As of May 2020, the number of scientific publications on probiotics is getting close to 28,000, yet their misrepresentation seems to perpetuate, affecting from general media to members of the medical community, and everyone in between. Several factors influence this confusing situation. However, a key concept, often unknown or misunderstood by those not versed in microbiology, lies at the heart of so much misunderstanding: the strain.

Almost everyone in the health and nutrition field has heard the definition of probiotics as “Live microorganisms which when administered in adequate amounts confer a health benefit on the host.” This definition has remained essentially unchanged in the last 20 years, having been endorsed by international expert panels including the Food and Agriculture Organization, International Probiotics Association, and the World Gastroenterology Organization.^1-3 However, these expert panels never limited themselves to simply issuing a definition; rather, they produced complete and comprehensive guidelines that concur in highlighting the importance of the strain concept.

Strain Matters … A Lot
Strains are often confused with species. Lactobacillus rhamnosus (soon to be renamed as Lactocaseibacillus rhamnosus) is not a strain, it’s a species. The strain is indicated by an alphanumeric code (e.g., Lactobacillus rhamnosus ATCC53103). Think of strains as dog breeds: all breeds belong to the same species (they all have a common ancestor and are genetically compatible), yet there is no doubt their properties can substantially differ.

The same goes for bacteria. As early as 1999, one of the seminal studies in probiotic science already demonstrated that strains of the same species could differ so much in their properties as different species.⁴ For instance, one strain of L. rhamnosus and one of L. plantarum could display a great adherence to intestinal cells, while other strains of L. rhamnosus and of L. plantarum could display moderate adherence or almost no adherence at all.

The importance of strains is not limited to probiotics. Clinical microbiologists (and often internists) have known for years that strain also matters for bacterial pathogens. In fact, in some cases, it can become a matter of life or death to the patient, as antibiotic resistance can change among strains of the same species.

However, the key role of strains is ignored again and again, not only by non-scientists, but also by well-meaning physicians who run a clinical trial with a cocktail of Bifidobacterium animalis strain A, Lactobacillus acidophilus strain B, Lactobacillus brevis C (soon to be renamed Levilactobacillus brevis), and Lactobacillus plantarum strain D (soon to be renamed Lactoplantibacillus plantarum); and because the study fails, they conclude all these species are worthless as probiotics.

Then a similar study with other strains from the same species achieves a positive result and leaves everyone scratching their heads wondering what’s going on, and whether clinical trials on probiotics can be trusted at all. The answer is simple: only studies using the same strains can be pooled together. In the logic of microbiology, statements like “probiotics work for condition X,” or even “Lactobacillus whatever species works for condition X,” are simply nonsense. Only statements like “Lactobacillus plantarum strain Z works for condition X” can be clinically tested to be true or not.

A Practical Example: Lactose Intolerance
The apparent contradiction between studies is nothing new. For instance, conflicting evidence on probiotics regarding lactose intolerance is more than a decade old. Lactose intolerance is fairly common among adults. It is often attributed to fading of lactase expression during aging (lactose malabsorption), although this reduced lactase activity is not enough to result in clinical symptoms per se.⁵

In the beginning of this century, a randomized, placebo-controlled trial elegantly demonstrated that yogurt containing live Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus was able to significantly reduce lactose malabsorption (as determined by lactose hydrogen breath test), albeit at a rather high dose of 500 grams/day for 15 days, while heating the same yogurt to kill the bacteria abrogated its effect.⁶

However, a few years later, a randomized study using yogurt with the same probiotic species and very similar dosing and duration of intervention failed to find any effect, despite using a set of study subjects that was 3.5-fold more numerous.⁷ Importantly, results of the first study were not marginal, their statistical significance was quite strong (p < 0.01, i.e. less than 1% probability of observing such result by chance). Yet the use of a much larger sample size in the second study made the lack of adequate sensitivity unlikely. The answer lies in the fact that although both studies used yogurts containing Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, these yogurts were made by different companies, using different strains.

Probiotic research is also plagued with clinical trials lacking a placebo group, and the field of lactose intolerance is no exception. We probiotic researchers also have our share of the blame. However, newer clinical studies have found probiotic strains capable of improving the symptoms that matter to patients (e.g., diarrhea, abdominal pain, flatulence) compared to placebo, such as the Intensive GI formula (also known as i.3.1) or the L. acidophilus DDS-1 single strain.^8-9 (The Intensive GI formula is composed of strains P. acidilactici KABP021 (CECT7483), L. plantarum KABP022 (CECT7484), and L. plantarum KABP023 (CECT7485).)

Clearly, improving intolerance is significantly more relevant to patients than just improving malabsorption (the amount of hydrogen exhaled after lactose intake). However, for as long as the key dependency of efficacy on the specific strains used in the clinical trials is not widely recognized, excellence in clinical trials will not readily translate into better sales.

Instead, mixed user experiences as well as confusing messages from media and doctors will keep flooding customers. Ultimately, no product can guarantee 100% efficacy, but taking the simple step of recognizing the importance of strains and using the ones supported by randomized, placebo-controlled trials can get us closer.

1. Araya M, Morelli L, Reid G, et al. Joint FAO/WHO Working Group: Guidelines for the Evaluation of Probiotics in Food. London Ontario (Canada); 2002.
2. 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. doi:10.1038/nrgastro.2014.66
3. Guarner F, Sanders ME, Eliakim R, et al. World Gastroenterology Organisation Global Guidelines: Probiotics and Prebiotics.; 2017.
4. Jacobsen CN, Nielsen VR, Hayford AE, et al. Screening of probiotic activities of forty-seven strains of Lactobacillus spp. by in vitro techniques and evaluation of the colonization ability of five selected strains in humans. Appl Environ Microbiol. 1999;65(11):4949-4956. doi:10.1128/aem.65.11.4949-4956.1999
5. Misselwitz B, Butter M, Verbeke K, Fox MR. Update on lactose malabsorption and intolerance: Pathogenesis, diagnosis and clinical management. Gut. 2019;68(11):2080-2091. doi:10.1136/gutjnl-2019-318404
6. Rizkalla SW, Luo J, Kabir M, Chevalier A, Pacher N, Slama G. Chronic consumption of fresh but not heated yogurt improves breath-hydrogen status and short-chain fatty acid profiles: a controlled study in healthy men with or without lactose maldigestion. Am J Clin Nutr. 2000;72(6):1474-1479. doi:10.1093/ajcn/72.6.1474
7. Ballesta S, Velasco C, Borobio MV, Argüelles F, Perea EJ. [Fresh versus pasteurized yogurt: comparative study of the effects on microbiological and immunological parameters, and gastrointestinal comfort]. Enferm Infecc Microbiol Clin. 2008;26(9):552-557. doi:10.1157/13128271
8. Cano-Contreras AD, Perez y Lopez N, Minero Alfaro JI, Medina-Lopez VM, Reyes-Huerta J. Efficacy of probiotic I3.1 symptomatic improvement in patients with lactose intolerance. Neurogastroenterol Motil. 2019;31(Suppl 3):e13658-48.
9. Pakdaman M, Udani J, Molina J, Shahani M. The effects of the DDS-1 strain of lactobacillus on symptomatic relief for lactose intolerance – a randomized, double-blind, placebo-controlled, crossover clinical trial. Nutr J. 2016;15(1):56.