What is the indicator used in Simmons citrate agar?

In Simmons citrate agar, bromthymol blue serves as the indicator. This medium is designed to determine an organism's ability to utilize citrate as its sole carbon source, leading to the production of alkaline byproducts. When the pH of the medium increases due to the utilization of citrate, bromthymol blue changes color from green to blue. This color change is indicative of a positive reaction, demonstrating that the organism can grow on citrate and produce alkaline products affecting the pH.

The other indicators mentioned are suited for different types of microbial tests. Phenol red, for example, is commonly used in carbohydrate fermentation tests and turns yellow in acidic conditions. Litmus is often utilized in milk fermentation tests and does not specifically indicate citrate utilization. Methyl red is used in the MR test for mixed acid fermentation and indicates a low pH rather than the utilization of citrate. Each of these indicators has its unique application based on the metabolic capabilities being tested, making bromthymol blue the correct choice for Simmons citrate agar.