Microbiology: Hydrogen sulfide test.

Hydrogen Sulfide Test (Cappuccino & Welsh, 2017).

Description. Some bacteria are able to reduce organic/inorganic sulfur sources (e.g. cysteine, thiosulfate) into hydrogen sulfide, H₂S. The ferrous ammonium sulfate in the SIM agar (also containing peptone and sodium thiosulfate as substrates in the reduction/hydrogenation of sulfur) deep medium acts as an indicator by forming black precipitate (positive result) in the presence of H₂S gas (Cappuccino & Welsh, 2017). The black precipitate “trail” also allows for observation of bacterial motility.

Clinical significance. The Hydrogen Sulfide test helps to separate Proteus/Salmonella (able to metabolize sulfur) from Shigella dysentariae (unable to metabolize sulfur).

Summary of main steps. It is assumed that the reader knows aseptic technique and basic inoculation.

1. Using aseptic technique and a transfer needle, inoculate a SIM agar deep tube via straight-line stab.
2. Incubate tube at 37°C for 24-28 hours. Observe results.

Reference

Cappuccino, J. G., & Welsh, C. (2018). Microbiology: A laboratory manual.