What are CFUs?
Colony forming units, or CFUs, have historically been the standard in biology for counting the capacity of bacterial cells to divide and form colonies. This is done by a technique called “plating”, or “plate counting”. It sounds just like what it is: studying a single bacteria as it replicates over and over until there are so many cells that they’re visible to the naked eye. Then the colonies are counted, and the number of multiplying bacteria are calculated. This number of bacteria in a probiotic dose is listed on the label, and often it’s assumed that the larger the number, the more effective the probiotic.
The higher the CFU number, the better? Not necessarily…
We know that probiotics need to be alive to efficiently provide health benefits. However, being alive doesn’t mean multiplying, and we now know that bacteria needs to be active in addition to being alive. Active bacteria are alive, but counting cells that are active but not multiplying is impossible with the plate counting method.
The CFU method doesn’t take into account the fact that bacteria can be active without multiplying. In fact, many of the mechanisms of action of probiotics, i.e. the efficacy of probiotics, require that the probiotic be active but not actively multiplying: production of specific metabolites that are continuously secreted outside the bacterial cell (SCFA, vitamins), the presence of cell wall compounds, and the production of specific proteins outside the cells. In order to be effective, probiotics need to be active in addition to being alive. Unfortunately, the CFU method can’t quantify the bacteria that are active but not multiplying. Furthermore, plate counting is limited by its dependence on a combination of culture conditions such as nutrients in the culture medium and temperature. Additionally, plating doesn’t provide information on the heterogeneity of a bacterial culture, namely they don’t detect the cells in a viable but not cultivable (VBNC) status.
What are AFUs?
So if counting CFUs isn’t effective for determining the efficacy of a probiotic, what is? Enter AFUs, or Active Fluorescent Units. This is a more advanced technique, and it uses flow cytometry to accurately quantify and distinguish between live, dead, and active cells. Basically, with flow cytometry, probiotic cells are marked with fluorescent light that is illuminated by a laser to show whether or not they’re viable. The machine then counts and gives a readout of how many microbes are alive and active.
At Pendulum, we use flow cytometry to measure AFUs for our products, because this method of measurement is an important parameter to better understand exactly how many live and active microorganisms are contained within each serving. In other words, it’s an accurate measurement to better provide probiotics that benefit you and your gut microbiome.