Modified oxidase test (microdase) is recommended for Gram-positive, catalase-positive cocci, only. The microdase test is a rapid method to differentiate Staphylococcus (-ve) from Micrococcus +ve) by detection of the enzyme oxidase. Filter paper disks impregnated with tetramethyl-p-phenylenediamine dihydrochloride (oxidase reagent) in dimethyl sulfoxide (DMSO) are used.
In the presence of atmospheric oxygen, the oxidase enzyme reacts with the oxidase reagent and cytochrome C to form the colored compound, indophenol.
Bacteria from a culture grown on Blood agar for 24 – 36 hours must be used. Cultures that are too young or too old may give inaccurate results. The DMSO renders the cells permeable to the reagent.
Modified oxidase test is performed in the same manner as the oxidase test, except that the reagent is 6% tetramethyl phenylenediamine hydrochloride in dimethyl sulfoxide.
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- Using sterile forceps, transfer a microdase disk from the stock bottle to a petri dish. Return the bottle of disks to the refrigerator, promptly.
- Using a wooden applicator stick, rub a small amount of several colonies of an 18-24 hour pure culture grown on blood agar onto the top of microdase disk.
- Incubate at room temperate for 2 minutes
- Positive test: A blue or purple-blue color change within 2 minutes
- Negative test: No change in color
- Positive: Micrococcus luteus
- Negative: Staphylococcus aureus
Modified oxidase test is used for differentiating Micrococcus from Staphylococcus. Micrococci should yield a positive result. Staphylococci should yield a negative result, with the exception of Staphylococcus sciuri, S. lentus and S. vitulinus