What is the hydrogen sulfide indicator used in triple sugar iron agar?

The hydrogen sulfide indicator used in triple sugar iron agar is sodium thiosulfate. This component acts as a sulfur source in the medium. When bacteria capable of producing hydrogen sulfide (H2S) grow on this agar, they metabolize sodium thiosulfate and generate H2S. This gas then reacts with iron salts present in the medium, forming a black precipitate of iron sulfide. This visible change indicates the reduction of sulfur and is crucial for identifying and differentiating various Enterobacteriaceae, as it helps in determining the metabolic capabilities of the organism.

The other options do not serve this specific purpose in triple sugar iron agar. Sodium nitrate is involved in other reactions related to nitrogen metabolism but does not indicate hydrogen sulfide production. Phenol red is a pH indicator that turns yellow in acidic conditions but does not relate to hydrogen sulfide detection. Litmus is used as a pH indicator in some media but also does not facilitate the detection of hydrogen sulfide. Thus, sodium thiosulfate is the correct answer for identifying hydrogen sulfide production in this medium.