Certain microbes have the ability to simplify amino acids by removing attached groups. Bacteria which produce cysteine desulfhydrase, a simplifying enzyme, are able to remove the sulfhydryl and amino groups from cysteine. This yields hydrogen sulfide, ammonia, and pyruvic acid. Both hydrogen sulfide and ammonia have unpleasant odors and can be detected by smell. Hydrogen sulfide smells like rotten eggs. Smell is not the safest way to identify things in the laboratory, so we resort to a secondary chemical reaction. Since hydrogen sulfide reacts with heavy metals such as iron, iron ions are suspended in the triple sugar iron (TSI) broth. Once an unknown bacterium is isolated and diluted in liquified agar medium, cysteine desulfhydrase production can be ascertained. Using aseptic technique, an inoculating needle is dipped in the bacterial medium and then stabbed into the TSI slant. After 48 hours of incubation time, the TSI slant is examined for any changes. If cysteine desulfhydrase was produced, cysteine would have been broken down into hydrogen sulfide, and this would have reacted with the iron ions to form the black precipitate ferrous sulfide, FeS. This is a positive test result for cysteine desulfhydrase. No change in color and absence of a precipitate is indicative of a negative reaction.