What is the indicator used in Simmons citrate agar?

Simmons citrate agar uses bromothymol blue as its pH indicator. This dye changes color in response to pH variations, which is crucial for assessing the ability of bacteria to utilize citrate as a carbon source. When bacteria that can metabolize citrate grow on this medium, they produce alkaline byproducts that raise the pH of the medium. As the pH increases, bromothymol blue transitions from green to blue, indicating a positive reaction for citrate utilization. This color change is fundamental in differentiating between bacteria that can or cannot utilize citrate, helping in the classification and identification of various bacterial species.

In contrast, the other indicators mentioned do not play a role in Simmons citrate agar, as they are used in different types of media for different biochemical tests.