Probiotic Strains Explained

One of the most useful things to understand about probiotics is how they are named, because the name tells you how specific - and therefore how meaningful - a product's claims can be. Probiotics are living microorganisms classified in a hierarchy: genus, then species, then strain. Effects observed in research are typically strain-specific, not shared across an entire genus or species. A label that says only "Lactobacillus" tells you very little; one that names the genus, species, and strain designation tells you exactly which microorganism has been used and lets you trace it to the research. Learning to read these names is the single best skill for evaluating a probiotic product.

Take an example such as Lactobacillus rhamnosus GG. Here Lactobacillus is the genus, rhamnosus is the species, and GG is the strain designation - a unique identifier for that specific cultured line. Another is Bifidobacterium longum BB536: genus Bifidobacterium, species longum, strain BB536. The strain is the level that matters most, because two strains of the same species can behave quite differently. A well-formulated product identifies every culture down to its strain designation. If only genus and species appear, you cannot link the product to the evidence, which is a meaningful limitation.

Lactobacillus is one of the most familiar groups, found in fermented foods and many supplements (note that some species have recently been reclassified into new genus names, though older names remain in common use). These bacteria produce lactic acid and are generally well studied. Commonly referenced strains include Lactobacillus rhamnosus GG, one of the most extensively researched probiotics globally, Lactobacillus plantarum 299v, and Lactobacillus acidophilus NCFM. Different strains have been studied for different purposes, which is exactly why the strain designation matters: "Lactobacillus acidophilus" alone could refer to many distinct strains with different documented properties.

Bifidobacterium is another major group and among the dominant beneficial bacteria in the large intestine, particularly prominent early in life. These species ferment fibre and contribute to short-chain fatty acid production in the colon. Commonly referenced strains include Bifidobacterium longum BB536, Bifidobacterium infantis 35624, and Bifidobacterium lactis strains used in many fermented dairy products. As with Lactobacillus, effects are tied to the specific strain rather than the genus, so the same principle applies when reading a label.

Not all probiotics are Lactobacillus or Bifidobacterium. Saccharomyces boulardii is a beneficial yeast rather than a bacterium and is naturally resistant to stomach acid. Spore-forming bacteria such as certain Bacillus species form protective spores that withstand heat, acid, and moisture, which can aid survival through the gut and on the shelf. These behave differently from typical lactic-acid bacteria, which is another reason precise naming and an understanding of the type help you interpret a product.

When assessing a product, look for: each culture named to strain designation, not just genus and species; the CFU (colony-forming unit) count guaranteed to the expiry date rather than only at manufacture; and ideally third-party testing confirming identity and potency. A shorter list of well-identified, well-studied strains is more informative than a long list of vaguely labelled ones or an eye-catching CFU number attached to unnamed cultures. Strain identity is the substance; CFU and marketing claims are secondary to it.

LOOM Gut Health specifies each culture to strain designation level, with CFU viability supported through to expiry and third-party testing for identity and potency, so you can see exactly what you are taking. Understanding strain naming helps you choose any probiotic on evidence rather than marketing. Probiotics support the gut microbiome as part of a balanced diet and lifestyle; they are not a treatment for any medical condition. Results vary from person to person, and anyone with ongoing digestive symptoms should speak to a GP.