What does a purple slant and blackened butt in lysine iron agar (LIA) indicate?

• A. E. coli
• B. Citrobacter spp.
• C. Salmonella spp.
• D. Proteus spp.

Answer: (C)

A purple slant and blackened butt in lysine iron agar (LIA) indicates that the organism can utilize lysine, resulting in the production of an alkaline environment (hence the purple color on the slant), and also produces hydrogen sulfide (H2S), which reacts with iron salts in the medium to form a black precipitate in the butt.

Salmonella species are well-known for their ability to produce hydrogen sulfide, indicating this metabolic pathway. The blackening of the butt represents the formation of insoluble iron sulfide as a result of the H2S produced during anaerobic respiration. Additionally, Salmonella can decarboxylate lysine, causing the alkaline reaction in the slant, which also contributes to the purple coloration.

This specific combination of reactions—alkaline slant and acid butt with H2S production—helps differentiate Salmonella from other Enterobacteriaceae that might produce similar reactions but do not produce H2S or exhibit different reactions in the two parts of the medium. Therefore, the presence of both these characteristics in the LIA tests confirms the identity of the organism as Salmonella spp.