Modified Oxidase Test (Microdase): Principle, Procedure and Uses

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.

Related topic: Oxidase test: Principle, Procedure, Results and Oxidase positive organisms 

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.


  1. Using sterile forceps, transfer a Microdase disk from the stock bottle to a petri dish. Return the bottle of disks to the refrigerator, promptly.
  2. 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.
  3. Incubate at room temperate for 2 minutes
Microdase Test (Modified Oxidase Test).
Right: Negative
Left: Positive


  1. Positive test: A blue or purple-blue color change within 2 minutes
  2. Negative test: No change in color

Quality Control:

  1. Positive: Micrococcus luteus
  2. 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