Nitrate discs are used as substrate for detection of nitrate reduction by microorganisms. The test involves detection of the enzyme nitrate reductase which causes the reduction of nitrate in the presence of a suitable electron donor to nitrite, which can be tested by an appropriate colorimetric reagent.Nitrate disc contains potassium nitrate as substrate which is broken down to nitrite when nitrate reductase positive culture is grown in presence of these discs. Nitrite production can be detected by using Nitrate Test Reagents - a-naphthylamine (R009) and Sulphanilic acid (R015). Escherichia coli ATCC 25922 Positive reaction:red or pink colour formation onaddition of nitrate test reagents. Acinetobacter calcoaceticus ATCC 43498b luxuriant negative reaction