Probiotics 101

"Lactobacillus" is not a thing you can buy. It is a genus containing hundreds of species and thousands of strains, and the evidence behind probiotic pills lives — or dies — at the strain level. This page covers the naming system, what happens to a capsule between your mouth and your colon, and what "billions of CFU" actually means next to the microbes already living there.

A Name, Not a Species

Microbes are named like addresses: genus, species, strain. The genus is the street, the species the building, the strain the apartment — and the evidence lives in the apartment. Lactobacillus rhamnosus GG, the most studied probiotic strain on Earth, was isolated in the 1980s by Sherwood Gorbach and Barry Goldin, who screened hundreds of candidates for acid and bile resistance before settling on one (the "GG" are their initials). Its documented effects — preventing antibiotic-associated diarrhea, shortening acute gastroenteritis in children — belong to that exact strain. A different L. rhamnosus strain is a different microbe with a different genome and, usually, no trial record at all. This is the core rule of the whole category: benefits transfer between strains about as reliably as recipes transfer between restaurants. The parent topic's scorecard is built entirely on this rule.

• 🧬 Same species, different job: two strains of the same species can differ by hundreds of genes — adhesion proteins, acid tolerance, metabolite output — which is why a positive trial of strain A says nothing about strain B.
• 📛 Designations that matter: "BB-12", "CNCM I-745", "299v", "DSM 17938" are strain designations. A label that stops at "Lactobacillus acidophilus" names a species but identifies no strain — and therefore no evidence.
• 🍶 Not all probiotics are bacteria: Saccharomyces boulardii is a yeast, not a bacterium, with its own strong trial record — a useful reminder that the category is about identity, not kingdom.
• 🏷️ Naming has a price: the strongest trial strains are usually proprietary, so the better-identified products are rarely the cheapest — identification is what you are paying for.

The Gauntlet: From Capsule to Colon

A capsule faces a hostile commute. Stomach acid at pH 1.5–3 can kill the acid-sensitive majority within an hour; bile salts in the small intestine strip the membranes of the survivors. The strains used in trials tend to be the ones that tolerate this route — LGG and S. boulardii survive gastric passage well, which is part of why they are the workhorses. But even for them, "survival" is not "settlement." The microbes that make it through do their work in transit, living alongside — not inside — the resident community, then exit in stool over the following days. That transit effect can still be real: transient strains compete with pathogens for adhesion sites, produce antimicrobial compounds, and nudge immune signaling while passing through. What they rarely do is move in permanently, which brings us to the number that matters most.

CFU Realism

"50 billion CFU" sounds enormous until you set it next to the 38 trillion bacteria already in residence — a ratio of roughly one to 760. Probiotic marketing leans on the bigness of billions; the honest framing is the opposite: the dose is a small, temporary ripple in a very large ecosystem. That does not make the dose trivial — the trial literature shows that a ripple of the right strain, at the right moment (mid-antibiotic-course, for instance), measurably changes outcomes. But it reframes what a probiotic is: not a new population, but a timed intervention by a specific microbe. Two dose-level facts follow. First, the trials that found effects used doses in the 1–100 billion CFU/day range — above that, "more" has no consistent extra benefit. Second, CFU counts on labels describe the product at manufacture; heat, humidity, and time erode viability, and most products are never independently re-tested at expiry. A 50-billion label can describe a 5-billion reality on the day you take it.

The Colonization Question

Do probiotics move in? The best direct evidence says: usually not. A systematic review of randomized trials in healthy adults found that most people showed no meaningful change in their fecal microbiota after supplementation — the incoming strains appeared transiently, if at all (Kristensen et al., Genome Medicine, 2016). A 2018 study went further: in about half of healthy volunteers given a standard 11-strain blend, the probiotic bacteria were rejected entirely, appearing in stool only while the capsules continued and vanishing afterward. The researchers found that a person's pre-existing gut signature predicted whether the strains would even persist in transit — they called it colonization resistance (Zmora et al., Cell, 2018). This is the biological backdrop for the whole series: probiotics act while passing through, which is why effects stop when the capsules stop, and why feeding the residents — who are already there, and stay — is the structurally different strategy. For the resident community itself, the microbiome topic owns the deep dive.

A Field Guide to Real Strains

A short, honest directory of the strains that keep appearing in the trial literature — and what each one is actually documented for. The effects column summarizes the demonstrated list; this page is about recognizing the names.

Questions, Answered Briefly

• 🔢 Is 100 billion CFU better than 10 billion? Not in any consistent way. Trial effects cluster between roughly 1 and 100 billion CFU/day, and head-to-head dose comparisons show no reliable "more is better" pattern above that band.
• 🕐 Do I have to take them forever? For most uses, effects last only while you take them — the strains are transients. That is why the question "what am I treating?" should come before "what am I taking?" (see the decision checklist).
• 🔥 Do they even survive my stomach? The studied strains are chosen partly for acid and bile tolerance and survive reasonably when taken with a meal; but survival rates by product are rarely published, which is part of why dose-at-expiry matters.
• 🫙 Aren't fermented foods the same thing? They deliver live cultures plus substrate, in a food matrix — related, but not interchangeable with a defined clinical strain. The fermented foods topic covers that side.

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