NITRATE DISCS

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 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 and Sulphanilic acid. Reduction of nitrate (NO3) to nitrite (NO2) and
subsequently to nitrogen gas (N2) usually takes place under anaerobic conditions, in which an organism derives its
oxygen from nitrate. Most facultative anaerobes can reduce nitrate in the absence of oxygen. This anaerobic
respiration is an oxidation process in which inorganic substances furnish oxygen to serve as an electron acceptor to
provide energy. The end product possibilities of nitrate reduction are many depending upon the bacterial species. The
more common end product via nitrite reduction is molecular nitrogen. Depending upon environmental conditions,
these products are usually not further oxidized or assimilated into cellular metabolism, but are excreted into the
surrounding medium.

Substrate for nitrate reduction