Probiotics Explained: Why Strain Matters More Than CFU Count

Why Most Probiotic Labels Are Misleading

Walk into any UK pharmacy and you will find shelves of probiotic supplements claiming billions of colony-forming units (CFUs) of "beneficial bacteria." The marketing implies that higher CFU counts equal greater benefit, and that any probiotic will broadly support gut health. Both propositions are incorrect. The science of probiotics is fundamentally about strain specificity — and understanding this distinction is the difference between spending money effectively and wasting it.

The Taxonomy Problem

Probiotics are classified in a three-level hierarchy: genus, species, and strain. For example, in Lactobacillus rhamnosus GG, "Lactobacillus" is the genus, "rhamnosus" is the species, and "GG" is the strain designation. The clinical evidence for a specific health effect is tied to the strain — not to the genus or species. A product containing Lactobacillus rhamnosus (without the GG strain designation) has no demonstrated claim to the same benefits as LGG, even though it has the same genus and species name.

This is not a minor technical nuance. Different strains of the same species can have entirely different properties, colonisation behaviour, and clinical effects. Probiotic products that list only genus and species, or that use vague descriptions like "multi-strain blend," provide no meaningful information about what evidence, if any, supports their claims.

Key Strains with Documented Evidence

Lactobacillus rhamnosus GG (LGG) is the most studied probiotic strain in the world. The evidence for its use in preventing and treating antibiotic-associated diarrhoea (AAD) is the most robust of any strain, with multiple meta-analyses confirming efficacy. A 2012 Cochrane review found that LGG significantly reduced the incidence of AAD. If you are taking antibiotics, LGG (taken at least two hours apart from the antibiotic) is the most evidence-based choice available without a prescription.

Bifidobacterium longum 35624 (sold as Alflorex in the UK, licensed as Align in the US) is the only probiotic strain to demonstrate consistent benefit specifically for irritable bowel syndrome (IBS) with bloating and abdominal discomfort in well-designed clinical trials. A 2006 randomised trial by O'Mahony et al. in Gastroenterology found significant improvements in abdominal pain, bloating, and bowel dysfunction compared to placebo. Subsequent trials have confirmed the effect. The dose is one capsule per day. It contains a much lower CFU count than most generic probiotics — demonstrating that strain quality and mechanism matter far more than CFU numbers.

Saccharomyces boulardii is technically a yeast rather than a bacterial probiotic. It has two primary evidence-based applications. First, it provides protection against Clostridioides difficile (C. diff) infection and recurrence in hospitalised patients on antibiotics — a clinically important use given the serious consequences of C. diff. Second, it has been shown to reduce antibiotic-associated diarrhoea and traveller's diarrhoea. Because it is a yeast, it is not killed by antibacterial antibiotics and can be taken simultaneously. It is also commonly used preventively when taking oral antibiotics.

CFU Counts: Context Matters

The emphasis on CFU counts in marketing is largely a distraction. A lower CFU count of a well-characterised, clinically studied strain is more valuable than a high CFU count of an uncharacterised mixture. The clinically effective dose of LGG in studies, for example, is 10 billion CFUs (1 × 10^10); higher doses have not consistently shown additional benefit. For B. longum 35624 (Alflorex), the effective dose is around 1 billion CFUs.

What matters more than the headline CFU figure is viability at point of consumption. A product may contain the stated CFUs at manufacture, but improper storage or inadequate enteric coating can result in near-total die-off before the capsule is even opened.

Refrigeration vs Shelf-Stable Formulations

Many lactobacillus and bifidobacterium strains require refrigeration to maintain viability. Shelf-stable formulations typically achieve stability through lyophilisation (freeze-drying) and protective packaging, but not all formulations achieve this equally. Saccharomyces boulardii is inherently more robust and is typically shelf-stable. When selecting a probiotic, confirming that the CFU count is guaranteed at expiry (not just at manufacture) and understanding the correct storage conditions is essential.