In the nitrate reduction test, what indicates a positive result after adding reagents?

In the nitrate reduction test, a positive result is indicated by the culture turning brick red after adding reagents. This color change occurs when nitrate (NO₃⁻) present in the growth medium is reduced to nitrite (NO₂⁻) by the organism being tested. The reagents, which typically include sulfanilic acid and naphthylamine, react with nitrite to produce a red azo dye, signaling a positive reaction.

The specific brick red coloration confirms that the organism has successfully reduced nitrate to nitrite during its metabolic processes. This is an essential differentiation method in microbiology to identify various bacteria based on their nitrate-reducing capabilities. The presence of nitrite is a critical indicator in determining an organism's metabolic pathway and its ability to utilize nitrate as an electron acceptor.