Enterobacteriaceae and Other Gram Negative Rods

(g)Nitrate reduction
This is used to see whether an organism can reduce nitrate to nitrite using nitrate reductase. The medium used in this test contains potassium nitrate. The reagents sulfanilic acid and a-naphthylamine are also added and if the nitrate is reduced the nitrate will react with the reagents to produce a red colour.
In the case of a negative result zinc dust is added. If there is still nitrate present the zinc will reduce it and turn a pink colour. However if there is no colour change it means that the nitrate has been reduced to other compounds apart from nitrite which gives a positive result.
(h)Amino acid decarboxylase
This test determines whether an organism can decarboxylate an amino acid i.e. is capable of removing the carboxyl group of an amino acid resulting in the production of an amine and carbon dioxide. An amino acid and bromcresol purple, a pH indicator, is added to the medium which is then sealed with mineral oil. This creates anaerobic conditions which encourages fermentation to take place resulting in acid products. If the amino acid is broke down it produces alkaline products which will then cause the indicator to turn purple, which is a positive result.
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