Here is a simple way to think of postbiotics: postbiotics are “good” dead bacteria. And with a little more detail: Postbiotics are beneficial inactivated microbes with or without their parts. Let’s dig into the details of the definition and its controversy, because it’s actually more complicated than that.
The term “postbiotics” has been debated in the technical world over the last few years. Researchers and non-researchers have used the term “postbiotics” to describe probiotics that have been inactivated (dead good bacteria) or to describe products produced by bacteria or probiotics (stuff good bacteria make).
Neither of these definitions are the same as “good dead bacteria,” which is, in part, how the International Scientific Association for Probiotics and Prebiotics (ISAPP) defined postbiotics in 2021.
Their technical definition of postbiotics is “a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host”.2 “Inanimate microorganisms” are basically dead cells. During inactivation, the dead cells may be split up into their parts (cell components or structures, like proteins or lipopolysaccharides that make up the cells).
However, cell byproducts – bioactive metabolites made by the microbes before they were inactivated – are not themselves defined as postbiotics by ISAPP, although such byproducts may be present within a postbiotic product.
There are two additional things to note here, for the science-lovers amongst us.
First, this definition does not require that postbiotics come from probiotics. For example, bacteria that may not act as probiotics (that is, they have not been tested and shown to provide a benefit, OR they did not provide a benefit when tested), could be inactivated and provide a benefit to human health only in their inactivated form.
Second, ISAPP doesn’t consider metabolites of probiotics to be postbiotics. Organic acids, vitamins, and short-chain fatty acids, such as butyrate, acetate or propionate, may be produced by some bacteria, but a “postbiotic” must include inactive cells or cell parts, not just metabolites.
One reason not to consider a molecule like butyrate as a postbiotic is that it can also be found independently from bacteria. Butyrate exists in butter fat, for example.3,4 Vitamin K, which is important for blood coagulation as well as possibly bone and heart health5 can be sourced from bacteria or from green vegetables.
Examples of metabolites or bacterial products that do not, by themselves, meet ISAPP’s definition of postbiotics (but may be PART of a postbiotic) include:2
- Short-chain fatty acids produced by bacteria, such as butyrate
- Vitamins produced by bacteria, such as vitamin K
- Organic acids including lactic acid
- Proteins, including antimicrobial peptides, bacteriocins, or enzymes secreted by bacteria
You may see butyrate and other products of bacteria still described as postbiotics since ISAPP only recently published this definition and it has not been universally accepted. Similarly, many items are currently called probiotics which do not fit the probiotic definition put forward in 2001 by the World Health Organization (WHO) and Food and Agriculture Organization (FAO).6
In summary, a postbiotic contains dead cells and may also contain other components or metabolites of the live microbes from which they are derived.
References:
1. Hill, C., et al. Nat Rev Gastroenterol Hepatol. 2014;11(8):506-514. doi:10.1038/nrgastro.2014.66
2. Salminen, S. et al. Nat Rev Gastroenterol Hepatol. 2021;18(9):649-667. doi:10.1038/s41575-021-00440-6
3. Danudol, A., & Judprasong, K. Foods. 2022;11(22):3606. doi:10.3390/foods11223606
4. Månsson, H. Food Nutr Res. 2008;52:10.3402/fnr.v52i0.1821. doi:10.3402/fnr.v52i0.1821
5. Mladěnka, P. et al. Nutr Rev. 2022;80(4):677-698. doi:10.1093/nutrit/nuab061
6. https://www.fao.org/3/a0512e/a0512e.pdf. Accessed March 22, 2024.