Nitrate Agar

Nitrate Agar is a solid medium recommended to differentiate microorganisms based on their ability to reduce nitrate to nitrite (denitrification). Members of the Enterobacteriaceae family use nitrate as the source of nitrogen, but it must be broken down to be utilized.

Depending on the bacterial species, the final product will be nitrite, ammonia or nitrogen gas. Potassium Nitrate is the nitrogen source in this medium. Nitrate breakdown to nitrite is detected by adding Sulfanilic Acid, which forms a diazonium salt when reacting with nitrite, and N,N-dimethyl-α-naphtylamine, which forms a red complex with the diazonium salt.

To detect microorganisms converting nitrate to ammonia or nitrogen gas, zinc dust must be added. Zinc reduces nitrate to nitrite, resulting in a red color. While the presence of red color indicates that nitrate is still present, its absence evidences nitrate breakdown further than nitrate by the bacteria.

The nutrients required for bacterial growth are provided by Peptic Digest of Animal Tissue and Beef Extract, being Agar the gelifying agent.