Modified Oxidase Test (Microdase): Procedure, Uses

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.

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 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.

Table of Contents


  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
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


  1. Tarrand, J. J., & Gröschel, D. H. (1982). Rapid, modified oxidase test for oxidase-variable bacterial isolates. Journal of clinical microbiology, 16(4), 772–774. 
  2. Faller, A., & Schleifer, K. H. (1981). Modified oxidase and benzidine tests for separation of staphylococci from micrococci. Journal of clinical microbiology, 13(6), 1031–1035.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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