why is it advisable to include a known oxidase-positive control
It is advisable due to following reason--
The oxidase test determines whether or not an organism has cytochrome oxidase in its electron transport chain. By adding a drop of oxidase reagent (p-phenylenediamine) to the culture.
The Oxidase test (also known as the Cytochrome Oxidase test) is used to look for oxidase enzymes produced by Organism. Oxidases catalyse electron transport between substrates acting as electron donors in the bacterium and tetramethyl-p-phenylenediamine OR dimethyl-p-phenylenediamine - a redox dye present as the hydrochloride or oxalate salt (the latter has a longer shelf-life). The dye is reduced to a deep violet-blue colour in the presence of oxidase enzymes.
Usually the oxidase reagent contains a chromogenic reducing agent, which is a compound that changes color when it becomes oxidized. If the test organism produces cytochrome oxidase, the oxidase reagent will turn blue or purple within 15 seconds.
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