Final answer:
When Phenol Red broth turns pink, it indicates a rise in pH due to the hydrolysis of substances by bacteria and the release of ammonia, signifying a positive urease test.
Step-by-step explanation:
When a Phenol Red broth turns pink, it indicates an increase in pH, typically due to the hydrolysis of compounds in the broth and the subsequent release of ammonia (NH3), which is an alkaline substance. The presence of ammonia raises the pH of the broth, and this change is detected by the pH indicator Phenol Red. When the pH becomes more alkaline (pH>7), Phenol Red turns from its original red color to a deep pink, which suggests that the microorganisms being tested are able to hydrolyze urea, displaying a positive urease test. This is a significant contrast to a scenario where, for instance, a Methyl Red (MR) test turns red post-incubation, which indicates a positive test for mixed acid fermentation due to the presence of stable acid end products generated by the bacteria.