Label literacy guide
What Is CFU in Probiotics? Label Accuracy, AFU, and What the Number Means
How the count is made, where it falls short, and how to read a label like a microbiologist
Colony-forming units are the standard measure of probiotic potency, but the number on the bottle only tells part of the story.
The short answer
CFU counts live bacteria that can grow in a lab test, but it doesn't guarantee what reaches your gut. Independent testing has found some products fall short of their labels.1,2 The labels worth trusting guarantee potency through expiration, name the strains, and come from brands that make quality visible.*
Pick up any probiotic bottle and you'll see a number: 30 billion, 50 billion, 100 billion. That's the CFU count, the closest thing the supplement aisle has to a potency standard. But it counts cells one way, at one moment, and labels vary in how they report it.
This guide covers what CFU means, how it's measured, why labels don't always match what's inside, and what AFU is when you see it. For how many CFU you might need, see our CFU dosage guide.
What CFU actually means
CFU stands for colony-forming units: a count of living bacteria that can multiply under lab conditions. The classic method is plate counting.
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1
Sample and dilute
A small amount of the probiotic is mixed into a sterile liquid and diluted step by step until it's countable.
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2
Plate and incubate
The dilution is spread on a nutrient dish and kept warm, usually for a day or more, depending on the species.
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3
Count and calculate
Each colony traces back to one live cell. Multiply the colonies by the dilution and you get CFU per serving.
The hidden limit of plate counting
Plate counts only see cells that grow on the plate. Some bacteria are alive but won't grow under lab conditions, a state called "viable but non-culturable."4 In a shelf-life study, plate counts fell faster than a membrane-based count, suggesting some cells had shifted into that state rather than died.5
Is more CFU better?
Not as a rule. A review of dose studies found a clear dose-response for antibiotic-associated diarrhea, but none clear for bowel function, IBS or several other outcomes.6 And across 65 trials, multi-strain mixtures were usually no more effective than single strains matched for the same condition.7 The authors' advice: choose based on trial evidence, not the numbers on the box. Our CFU dosage guide shows the doses specific studies actually used.
The label problem
The FDA doesn't pre-approve supplement labels, so potency claims come from the manufacturer. Independent testing shows why that matters. When researchers tested 17 commercial probiotics, 5 had lower live counts than labeled and 4 were mislabeled for which bacteria they contained.1 In an audit of 25 animal probiotics, only 4 of the 15 that listed a count met it, and about a third misspelled the organisms on the label.2
How to read a probiotic label in 30 seconds:
- Is CFU guaranteed through expiration? "At time of manufacture" doesn't promise that dose when you take it. Brands that guarantee through expiration add extra at the factory so the stated dose survives to the last capsule.
- Are the strains named? "L. rhamnosus" is a species; "L. rhamnosus GG" is a strain. The research is strain-specific.8
- Is storage clearly stated? Store it the way the label says. Our refrigerated vs. shelf-stable guide explains why.
- Is it made under cGMP? Current Good Manufacturing Practice is the baseline for a supplement you can trust.
CFU over shelf life
Live bacteria slowly die off between the factory and your cabinet. How fast depends on how well the product keeps out moisture. In a two-year study of commercial capsules with freeze-dried bacteria stored at room temperature, counts fell by only 0.12 to 0.26 log (roughly 25% to 45% of cells), and falling counts tracked with rising moisture in the capsule.3 Well-made, dry products can hold up well; poorly made ones may not.
Some species are tougher than others. Spore-forming Bacillus coagulans survived heat, cooking and simulated stomach acid far better than Lactobacillus strains in lab tests.9
AFU: the other number you might see
AFU stands for active fluorescent units. It comes from flow cytometry, a laser-based method standardized for lactic acid bacteria under ISO 19344.3 Instead of growing colonies, it stains cells with fluorescent dyes that show whether each cell's membrane is intact, and counts them as they pass a laser.
AFU can catch cells plate counts miss, including viable but non-culturable ones.4,5 On the other hand, CFU is the measure authorities ask for,10 so it's still the better number for comparing products. Treat AFU as useful extra information when a brand provides it.
| CFU | AFU | |
|---|---|---|
| Stands for | Colony-forming units | Active fluorescent units |
| Method | Plate counting on nutrient dishes | Flow cytometry (ISO 19344) |
| Counts | Cells that grow into colonies | Cells with intact membranes |
| Time | A day or more | Much faster |
| Blind spot | Can miss live cells that won't grow on a plate | Less familiar, and not what most trials report |
Strains studied at lower doses
If dose were everything, every probiotic would race to 100 billion. Strain identity carries more weight than raw count.8 Some strains have shown results at modest doses:
| Strain | Dose studied | What happened |
|---|---|---|
| B. infantis 35624 | 100 million | Beat placebo for IBS symptoms over 4 weeks in 362 women; the 10 billion dose didn't11 |
| B. coagulans LBSC | 6 billion | Improved bloating, cramping, pain and stool consistency over 80 days in 40 adults with IBS12 |
| B. coagulans GBI-30, 6086 | Once daily | Pain and bloating improved from baseline over 8 weeks in 44 adults with IBS. The groups started out different, so only within-group changes were compared13 |
| L. rhamnosus GG | Varied | Across 12 trials, it lowered antibiotic-associated diarrhea risk from 22% to 12%; the effect was clear in children, less so in adults14 |
The takeaway: named strains with trial evidence, and a potency you can trust through expiration, tell you more than the biggest number on the shelf.
Three points on the potency range
Organic
Gut Balance | Organic Flora
30B cultures, 60 strains, USDA Organic
An easy place to start if you're new to probiotics.*
Ultra Daily | Vital Flora
60B cultures, 60 strains, 7 organic prebiotics
Balanced 30B Lacto + 30B Bifido in a delayed-release capsule. Where most people start.*
Advanced Biome | Vital Flora
100B cultures, 100 strains, 10 organic prebiotics
Our most potent, most diverse formula, in a delayed-release capsule, shelf-stable or refrigerated.*
Vital Flora probiotics are made in a cGMP-compliant facility. Looking for support for a specific concern? Browse all probiotics.
Where should you start?
One question
Based on how each formula is designed. Not a diagnosis, and not a substitute for your provider's advice.
What are you looking for?
Your starting point
Start with Organic Flora
A moderate 30 billion, USDA Organic, is an easy place to begin. You can step up later.*
Your starting point
Vital Flora Ultra Daily
A balanced 60 billion across 60 strains with prebiotics, in a delayed-release capsule, for everyday digestive and immune support.*
Your starting point
Match the formula to the concern
For a specific concern, the strains matter as much as the count. Browse our targeted formulas, or use the finder in our CFU guide.*
Your starting point
Vital Flora Advanced Biome
100 billion across 100 strains with 10 organic prebiotics. Our most potent, most diverse formula.*
Frequently asked questions
Colony-forming units: a count of the live bacteria in a serving that can grow and multiply in a lab test. It's measured by plating a diluted sample and counting the colonies that grow. It's the measure authorities ask for.10
Not automatically. Higher doses clearly helped for antibiotic-associated diarrhea, but no clear dose-response showed up for bowel function, IBS or several other outcomes.6 Which strains are in the product, and whether the count is guaranteed through expiration, matter as much as the number.
Yes, some cells die off over time. How much depends on the product: in a two-year study of well-made freeze-dried capsules at room temperature, counts dropped only modestly, and the drop tracked with moisture.3 That's why "guaranteed through expiration" matters: it means the brand added extra at the factory so the label holds to the end.
Not necessarily. Each format is designed for its storage. Some species, like spore-forming Bacillus coagulans, tolerate heat far better than others.9 See our refrigerated vs. shelf-stable guide.
References
- Ullah M, et al. Viability and Composition Validation of Commercial Probiotic Products by Selective Culturing Combined with Next-Generation Sequencing. Microorganisms. 2019;7(7). PubMed
- Weese JS, Martin H. Assessment of commercial probiotic bacterial contents and label accuracy. Can Vet J. 2011;52(1):43-6. PubMed
- Sielatycka K, et al. Dynamics of Active Fluorescent Units (AFU) and Water Activity (a(w)) Changes in Probiotic Products-Pilot Study. Foods. 2023;12(21). PubMed
- Pazos-Rojas LA, et al. The Viable but Non-Culturable (VBNC) State, a Poorly Explored Aspect of Beneficial Bacteria. Microorganisms. 2023;12(1). PubMed
- Visciglia A, et al. Assessment of shelf-life and metabolic viability of a multi-strain synbiotic using standard and innovative enumeration technologies. Front Microbiol. 2022;13:989563. PubMed
- Ouwehand AC. A review of dose-responses of probiotics in human studies. Benef Microbes. 2017;8(2):143-151. PubMed
- McFarland LV. Efficacy of Single-Strain Probiotics Versus Multi-Strain Mixtures: Systematic Review of Strain and Disease Specificity. Dig Dis Sci. 2021;66(3):694-704. PubMed
- McFarland LV, et al. Strain-Specificity and Disease-Specificity of Probiotic Efficacy: A Systematic Review and Meta-Analysis. Front Med (Lausanne). 2018;5:124. PubMed
- Majeed M, et al. Comparative evaluation for thermostability and gastrointestinal survival of probiotic Bacillus coagulans MTCC 5856. Biosci Biotechnol Biochem. 2021;85(4):962-971. PubMed
- Wendel U. Assessing Viability and Stress Tolerance of Probiotics-A Review. Front Microbiol. 2021;12:818468. PubMed
- Whorwell PJ, et al. Efficacy of an encapsulated probiotic Bifidobacterium infantis 35624 in women with irritable bowel syndrome. Am J Gastroenterol. 2006;101(7):1581-90. PubMed
- Gupta AK, Maity C. Efficacy and safety of Bacillus coagulans LBSC in irritable bowel syndrome: A prospective, interventional, randomized, double-blind, placebo-controlled clinical study [CONSORT Compliant]. Medicine (Baltimore). 2021;100(3):e23641. PubMed
- Hun L. Bacillus coagulans significantly improved abdominal pain and bloating in patients with IBS. Postgrad Med. 2009;121(2):119-24. PubMed
- Szajewska H, KoĆodziej M. Systematic review with meta-analysis: Lactobacillus rhamnosus GG in the prevention of antibiotic-associated diarrhoea in children and adults. Aliment Pharmacol Ther. 2015;42(10):1149-57. PubMed