Nitrate reagent discs are used 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. Almost all Enterobacteriaceae reduce nitrate. Nitrate reagent discs when placed on suspected colony turn red-pink in case of nitrate reduction (positive reaction), when a drop or two of Part B (Rehydrating fluid) is added to the disc. Escherichia coli ATCC 25922 Positive reaction:red or pink colour formation onaddition of nitrate test reagents. Acinetobacter calcoaceticus ATCC 43498b luxuriant negative reaction