CFU (Colony Forming Units)

Definition

CFU (Colony Forming Units) is a measure of the number of viable bacteria in a probiotic product — specifically, the number of bacteria capable of dividing and forming a visible colony on a growth medium. A probiotic labelled “10 billion CFU” contains approximately 10 billion living, colony-forming bacteria per dose.

Why CFU Is Primarily a Marketing Metric

How CFU Is Measured

CFU is counted by diluting a sample and plating it on a growth medium. After incubation, visible colonies are counted. The limitation: CFU measures bacteria that can form colonies under lab conditions, not all viable bacteria. Some viable bacteria may fail to form colonies due to stress or growth conditions — leading to under-counting.

CFU vs AFU

AFU (Active Fluorescent Units) — used by Seed DS-01 — counts both colony-forming and dormant-but-viable bacteria using flow cytometry and fluorescent staining. AFU typically reads higher than CFU for the same sample because it captures more of the viable population.

Neither unit is definitively superior for predicting clinical outcomes; both are valid measurements of bacterial viability. The key point is that neither unit, at any level, substitutes for strain-indication evidence.

What CFU Does Tell You

CFU is useful for:

• Comparing products within the same strain: 50B CFU LGG vs 10B CFU LGG is a meaningful difference for that specific strain.
• Assessing label accuracy: Third-party testing (ConsumerLab, NSF) checks whether the stated CFU count is present at expiry — a real quality-control check.
• Post-antibiotic dosing: Some evidence suggests higher doses (25-50B CFU) may be appropriate for antibiotic-associated diarrhoea recovery, where gut bacteria have been significantly depleted.

Bottom Line

When evaluating a probiotic, ask: which specific strain does this product contain, and does that strain have RCT evidence for my specific indication? CFU count is secondary — check it only after the strain question is answered.