What is the indicator in Bismuth-sulfite agar?

Bismuth-sulfite agar is a selective and differential medium primarily used for isolating and identifying Salmonella typhi and other bile-resistant gram-negative bacteria. The key indicator in this medium is ferrous sulfate, which reacts with hydrogen sulfide (H2S) produced by certain bacteria during fermentation.

When H2S is produced, it reacts with the ferrous sulfate to form a black precipitate of ferrous sulfide, indicating growth of bacteria that produce this sulfide. This characteristic blackening is what allows for the differentiation of H2S-producing organisms from those that do not produce it, making ferrous sulfate an essential component of the medium's function in identification processes. The other options listed, such as neutral red, phenol red, and bromothymol blue, serve as indicators in different culture media but are not used in Bismuth-sulfite agar.