AFU In Probiotics


AFU is already on some probiotic supplement labels, and may be on more. As if there is not enough confusing information on food, drug, and supplement labels, AFU is the new acronym you may see, and this page will help you to understand it.

For years probiotics were measured in grams, which told us nothing about how many viable cells there might be in a product. Then came CFU, colony-forming units. This was a much better way to quantify probiotics and allow for product comparisons.

CFU is an estimated measurement of those microbes that can form individual colonies, that is, those that shown to be able to reproduce. It does not measure inactive, dead, or nonviable organisms. CFU measurement requires plating microbes in a growth medium, diluting the medium and microbes to allow visual counting of between 30 and 300 colonies, for example, and then back-calculating how many microbes would be in one milliliter of the sample.

CFU measurement is a time-consuming process, and results can vary depending on the dilution factor, how homogenous the sample was, and how well a person can read the counts. Many probiotic manufacturers will add more CFU than is stated on the label to account for differences in sampling and reading and to insure that their product contains at least the amount shown on the label.

What is AFU?

AFU, active fluorescent unit, is another way to measure microbes. AFU is measured with flow cytometry, a method that is much faster than culture plating. Flow cytometry is a technology that originated in 1968. In flow cytometry, probiotic cells are tagged with fluorescent markers and passed one-at-a-time through a funnel-like device with lasers and detectors that allows each cell to be measured. The light signals are converted into electronic signals that are analyzed by a computer. The total count in the sample is reported as total fluorescent units (TFU).

Flow cytometry can distinguish between dead, damaged, and alive cells. It counts viable but not culturable (VBNC) cells that may not have been culturable on the CFU plates, but that may still have benefits of live organisms. It also detects inactive organisms that may re-activate in the gastrointestinal tract. The end result is active fluorescent unit, AFU, those microbes that may be active.

AFU is helpful for microbes that are very hard to culture, such as strict anaerobes that would die if exposed to the tiniest bit of oxygen.

Which is Best, AFU or CFU?

Both AFU measurements and CFU measurements have their merits.  Almost all probiotics studies used/use CFU as the measurement. Theoretically, CFU may equal AFU in a freshly-prepared probiotic sample, but that may not hold true over time. Therefore, if only AFU is stated, only studies using AFU can be used to determine effectiveness. The studies that were performed with CFU would be meaningless, as currently there is no set formula to convert from CFU to AFU. Both metrics must be measured at this time.

For now, CFU is the measurement used on most, but not all products Some manufacturers may list only AFU, such as Seed Probiotics and Pendulum Probiotics. Other manufacturers may list only CFU, or you may see CFU and AFU listed on a product.

Since almost all probiotic studies were/are performed using CFU measurements, when choosing a probiotic supplement, use the specific microbes at the specific CFU used in the study relating to your condition. If you rely on AFU, be sure to find a study that used the dosage you are considering.

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