Answering "Describe a method to rapidly identify Enterobacteria"

And the answer is The Indole Test, a test that makes use of the capacity certain bacteria have to metabolise the amino acid Tryptophan using a series of intracellular enzymes. This results in the release of a substance known as Indole that is therefore combined with a reactive agent that might accuse colour change.

And how exactly is this accomplished?

For a facultatively anaerobic organism like E. coli all they do is conduct an exothermic reaction to force a reductive deamination (release of -NH2) of the Tryptophan with the aid of the enzyme Tryptophanase (and the coenzyme factor Pyridoxal phosphate). They mix all these very well in their internal blenders and the result is theformation of three main elements:

- Ammonium

- Pyruvic acid

- Indole!

Than all we have to do is join the indole produced with an agent containing a mix of HCl and p-dimethylaminobenzaldehyde in amyl alcohol - nothing much!!, if you can say this long description p-dimethylaminobenzaldehyde in amyl alcohol, you're half way through to get it over the counter... Hahah! Actually this very long-named substance is an organic compound, shall we call it a 'floater' (not water soluble)!!!

And than, bammmmmm, the solution changes colours from yellow to red in a positive treatment. The red solution will top the remaining layers in an indole positive treatment whereas in your negative control (no indole present) it will remain yellowish.

Simples!

The Indole test protocol [1] can be found here thanks to the contribution of Sagal Aryal.

[1] Indole test, Microbiology Notes, [http://www.microbiologyinfo.com/indole-test-principle-reagents-procedure-result-interpretation-and-limitations/], last visited on the 15th of October 2015, last published on the 6th of September 2015.

Image taken from [1].