You've seen CFU — colony-forming units — on the back of every probiotic bottle. It's how we count living bacteria. But there's a lesser-known counterpart that measures something bacteria can't do to themselves: get hunted down and killed, one species at a time.
That number is the PFU — the plaque-forming unit — and it's how scientists count bacteriophages: viruses that infect and destroy specific bacteria, and nothing else.
If CFU is the headcount of the good guys you add to your gut, PFU is the headcount of the precision tools you might one day use to subtract the troublemakers. Here's what it means, how it's measured, and why researchers are watching it closely for hard problems like SIBO and the autism gut.
What is a PFU? (the short answer)
A plaque-forming unit (PFU) is a measure of how many infectious virus particles — usually bacteriophages — are in a sample. One PFU is the smallest amount of phage that can infect a bacterial cell and start a chain reaction of destruction, clearing a visible hole, called a plaque, in a lawn of growing bacteria. PFU is to viruses what CFU is to live bacteria: a count of what's actually active, not just present.
What is a bacteriophage?
A bacteriophage (or just “phage”) is a virus that only infects bacteria — it's completely harmless to human cells. The name literally means “bacteria eater.” Phages are the most abundant biological entity on Earth, and your gut already contains trillions of them, quietly shaping which bacteria thrive.
Their defining trait is specificity. A given phage typically infects just one species of bacteria — sometimes just one strain. A phage that hunts a problem bacterium will leave the hundreds of beneficial species around it untouched. That's the opposite of a broad-spectrum antibiotic, which carpet-bombs the whole neighborhood.
That precision is the entire reason PFU is worth understanding.
How is a PFU measured? (the plaque assay)
PFU comes from a classic lab technique called the plaque assay:
- A layer (“lawn”) of the target bacteria is grown across a dish of soft agar.
- A diluted sample of phage is added on top.
- Wherever a single infectious phage lands, it hijacks a bacterial cell, replicates, bursts it open, and releases dozens of new phages — which infect the neighbors, and so on.
- Within hours, that chain reaction eats a clear circular plaque in the cloudy bacterial lawn.
- Count the plaques, adjust for the dilution, and you get PFU per milliliter (PFU/mL).
Each plaque traces back to one original infectious phage — so PFU counts functional, bacteria-killing viruses, not broken or inactive particles. That's the same logic behind CFU counting only live bacteria, not dead cells.
CFU vs. PFU at a glance
| CFU (colony-forming unit) | PFU (plaque-forming unit) | |
|---|---|---|
| Counts | Live bacteria | Infectious viruses (phages) |
| Shows up as | A colony growing on a plate | A plaque (clear hole) in a bacterial lawn |
| What it tells you | How many microbes can grow | How many phages can infect and kill |
| Role in the gut | What you add (probiotics) | What you might remove (targeted) |
| Specificity | Broad — a strain colonizes widely | Narrow — a phage targets one species/strain |
| On a label today | Standard on probiotics | Rare; mostly research & phage products |
Same idea — count only what's biologically active — pointed in opposite directions. (A third relative, AFU, measures the same “is it alive?” question as CFU, just more directly.)
What counts as a “good” PFU?
Unlike CFU, there's no tidy consumer benchmark yet, because phage products aren't mainstream. But a few anchors from the research world:
- Concentration: Published phage-therapy studies commonly report doses in the range of roughly 10⁷ to 10⁹ PFU per dose, though the “right” number depends heavily on the target bacterium, the route, and the goal. Optimal dosing is still being established.
- Higher isn't automatically better. Because phages self-amplify — one infection makes dozens more — the dynamics aren't linear like a static drug. A modest PFU can bloom if its target is abundant, and fizzle if the target is scarce.
- Match matters more than magnitude. A billion PFU of a phage that doesn't recognize your strain does nothing. A well-matched phage at a lower PFU can be far more effective. Specificity is the whole game.
So “a good PFU” is less about a big number and more about the right phage, correctly matched to the bacterium you're trying to reduce, at a dose the research supports.
Where PFU gets interesting: precision use cases
Phages matter most exactly where broad antibiotics fail — when you need to knock down one overgrown bug without wrecking everything else. Two areas researchers are watching:
SIBO (small intestinal bacterial overgrowth)
SIBO is a problem of too much bacteria in the wrong place — the small intestine — often dominated by specific gas-producing species. The standard tools are broad antibiotics like rifaximin, which reduce the overgrowth but don't discriminate, and relapse is common.
Phages offer a compelling research alternative: target the overgrown species specifically, thin it out, and leave the beneficial background flora intact. PFU is how you'd measure and dose that intervention. This is early, experimental science — not a treatment you can buy — but it's a natural fit for SIBO's core problem: precision, not carpet-bombing.
The autism gut
Many children on the autism spectrum have a distinctly altered gut microbiome, with overrepresentation of certain bacterial groups. Researchers studying the gut-brain axis are interested in whether precisely modulating specific taxa — rather than blanket antibiotics, which can worsen dysbiosis — could matter.
Phage-based modulation is one hypothesized precision tool here, and it's very much research-stage. (Flore's own autism work is a microbiome diagnostic in development, not a phage therapy — and the published probiotic study in this space is an early pilot, not a randomized trial.) The through-line is the same: the autism microbiome rewards targeted approaches over broad ones, and PFU is the language of targeting.
Why this matters to how Flore thinks
Whether the unit is CFU or PFU, the philosophy is identical: your gut isn't a problem to be blasted — it's an ecosystem to be tuned. Adding the right beneficial strains (measured in CFU) and, someday, precisely dialing back the overgrown ones (measured in PFU) are two halves of the same precision-microbiome idea.
Phage products aren't part of Flore today. But understanding PFU tells you where personalized microbiome science is heading — away from one-size-fits-all, toward interventions matched to your specific gut. If you want the full picture of how a probiotic gets measured and verified, read our companion pieces on what a CFU is and how viability is confirmed with AFU.
Frequently asked questions
Is a PFU the same as a CFU?
No. A CFU counts live bacteria; a PFU counts infectious viruses (bacteriophages). Both measure only biologically active units — a colony you can grow, or a plaque a phage can clear — rather than total particles present.
Are bacteriophages dangerous to humans?
Bacteriophages infect only bacteria and cannot infect human cells. They're a natural, abundant part of your existing gut ecosystem.
What's a good PFU count?
There's no consumer standard yet. Research doses often fall around 10⁷–10⁹ PFU, but the match between the phage and the target bacterium matters more than the raw number.
Can phages treat SIBO or autism?
Not as an approved, available therapy. Both are active areas of research precisely because phages can target specific bacteria without disrupting the whole microbiome. Talk to your doctor about evidence-based options; nothing here is medical advice.
Why do probiotics use CFU instead of PFU?
Probiotics are live bacteria you're adding, so they're counted in CFU. PFU only applies to viruses (phages), which aren't part of standard probiotic products.