What Are Probiotics? The Official Definition and What Does and Does Not Qualify
The scientific definition of a probiotic, its four working parts, and what does and does not qualify, from yogurt and heat-killed products to fiber and purified metabolites.
Content last updated . This page has not been medically reviewed.
In plain terms, "contains live bacteria" is not enough. A product needs live organisms, a documented benefit and a high enough amount, and the benefit belongs to a specific strain. This page is information only, not medical advice. For the wider picture see probiotic supplements explained.
The four working parts
| Part | What it means | Source |
|---|---|---|
| Live | The microorganism must be alive when administered | [2] |
| Documented benefit | It must be tested and shown to give a health benefit | [2] |
| Adequate amount | It must be given at high enough levels to produce that benefit | [2] |
| A specific, characterized organism | Different strains have different effects, so the organism has to be well defined | [2][3] |
Why "contains live bacteria" is not enough
ISAPP's 2021 consensus on fermented foods says the word "probiotic" should be used only when a demonstrated health benefit comes from well-defined, characterized live microorganisms. The benefit must come at least in part from the live organisms and go beyond any nutritional value of the food itself. [3] The NHS (UK) makes a related point: probiotics are generally classed as food rather than medicine, and because of how they are regulated we cannot always be sure a product has the bacteria on its label, enough of them, or bacteria that survive to reach the gut. [4]What does and does not qualify
| Item | Does it qualify as a probiotic? | Why | Source |
|---|---|---|---|
| Yogurt, kefir, miso, natto, tempeh, most cheeses | Not automatically | ISAPP lists yoghurt, kefir, miso, natto, tempeh and most cheeses among fermented foods with live microorganisms present, but the terms "fermented food" and "probiotic" are not interchangeable | [3] |
| Bread and other foods heated after fermenting | No | Baking after fermentation effectively kills the fermentation microorganisms | [3] |
| Heat-killed bacteria | No | The AGA noted that heat-killed preparations do not meet the strict definition of probiotics; ISAPP's 2021 consensus places inactivated preparations under the term "postbiotic" instead | [5][6] |
| Prebiotic fiber | No | A prebiotic is a substrate that host microbes use selectively; it is not a live organism | [7] |
| Purified microbial products such as butyric acid or lactic acid | No | ISAPP's 2021 consensus says purified metabolites are not postbiotics either | [6] |
| A live strain tested in people for a stated outcome | Yes, for that strain, outcome, amount and group | Probiotic status comes from documented benefit, not from the genus name | [2] |
| Live bacteria with no human study of benefit | Not shown to qualify | Live microorganisms must be tested and shown to have a health benefit | [2] |
Reading the name on a label
A probiotic name has a genus, a species and a strain code. NCCIH (US) says the most common probiotic bacteria belong to the groups Lactobacillus and Bifidobacterium, and that yeasts such as Saccharomyces boulardii are also used. [8] NCCIH also says different types may have different effects. [8]Names have changed. A 2020 taxonomy study proposed splitting the genus Lactobacillus into 25 genera, and said the everyday word "lactobacilli" would remain useful for the whole group. [9] Labels and papers may therefore show an older or a newer name for the same organism. For example, a 2026 review gives one strain as Lactiplantibacillus plantarum 299v in its abstract and as Lactobacillus plantarum 299v in its conclusions. [10] The same review writes another strain as Bifidobacterium longum (formerly Bifidobacterium infantis) 35624. [10] Our pages on strain names and taxonomy changes and how to read a probiotic label go further. Group profiles are at Lactobacillus and Bifidobacterium probiotics.
Official intake levels
The sources we read do not describe an official recommended intake (RDA, AI or upper limit) for probiotics.Frequently asked questions
What is a probiotic in one sentence?
Live microorganisms that, given in adequate amounts, give the host a health benefit. [1]
Is yogurt a probiotic?
Not automatically. Yogurt is a fermented food that usually has live microorganisms present, but ISAPP says the terms fermented food and probiotic cannot be used interchangeably. [3]
Is a heat-killed bacteria product a probiotic?
No. The AGA noted that heat-killed preparations do not meet the strict definition of probiotics. [5]
Does "probiotic" on a label mean the product works?
No. A probiotic must be tested and shown to have a benefit, and different strains have different effects. [2] The NHS adds that we cannot always be sure a product contains what the label says. [4]
Are probiotics and prebiotics the same thing?
No. Prebiotics are nondigestible food components that selectively stimulate desirable microorganisms, while probiotics are live microorganisms. [8]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Who defines it, and who regulates it
Safety and who should ask first
References
- Hill C, et al. The ISAPP consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506-14. Abstract read 2026-10-22.
- International Scientific Association for Probiotics and Prebiotics (ISAPP). Probiotics. Read 2026-10-22.
- Marco ML, et al. The ISAPP consensus statement on fermented foods. Nat Rev Gastroenterol Hepatol. 2021;18(3):196-208. Abstract read 2026-10-22; full text read via PubMed Central (PMC7925329).
- NHS (UK). Probiotics. Page last reviewed 24 November 2022. Read 2026-10-22.
- Preidis GA, et al. AGA Technical Review on the Role of Probiotics in the Management of Gastrointestinal Disorders. Gastroenterology. 2020;159(2):708-738. Full text read via PubMed Central (PMC8018518). Read 2026-10-22.
- Salminen S, et al. The ISAPP consensus statement on the definition and scope of postbiotics. Nat Rev Gastroenterol Hepatol. 2021;18(9):649-667. Abstract read 2026-10-22; full text read via PubMed Central (PMC8387231).
- Gibson GR, et al. The ISAPP consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol. 2017;14(8):491-502. Abstract read 2026-10-22.
- NCCIH (US National Center for Complementary and Integrative Health). Probiotics: Usefulness and Safety. Read 2026-10-22.
- Zheng J, et al. A taxonomic note on the genus Lactobacillus. Int J Syst Evol Microbiol. 2020;70(4):2782-2858. Abstract read 2026-10-22 (via Europe PMC).
- Maslennikov R, et al. Strain-specific systematic review with meta-analysis of probiotics efficacy in the treatment of irritable bowel syndrome. J Clin Med. 2026;15(3):1152. Full text read via PubMed Central (PMC12898053). Read 2026-10-22.