisms need viability, and it is open. The second is a labelling question, and it is not open at all.
In a review I published this year with a colleague, we looked at the state of the category. Most commercial products described as probiotic skincare contain non-viable microbial derivatives rather than live, strain-identified organisms, largely because of formulation, stability and regulatory constraints rather than any decision about mechanism.5
Worth saying plainly
My objection has never been that non-living ingredients are useless. It is that two different things are being sold under one word, and the word chosen is the one describing the harder, rarer option. If a product contains inactivated material, the honest move is to say so and defend it on its own evidence. Plenty of it would survive that test.
✗ The myth
“Dead probiotics work just as well, so viability is a marketing gimmick.”
✓ The fact
Some effects survive inactivation and some do not. Both statements are supported. Neither makes it accurate to call an inactivated preparation a probiotic.
Key takeaways
✓ Deliberately inactivated microorganisms do produce measurable biological responses. This is documented, and it is called the probiotic paradox.
✓ Structural components survive inactivation. Cell wall material, surface proteins and exopolysaccharides are all still there.
✓ Continuous production, response to conditions, and competition for space and nutrients all require a metabolically active cell.
✓ A live preparation contains non-viable cells over time as well, so choosing live adds mechanisms rather than trading them away.
✓ Whether viability is required for a given effect is a scientific question. Whether it is required to use the word probiotic is not. It is the definition.
✓ Most products marketed as probiotic skincare contain non-viable derivatives.
References
Links go to the peer-reviewed record. You are welcome to check my work.
1. Adams CA. The probiotic paradox: live and dead cells are biological response modifiers. Nutr Res Rev. 2010;23(1):37–46. PubMed 20403231
2. 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
3. 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
4. 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
5. 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.
