Modified oxidase test (microdase) is recommended for Gram-positive, catalase-positive cocci, only. The microdase test is a rapid method to differentiate Staphylococcus from Micrococcus 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 which are too young or too old may give inaccurate results. The DMSO renders the cells premeable to the reagent.
- 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