Very long time ago… This is a photo from the beginning of my work as a future/would-be mycologist. By the color of the test tubes, you can easily see that we are dealing with a medium with urea. In bacteriology, the urea hydrolysis test is used routinely for almost all bacteria. In turn, in the case of dermatophytes, although it is a differentiating feature, it usually gives unexpected results. By accident I managed (them) to get quite a nice color gradient from yellow (which doesn't happen often) to very pink (fuchsia/bishop color). And the culprit was Trichophyton verrucosum, which according to the literature should be urea positive. This yellow color (i.e. the opposite of the intended effect) is due to the fact that this strain of T. verrucosum very quickly used up a small amount of dextrose in the medium, which acidified the medium so much that the phenol red from a slightly orange color turned yellow. In general, we all use the term Christensen's medium, but every lab's composition is a little different. Note that there is such a thing as Stuart's medium for detecting urease, and it is not much different from Christiansen's medium. Virtually every laboratory uses a combination of these media so as not to add dextrose but at the same time the medium is "rich" enough for other microbes to grow on it. Hence, there are often considerable discrepancies - especially in mycology. Believe it or not, the same strain that gives a typical positive result in lab A will give +/- or the opposite in lab B.
