Can someone explain the color-changing unit (CCU) to me?

Question

I've been physically carrying out serial tenfold dilutions on samples of Ureaplasma to work out the color-changing units (CCU). As a definition, the CCU is the highest dilution at which there is a color change. If the highest dilution is 10^3, apparently that means that there are 10^3 cells per ml in the original sample. But this is what I do not understand. How are we sure of this? I feel like there is a lot that I am missing. Any help would be greatly appreciated.

Answer 1

The color-changing unit for Ureaplasma were initially developed based on correlating them to colony-forming units (CFU). By looking at how many cells grow from a given CCU₅₀, we established a curve for correlating them. This was later verified by comparing the amount of DNA in a sample to its CCU₅₀ and its CFU. The CCU₅₀ actually correlated better because the enzymatic activity is not decreased even if cells die. So while no test is perfect, we have pretty good correlation. In fact, most lab tests are based on color changes or optical densities which we then correlate to known concentrations.