Oxidase test 

Principle certain bacteria produce an enzyme cytochrome c oxidase shortly called oxidase, which is a component of the electron transport chains of bacteria.On addition of a colourless substrate tetramethyl p phenylene diamine dihydro chloride, the bacteria that produce oxidase enzyme will form a dark purple product.

Methodology a sterile filter paper disc is impregnated with oxidase reagent and a small portion of colony is applied on it with a sterile loop.The production of purple colour in the swab indicates oxidase positive.

Example 

Oxidase positive pseudomonas species and neisseria species.

Oxidase negative Escherichia coli and klebsiella species.

Indole methyl red voges proskauer citrate test 

The four test which are popularly called Imvic test are done to differentiate the various members of enterobacteriaceae.

Indole test 

Principle some bacteria can metabolize tryptophan to indole.The production of indole is detected by the addition of Kovacs reagent which produces a red colour.

Methodology the bacterium is inoculated in tryptone water which contains tryptophan and is incubated at 37 degree Celsius for 24 h.After incubation 1 ml of Kovacs reagent is added to the tube.a bright red colour in the top layer indicates indole positive and the absence of colour indicate indole negative.

Example it is used to differentiate Escherichia coli from klebsiella species 

Indole positive e coli 

Indole negative kleb species 

Methyl red and voges proskauer test 

Principle

These two test use Mr vp broth with contain peptones buffer and glucose bacteria can convert glucose to pyruvate by different metabolic pathways.Two important path way are the acid path way and the neutral path way.

In acid pathway the end product are lactic acetic acid and formic acid.This pathway is identified by methyl red test.In this test the pH indicator Mr is added to the inoculated tube of mr and vp broth.if stable acid products are present the Mr will stay red this indicates Mr positive.

In the neutral pathway either unstable acidic product are formed or the bacteria produce neutral end product like acetoin when voges proskauer reagent are added to the inoculated broth the acetoin end product is oxidised to diacetyl in the presence of potassium hydroxide the diacetyl will then react to produce a red colour which indicates positive results 

Methodology inoculate the bacterium in two Mr vp broth and incubate 37 degree celcius for 24 h in one tube add Mr and in another tube add vp.reagent and look for the appearance of red colour. 

Example methyl red 

positive e coli 

Negative kleb species

Voges proskauer 

Postive kelb species

Negative e coli.