A number on a probiotic label is a promise: this many living, beneficial microbes, of exactly these strains, and nothing else. The hard part isn't printing the number — it's proving it's true. Here's how a probiotic earns that trust, and why the field is learning to look past the one number everyone else prints.
The problem with counting CFU alone
CFU — colony-forming units — has been the industry standard for decades. You grow the probiotic on a plate and count the colonies that appear. It works, but it has a real blind spot: it only counts cells that will divide into a visible colony in a lab dish over several days.
The catch is that many probiotic cells are alive, metabolically active, and beneficial — but temporarily won't form a colony under lab conditions. Scientists call these viable-but-non-culturable (VBNC) cells. To a plate count, they're invisible. So CFU can undercount the living probiotics you're actually getting — and it takes days to get an answer.
That's why the field is moving toward a faster, more complete measure.
What is AFU?
An Active Fluorescent Unit (AFU) is a count of living, active probiotic cells measured by flow cytometry rather than by growing colonies on a plate. Cells are tagged with fluorescent viability stains that reveal which ones have intact, functioning membranes, then counted one by one as they flow past a laser. AFU captures every viable cell — including the viable-but-non-culturable ones that CFU misses.
Because it counts cells directly instead of waiting for them to grow, AFU is:
- More complete — it sees viable cells CFU can't, so it reflects true viability.
- Faster — results in hours, not days.
- More reproducible — less swing from plating conditions.
- Standardized — flow-cytometry enumeration of probiotics is defined under the international standard ISO 19344 / IDF 232, which is where the “Active Fluorescent Units” term comes from.
AFU vs. CFU at a glance
| CFU (colony-forming unit) | AFU (active fluorescent unit) | |
|---|---|---|
| Method | Grow colonies on a plate | Flow cytometry + viability stains |
| Counts | Only cells that grow a colony | All viable, membrane-intact cells |
| Sees VBNC cells? | No | Yes |
| Time to result | Days | Hours |
| Standard | Long-standing convention | ISO 19344 / IDF 232 |
| Blind spot | Undercounts true viability | — |
AFU doesn't replace the idea behind CFU — it just measures it more honestly. (For the phage counterpart to both, see PFU.)
The three questions every probiotic has to answer
A trustworthy count is only one of three questions a probiotic has to answer. A rigorous verification standard checks all three: is it what it claims, is it alive, and is it pure?
1. Identity — is it really this strain? (16S rRNA sequencing)
The exact strain of every organism in a well-made probiotic is established up front through the manufacturer's Certificate of Analysis (COA). On the finished product, 16S rRNA gene sequencing can then confirm identity down to the species level — verifying by DNA that the organisms named on your formulation are the ones actually in it, not an assumption. Strain is known by COA; 16S is an independent species-level confirmation on the product itself.
2. Viability — is it actually alive? (AFU / flow cytometry)
A probiotic only benefits you if the cells reach you alive, so viability is non-negotiable. The most complete way to measure it is to count living cells directly with AFU / flow cytometry, so the number reflects living, active cells — including the viable-but-non-culturable ones a plate count would miss — rather than a figure inflated by dead cells or deflated by lab-shy survivors. That true-viability standard is what a personalized probiotic should be held to.
3. Purity & contamination — is it only this?
A clean probiotic contains the declared organisms and nothing else. The principle is simple: screen for purity and contamination so the product isn't carrying unwanted microbes, pathogens, or adulterants. What's on the label should be what's in the capsule — and only that.
Why this matters even more for a personalized probiotic
When your formulation is built for your gut, precision isn't a nice-to-have — it's the entire product. The right strains, actually alive, and genuinely pure is the difference between a personalized probiotic that does what it's designed to and an expensive placebo. Testing to species identity, real viability, and verified purity is how a personalized promise stays honest. It's the same philosophy behind every unit in the family — CFU, PFU, and AFU: count only what's biologically real.
Frequently asked questions
What is AFU in probiotics?
AFU (Active Fluorescent Unit) is a measure of living probiotic cells counted by flow cytometry using fluorescent viability stains, standardized under ISO 19344/IDF 232. Unlike CFU, it captures viable cells that won't grow a colony on a plate, giving a more complete and faster viability count.
Is AFU better than CFU?
AFU is more complete (it sees viable-but-non-culturable cells CFU misses), faster (hours vs. days), and more reproducible. CFU remains the familiar label convention, but AFU measures true viability more directly.
How do you know a probiotic contains the strains it claims?
The strain of every organism is established up front through the manufacturer's Certificate of Analysis (COA). On the finished product, DNA-based identity testing — 16S rRNA gene sequencing — independently confirms the organisms present down to the species level.
How are probiotics tested for contamination?
Purity and contamination screening verifies the product contains only the declared organisms and is free of unwanted microbes, pathogens, or adulterants.
Why does viability testing matter?
A probiotic only benefits you if the cells are alive when you take them. Viability testing (AFU/flow cytometry) reflects living, active cells — not dead ones inflating the count.