Background
The urease – Indole test is used to screen for potential Salmonella and Shigella colonies on enteric culture media.
The urease test will determine the ability of an organism to split urea, through the production of the enzyme urease. Two units of ammonia are formed with resulting alkalinity in the presence of the enzyme, and the increased pH is detected by a pH indicator.
The Indole test identifies organisms capable of degrading the amino acid tryptophan which results in the release of indole. The addition of Kovacs Reagent produces a deep red colour ring on the surface of the tube in the presence of indole.
Salmonellae and Shigellae are urease negative whilst Enterobacterales such as Proteus, Morganella, Klebsiella and some Citrobacter are strongly urease positive. The production of indole from tryptophan is a characteristic absent in Salmonella but present in Escherichia coli, Morganella and some species of Klebsiella, Aerobacter and Citrobacter.
The test involves preparing a heavy suspension of the test organism in the presence of urea and tryptophan. Following incubation the development of a pink or reddish colour is a positive reaction for urease, the development of a red surface ring following the addition of Kovacs Reagent is a positive reaction for indole.