AFU (Active Fluorescent Units)
AFU (Active Fluorescent Units) is an alternative measure of probiotic potency that counts both colony-forming and dormant-but-viable bacteria using flow cytometry. Used by Seed DS-01.
Definition
AFU (Active Fluorescent Units) is a measure of probiotic viability that uses flow cytometry and fluorescent dyes to count bacteria. Unlike CFU (Colony Forming Units), AFU detects all viable cells — including those that are metabolically active but temporarily dormant and unable to form colonies under lab conditions.
How AFU Is Measured
Flow cytometry passes individual cells through a laser beam. Fluorescent dyes bind to cells with intact membranes (indicating viability). The detector counts each fluorescent signal. This method is faster, more precise, and counts a broader population than traditional plate-count CFU methods.
Why AFU Typically Reads Higher Than CFU
For the same probiotic sample, AFU counts are typically 20-40% higher than CFU counts. This is because:
- CFU only counts bacteria that survive plating conditions and form visible colonies
- AFU counts all viable cells, including those stressed by freezing, desiccation, or encapsulation
Neither method is categorically “right” — they measure different aspects of bacterial viability.
Does AFU vs CFU Matter for Clinical Outcomes?
The practical significance of AFU vs CFU for consumer outcomes is uncertain. Most probiotic RCTs use CFU-counted products. Whether the additional viable cells counted by AFU produce additional clinical benefit has not been directly studied in human trials.
The AFU vs CFU debate should not distract from the more important question: does this product contain a strain with evidence for your specific health goal?
Who Uses AFU?
Currently, Seed DS-01 is the most prominent consumer probiotic brand using AFU as its primary potency measure. The company argues AFU is a more accurate reflection of viable bacterial load in the capsule at time of consumption.
Bottom Line
AFU is a scientifically valid measurement that captures a broader slice of viable bacteria than CFU. It does not, however, substitute for strain-indication evidence. A high AFU product with strains that lack evidence for your condition will not outperform a low-CFU product with a matched, well-evidenced strain.