What indicates the presence of hydrogen sulfide in triple sugar iron agar (TSIA)?

In triple sugar iron agar (TSIA), the presence of hydrogen sulfide (H2S) is indicated by the formation of a black precipitate in the medium. This reaction occurs when sodium thiosulfate is reduced to hydrogen sulfide by certain bacteria. The hydrogen sulfide then reacts with ferric ions present in the medium, specifically from ferric ammonium citrate, forming insoluble ferrous sulfide that appears as a black precipitate.

This method of detecting hydrogen sulfide is a vital part of identifying specific Enterobacteriaceae species, which can produce H2S. In contrast, tryptophan and alpha-naphthol do not play a direct role in H2S production or detection in this specific agar medium. Tryptophan is more associated with indole production, while alpha-naphthol is typically used in other biochemical tests, such as the Voges-Proskauer test. Thus, sodium thiosulfate is the correct choice because of its role in facilitating the reaction that leads to the visible black precipitate indicating hydrogen sulfide presence.