How to Choose a Probiotic: Strains, CFUs, and What Actually Works

Why Strain Specificity Is Everything in Probiotics. The most important concept in probiotic science is strain specificity: the health effects of a probiotic are specific to the exact strain, not the species or genus. Lactobacillus acidophilus NCFM and Lactobacillus acidophilus La-5 are both Lactobacillus acidophilus — but they have different genomes, different mechanisms of action, and different clinical evidence profiles. A study showing that Lactobacillus rhamnosus GG reduces antibiotic-associated diarrhoea tells you nothing about whether Lactobacillus rhamnosus ATCC 53103 (a different strain) does the same. This is why the evidence base for probiotics is so fragmented: hundreds of strains have been studied, each for specific conditions, and the results cannot be generalised across strains. When evaluating a probiotic, the relevant question is not 'does this species work for my condition?' but 'does this specific strain have clinical evidence for my specific condition?' The strain is identified by three parts: genus (e.g. Lactobacillus), species (e.g. rhamnosus), and strain designation (e.g. GG or ATCC 53103). A product that lists only genus and species without a strain designation cannot be evaluated against the clinical literature — and this is a significant red flag.

The Most Evidence-Backed Probiotic Strains by Condition. Antibiotic-associated diarrhoea: Lactobacillus rhamnosus GG (LGG) — the most studied probiotic strain in the world; a 2012 Cochrane review of 63 RCTs found LGG reduced antibiotic-associated diarrhoea risk by 51%; Saccharomyces boulardii CNCM I-745 — a yeast probiotic with strong evidence for antibiotic-associated diarrhoea and C. difficile infection; not affected by antibiotics (being a yeast, not a bacterium). IBS (Irritable Bowel Syndrome): Bifidobacterium infantis 35624 (Align) — a 2006 RCT found it significantly reduced IBS symptoms including bloating, pain, and bowel habit; Lactobacillus plantarum 299v — multiple RCTs showing reduction in IBS pain and bloating; VSL#3 (a multi-strain product) — evidence for IBS-D and inflammatory bowel disease. Traveller's diarrhoea: Saccharomyces boulardii CNCM I-745 — a 2005 meta-analysis found it reduced traveller's diarrhoea risk by 21%; Lactobacillus rhamnosus GG — some evidence for prevention. Vaginal health: Lactobacillus rhamnosus GR-1 + Lactobacillus reuteri RC-14 (RepHresh Pro-B, Fem-Dophilus) — the most studied combination for bacterial vaginosis and vaginal health; multiple RCTs showing restoration of normal vaginal flora. Immune function: Lactobacillus rhamnosus GG — reduces duration of respiratory infections in children; Bifidobacterium animalis subsp. lactis BB-12 — reduces cold and flu duration; Lactobacillus casei Shirota (Yakult) — multiple studies showing immune modulation. Mental health (gut-brain axis): Lactobacillus helveticus R0052 + Bifidobacterium longum R0175 (Lallemand's Probio'Stick) — a 2011 RCT found this combination reduced anxiety and depression scores; Lactobacillus rhamnosus JB-1 — reduced anxiety behaviour in animal models; human evidence is emerging. Infant colic: Lactobacillus reuteri DSM 17938 — multiple RCTs showing reduction in crying time in breastfed infants with colic.

CFU Count: How Much Is Enough? CFU (Colony Forming Units) is the measure of viable bacteria in a probiotic. The appropriate CFU count depends on the strain and condition — there is no universal 'more is better' rule. Most clinical trials use doses of 1–10 billion CFU/day for general gut health and immune support. Some conditions require higher doses: VSL#3 for IBD uses 450–900 billion CFU/day. Some strains are effective at lower doses: Lactobacillus reuteri DSM 17938 for infant colic is effective at 100 million CFU/day. The critical issue is not the CFU count at manufacture but the CFU count at the time of consumption. Probiotic bacteria die during storage — heat, moisture, and oxygen all accelerate die-off. A product that contains 50 billion CFU at manufacture may contain far fewer by the time you take it, particularly if it has been stored incorrectly. Look for products that: guarantee CFU count at expiration (not at manufacture); are stored in the refrigerator (for non-spore-forming strains); use moisture-resistant packaging (blister packs are better than bottles for humidity-sensitive strains); and include enteric coating or delayed-release capsules to protect bacteria from stomach acid.

Survivability: Getting Bacteria to the Colon Alive. The colon is where probiotics exert most of their effects — but to get there, bacteria must survive the acidic environment of the stomach (pH 1.5–3.5) and the bile salts of the small intestine. Not all strains survive this journey. Strategies that improve survivability: Enteric-coated capsules — dissolve in the small intestine rather than the stomach, bypassing stomach acid; Delayed-release capsules (DRcaps) — similar mechanism; Spore-forming bacteria (Bacillus coagulans, Bacillus subtilis) — form protective spores that survive stomach acid, heat, and moisture; do not require refrigeration; increasingly used in shelf-stable products; Taking probiotics with food — the buffering effect of food raises stomach pH and improves bacterial survival; some studies suggest taking probiotics 30 minutes before a meal or with a meal improves survival compared to taking on an empty stomach. Strains with naturally high acid and bile tolerance: Lactobacillus rhamnosus GG, Lactobacillus acidophilus NCFM, Bifidobacterium animalis subsp. lactis BB-12. Strains that require enteric protection: most Bifidobacterium species have lower acid tolerance and benefit from enteric coating.

Multi-Strain vs. Single-Strain: Which Is Better? The marketing of probiotic products heavily favours multi-strain formulations — '10 strains, 50 billion CFU' sounds more impressive than a single strain. But the evidence does not consistently support multi-strain superiority. For conditions with strong single-strain evidence (antibiotic-associated diarrhoea with LGG, IBS with B. infantis 35624), a single well-studied strain at an appropriate dose is preferable to a multi-strain product where each strain is present at a sub-therapeutic dose. For general gut health and immune support, multi-strain products that include both Lactobacillus and Bifidobacterium species may provide broader benefit. The key principle: a multi-strain product is only as good as the evidence for each individual strain it contains. A product with 10 strains that have no clinical evidence is inferior to a single-strain product with robust RCT data. When choosing a multi-strain product, look for products where each strain is identified to the strain level (not just species) and where the manufacturer can point to clinical evidence for each strain.