Probiotics

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host — that's the WHO/FAO definition, and the phrase "adequate amounts" matters as much as "live." They're the supplement-form deposits of the same families of bacteria (and one yeast) that make up your gut microbiome: mostly Lactobacillus and Bifidobacterium species, plus the yeast Saccharomyces boulardii. The microbiome itself is the dense community of trillions of microbes living in your gut, and it does real metabolic work — fermenting fibre into short-chain fatty acids, training the immune system, and crowding out pathogens. Probiotic supplements are an attempt to nudge that ecosystem in a helpful direction.

The single most important thing a buyer needs to understand is strain-specificity. A probiotic is not a generic commodity like vitamin C, where one molecule does one job. The clinical evidence attaches to *named strains* — not to "probiotics" in general, not even to a species, but to a specific strain identified by a code. Lactobacillus rhamnosus GG (LGG) is a different product from Lactobacillus rhamnosus in general; Bifidobacterium infantis 35624 is the strain with the IBS trial behind it; Saccharomyces boulardii is the yeast with the strongest antibiotic-associated-diarrhoea data. Two capsules can both say "Lactobacillus" on the front and do completely different things — or nothing — depending on the exact strain inside. This is why "Probiotic, 10 strains, 50 billion CFU" on a budget label tells you almost nothing: the label is naming a category, not the studied product.

So the practical definition of a useful probiotic is: a *named, strain-identified* culture, dosed at the level used in its trials, delivered alive to the gut. The dose is measured in CFU (colony-forming units — the count of viable, reproduction-capable cells). And because the cells are alive, they degrade: many products need refrigeration, while better-engineered ones are shelf-stable. "Live cultures" on the box is only meaningful if they're still live at the end of shelf life, in the part of the gut where they act.

Probiotics don't permanently "recolonise" your gut — for most strains they're transient passengers, present while you take them and largely gone within weeks of stopping. The benefit comes from what they do *during* that transit. The best-characterised mechanisms are: competitive exclusion (live cultures occupy attachment sites and consume nutrients pathogens would use, crowding bad bugs out); production of short-chain fatty acids and other metabolites that lower gut pH and feed the cells lining the colon; reinforcement of the gut barrier (tightening the junctions between epithelial cells so less inflammatory material leaks through); and immune modulation (signalling to gut-associated immune tissue to damp down inflammatory responses). Different strains lean on different mechanisms, which is exactly why the outcome is strain-specific.

The clearest clinical payoff is antibiotic-associated diarrhoea (AAD). Antibiotics carpet-bomb the microbiome along with the target infection, and the resulting disruption causes diarrhoea in a large minority of people. Taking the right probiotic alongside the antibiotic refills the ecosystem fast enough to blunt this. McFarland's 2006 meta-analysis (PMID 16635227) pooled the RCTs and found probiotics significantly reduced AAD and helped prevent Clostridioides difficile disease; the two strains with the strongest AAD evidence are Saccharomyces boulardii (McFarland 2010, PMID 20458757) and Lactobacillus rhamnosus GG (Szajewska 2015, PMID 26365389). This is the use-case where the data are strong enough that many clinicians actively recommend it.

The second well-studied target is irritable bowel syndrome (IBS) — the abdominal pain, bloating, and irregular bowel habit that affect a large share of adults. Here probiotics modulate the gut-brain axis and the low-grade inflammation thought to drive symptoms. Ford's 2014 systematic review (PMID 25070051) found probiotics as a class improved overall IBS symptoms versus placebo — while explicitly noting that *which* strains work best remains unclear. Whorwell 2006 (PMID 16863564) is the landmark single-strain trial: Bifidobacterium infantis 35624 at 1×10⁸ CFU/day significantly beat placebo for abdominal pain, bloating, and bowel dysfunction. The 2018 Hungin international consensus (PMID 29460487) pulled this together for clinicians: specific probiotics help reduce overall symptom burden and abdominal pain in some IBS patients, and specified strains are useful adjuvants to prevent AAD. The throughline of all of it: the benefit is real, but it's the *strain*, dose, and indication that decide whether you get it.