Strain Specificity: Why One Strain's Result Does Not Transfer to Another
Why a result for one probiotic strain should not be assumed for another strain, species, product or food, and where expert panels say some effects are shared.
Content last updated . This page has not been medically reviewed.
Quick answer
A probiotic result belongs to the exact strain or product that was tested, in the people and at the amount studied. The US National Center for Complementary and Integrative Health (NCCIH) gives the plain example: if a specific kind of Lactobacillus helps prevent an illness, that does not necessarily mean another kind of Lactobacillus, or any Bifidobacterium probiotic, would do the same thing. [1] The NHS (UK) puts it the same way: a type that helps with one problem does not mean it will help other problems, or that other types will work just as well. [2]
This page is information only. It does not recommend any strain or product.
How a probiotic is named
Names go from broad to narrow: genus (for example Lactobacillus or Bifidobacterium), then species, then strain, which is a specific lineage with its own code. A label that stops at genus and species leaves out the part that studies are attached to. Scientific names can change as organisms are reclassified, so an older paper and a current label may not use the same name; see Strain names and taxonomy changes and Probiotic strains and labels.What the sources we read say
| Source | What it says | Strength |
|---|---|---|
| NCCIH (US) [1] | Different types of probiotics may have different effects. | Official public-health information |
| NHS (UK), reviewed 2022 [2] | Helping one problem does not mean helping others; other types may not work as well. | Official public-health information |
| Health Canada notice, January 2010, archived [3] | Not all strains of a species have probiotic activity; changing the strain is treated as a fundamental change to the medicinal ingredients; claims are expected to rest on strain-specific evidence in humans. | Regulator guidance; archived, not updated |
| AGA technical review, 2020 [4] | The overall evidence is promising for probiotics in general and for certain strains and combinations in particular, in preventing C. difficile-associated diarrhea after antibiotics; the reviewers analysed individual species and strains separately. | Systematic review, graded certainty |
| ISAPP expert panel, 2014 [5] | Some effects are shared across many strains; others, including immune effects, are more likely to be strain-specific. | Expert consensus, not a trial |
Where experts say effects may be shared
Strain specificity is not absolute, and it helps to know the debate. The 2014 panel of the International Scientific Association for Probiotics and Prebiotics (ISAPP) concluded that certain effects can be ascribed to probiotics as a general class, for well-studied species at a functional dose. [5] It also wrote that other effects, including immune effects, are more likely to be strain-specific, and that claims for those can only be made for strains or species in which the mechanism has been shown. [5] For newer, little-studied probiotics, it advised proceeding strain by strain until there is enough information to generalize. [5] It gave an example from the EU: EFSA accepted species-level characterization for yogurt cultures and lactose digestion because the mechanism, microbial production of an enzyme that helps digest lactose, is well understood. [5]The takeaway: a shared effect needs a known, shared mechanism and evidence, not just a shared genus name.
Strain specificity inside a person
A 2018 study that sampled the gut lining of people who took an 11-strain probiotic combination or placebo found person-, region- and strain-specific colonization patterns, which probiotic presence in stool did not tell apart. [6] That is one study of one combination, so we treat it as early evidence about why responses vary. See Individual variation in probiotic response.Worked example
In 2018, a trial of Lactobacillus rhamnosus GG in young children with gastroenteritis (1 x 10^10 CFU twice daily for 5 days) and a trial of a combination of L. rhamnosus R0011 and L. helveticus R0052 (4.0 x 10^9 CFU twice daily) both found no difference from placebo on severity of gastroenteritis. [7][8] Neither trial tells us about other strains, other ages or other illnesses. The details and what they do and do not show are in Why probiotic trials disagree.Foods are not supplements
A yogurt or kefir that contains live microbes is not the same as a tested strain in a capsule. The ISAPP panel wrote that in studies of fermented foods it is not always possible to distinguish the contribution of the live microbes from that of the food matrix. [5] Do not carry a result from a food to a supplement, or the reverse. See Fermented foods vs probiotic supplements.What to check before relying on a claim
- The full strain name (genus, species, strain code), not only "Lactobacillus".
- The product or formulation tested, including whether it was a single strain or a mix.
- The amount (CFU), the duration and the comparator the trial used.
- The group studied: age, health and the problem being treated.
- Whether the claim is about the same outcome the trial measured.
Evidence for named strains by condition is gathered in Probiotics for specific needs: strain names in trials.
Frequently asked questions
What is a probiotic strain?
It is a specific lineage within a species, identified by a code after the species name. NCCIH notes different types of probiotics may have different effects. [1]
If a probiotic helped in a study, will any product with the same genus work?
Not necessarily. NCCIH says helping with one illness does not mean another kind of Lactobacillus or any Bifidobacterium probiotic would do the same thing. [1]
Are all effects strain-specific?
No. A 2014 expert panel said some effects can be ascribed to well-studied probiotics as a general class, while others, such as immune effects, are more likely to be strain-specific. [5]
Does a result from yogurt apply to a probiotic capsule?
Not on the evidence we read. In food studies it is not always possible to separate the live microbes from the food matrix. [5]
Why does the label need the strain code?
Because evidence is attached to the strain. Health Canada's archived notice said strain-level evidence is expected for applications and that claims should be supported by strain-specific evidence in humans. [3]
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.
Safety and who should ask a clinician
Check the safety information for your exact product, not a different strain. NCCIH says the risk of harmful effects is greater in people with severe illnesses or compromised immune systems. [1] The NHS advises talking to a doctor before any probiotic supplement if you have an existing health condition or a weakened immune system. [2] Ask a clinician or pharmacist about pregnancy, children, kidney or liver conditions, surgery and interactions with medicines, and see Probiotic safety and Medication interactions. Do not stop a prescribed medicine to try a probiotic.
References
- NCCIH (US National Center for Complementary and Integrative Health). Probiotics: Usefulness and Safety. Page last updated August 2019 as shown on the page. Read 2026-10-23.
- NHS (UK). Probiotics. Page last reviewed 24 November 2022. Read 2026-10-23.
- Health Canada. Notice: Product Licence Applications for Natural Health Products Containing Probiotics. Notice dated January 2010; the page is archived and states it will not be updated. Read through a web-fetch tool 2026-10-23.
- 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 2026-10-23.
- Hill C, et al. 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. Full text read 2026-10-23.
- Zmora N, et al. Personalized Gut Mucosal Colonization Resistance to Empiric Probiotics Is Associated with Unique Host and Microbiome Features. Cell. 2018;174(6):1388-1405. Abstract read 2026-10-23.
- Schnadower D, et al. Lactobacillus rhamnosus GG versus Placebo for Acute Gastroenteritis in Children. N Engl J Med. 2018;379(21):2002-2014. Abstract read 2026-10-23.
- Freedman SB, et al. Multicenter Trial of a Combination Probiotic for Children with Gastroenteritis. N Engl J Med. 2018;379(21):2015-2026. Abstract read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.