How does the presence of H2S affect lysine iron agar?

The presence of hydrogen sulfide (H2S) in lysine iron agar is indicative of specific microbial metabolic activities. When H2S is produced by certain bacteria, it reacts with the iron salts present in the medium, forming iron sulfide, which appears as a black precipitate. This blackening typically occurs in the butt of the agar where the environment is anaerobic, allowing the sulfate-reducing bacteria to survive and produce H2S.

This reaction is a key feature used to identify and differentiate enteric bacteria, particularly those in the family Enterobacteriaceae, which can reduce sulfur compounds. The formation of a blackened butt in lysine iron agar signifies a positive test for H2S production, aiding in the identification of specific bacterial species based on their metabolic capabilities. The other options do not accurately reflect the biochemical reactions associated with H2S in this medium.