Which H2S indicator is used in Xylose-Lysine-Deoxycholate agar?

In Xylose-Lysine-Deoxycholate (XLD) agar, the indicator for hydrogen sulfide (H2S) production is ferric ammonium citrate. This compound serves as a detection agent that reacts with hydrogen sulfide produced by specific bacteria, resulting in the formation of a black precipitate when H2S is present. This characteristic blackening aids in the identification of certain enteric bacteria, such as Salmonella, which can produce H2S.

The role of ferric ammonium citrate is crucial, as it effectively indicates the metabolism of sulfur compounds by the bacteria being cultured, thus helping in the differentiation among different species. Other options listed, while they may be associated with various other media or biochemical tests, do not serve as the H2S indicator in XLD agar specifically. For instance, sodium thiosulfate is another sulfur source used in some media, but it does not produce the black precipitate directly on XLD. Similarly, ferrous sulfate can be used in some other media for H2S detection but is not the choice for XLD agar, and brilliant green is typically used as an indicator in different cultures.