There is no single definitive count of probiotic strains, but researchers have identified hundreds of distinct strains used in supplements, foods, and clinical settings. The number keeps growing as scientists isolate new strains from the human gut, fermented foods, and other environments. Understanding what “strain” actually means, and why the differences between strains matter, is more useful than chasing a precise total.
What Counts as a Probiotic Strain
Probiotics are classified using a three-part naming system: genus, species, and strain. Think of it like an address. The genus is the city, the species is the street, and the strain is the specific house number. Two probiotics can share the same genus and species but behave very differently because they belong to different strains.
For example, Lactobacillus rhamnosus is a species. But Lactobacillus rhamnosus GG is a specific strain within that species, and it has a clinical track record that other strains of L. rhamnosus may not share. The alphanumeric code at the end (like “GG” or “I-745” or “35624”) is what identifies the exact strain. When a probiotic supplement doesn’t list the strain designation on its label, you have no way of knowing whether it matches the strain actually studied for a given health benefit.
How Many Genera and Species Are Used
Most commercial probiotics come from a handful of well-studied genera. The most common include Lactobacillus, Bifidobacterium, and the yeast Saccharomyces. Other genera like Streptococcus, Enterococcus, and Bacillus also appear in certain products. Within each genus, there are multiple species, and within each species, potentially dozens of individually characterized strains.
The landscape got more complicated in 2020, when scientists reclassified the massive Lactobacillus genus into 25 separate genera, including 23 entirely new ones. Species that were once all called “Lactobacillus” now carry names like Lacticaseibacillus, Lactiplantibacillus, and Limosilactobacillus. So the familiar Lactobacillus rhamnosus is now officially Lacticaseibacillus rhamnosus, and Lactobacillus plantarum became Lactiplantibacillus plantarum. Many supplement labels still use the old names, which can make comparison shopping confusing.
This reclassification didn’t create new organisms. It just reorganized existing ones based on genetic analysis. But it illustrates how diverse even a single traditional probiotic “group” really is, spanning enough genetic variety to fill 25 distinct genera.
Why the Exact Number Keeps Changing
New probiotic strains are continually being isolated, characterized, and patented. Companies and research labs identify strains from sources like human breast milk, infant gut samples, traditional fermented foods (kimchi, kefir, miso), and even insect guts. Each time a new strain is shown to survive digestion and produce a measurable health effect, it can be added to the probiotic catalog. There is no central registry that tracks every probiotic strain globally, which is part of why you won’t find a single authoritative number.
A reasonable estimate is that several hundred distinct strains are currently sold in commercial products or actively studied in clinical trials worldwide. But that number represents a tiny fraction of the bacterial diversity in the human gut alone, which contains trillions of microorganisms across more than a thousand species.
Strain Differences Have Real Health Implications
Not all probiotic strains do the same thing. Clinical evidence is tied to specific strains, not to broad categories. A systematic review and meta-analysis published in Frontiers in Medicine examined this question directly and found clear patterns of strain-specific effectiveness across multiple conditions.
Saccharomyces boulardii I-745, a probiotic yeast, has the strongest evidence for preventing antibiotic-associated diarrhea in both adults and children, treating acute diarrhea in kids, and reducing traveler’s diarrhea. Lactobacillus rhamnosus GG (now Lacticaseibacillus rhamnosus GG) shows significant results for preventing antibiotic-associated diarrhea in children and reducing hospital-acquired infections. For irritable bowel syndrome, Bifidobacterium infantis 35624 and Lactobacillus plantarum 299v (now Lactiplantibacillus plantarum 299v) each had more positive trials than negative ones. Lactobacillus casei DN114001 showed consistent benefits for treating acute diarrhea in children across multiple trials.
The key takeaway: swapping one strain for another, even within the same species, can mean the difference between a product that works for your specific concern and one that does nothing. This is why reading beyond the genus and species on a label matters.
Single-Strain vs. Multi-Strain Products
Probiotic supplements range from single-strain formulas to blends containing 10, 15, or even 30 different strains. A review from the University of Reading analyzed the limited head-to-head comparisons available and found that in 12 out of 16 studies (75%), multi-strain combinations outperformed any of their individual component strains used alone. The researchers concluded that multi-strain probiotics generally appear more effective, though the evidence base remains small.
That said, “more strains” doesn’t automatically mean better. A well-studied single strain with strong clinical evidence for your specific concern can be a smarter choice than a 20-strain blend where none of the individual strains have been tested for that condition. The quality of the evidence behind each strain matters more than the total count on the label.
What to Look for on a Label
When you pick up a probiotic supplement, check whether the label lists the full strain designation, not just the genus and species. A product that says “Lactobacillus rhamnosus” without the strain code gives you no way to verify whether it matches the strain studied in clinical trials. A product listing “Lactobacillus rhamnosus GG” tells you exactly which organism you’re getting.
Also look for the number of colony-forming units (CFUs), which tells you how many live organisms are in each dose. Effective doses in clinical research typically range from 1 billion to tens of billions of CFUs, depending on the strain and the condition. More isn’t always better here either. What matters is whether the specific CFU count matches what was used in studies showing benefit.
If a product uses updated genus names (like Lacticaseibacillus instead of Lactobacillus), that’s actually a good sign. It suggests the manufacturer is keeping up with current science. But many reputable products still use the older names, so don’t rule out a product solely based on which naming convention it follows.