Microdase (Modified Oxidase) Test- Principle, Procedure, Results

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Objective of Microdase (Modified Oxidase) Test

This test is used to differentiate gram-positive, catalase-positive cocci (micrococci from staphylococci).

Principle of Microdase (Modified Oxidase) Test

The microdase test, also known as modified oxidase test is a rapid test to differentiate Staphylococcus from Micrococcus which are Gram positive cocci possessing catalase enzyme. The differentiation is based on the detection of oxidase enzyme. For the detection of oxidase enzyme a filter paper circular disks impregnated with tetramethyl-p-phenylenediamine dihydrochloride (oxidase reagent) in dimethyl sulfoxide (DMSO) are used. The modified oxidase reagent is prepared as 1% (w/v) tetramethyl-p-phenylenediamine in certified grade dimethyl sulfoxide. DMSO aids in the permeability of cells to the reagent. In the presence of atmospheric oxygen, the oxidase enzyme reacts with the oxidase reagent and cytochrome C to form the colored compound, indophenol indicated as blue or purplish blue coloration on the disc after the introduction of bacterial colony on the disc.

Media Used in Microdase (Modified Oxidase) Test

Oxidase Disc

Filter paper disks impregnated with tetramethyl-p-phenylenediamine dihydrochloride (oxidase reagent) in dimethyl sulfoxide (DMSO).

Procedure of Microdase (Modified Oxidase) Test

  1. Using sterile forceps transfer a Microdase disk from the stock bottle to a petri dish.
  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
  4. Observe for the colour development

Result Interpretation of Microdase (Modified Oxidase) Test

Result Interpretation of Microdase (Modified Oxidase) Test

Positive test: development of blue or purple blue on the disc within 2 minutes 

Negative test: no color change


  • Staphylococci should yield a negative color change, except for S. sciuri, S. lentus, and S. vitulus.

Quality Control

Positive: Micrococcus luteus (ATCC10240)

Negative: Staphylococcus aureus (ATCC25923)


  1. Tille P.M (2014). Bailey and Scott’s diagnostic microbiology.Thirteen edition. Mosby, Inc., an affiliate of Elsevier Inc. 3251 Riverport Lane, St. Louis, Missouri 63043
  2. Faller A, Schleifer KH. Modified oxidase and benzidine tests for separation of staphylococci from micrococci. J Clin Microbiol. 1981 Jun;13(6):1031–1035.
  3. Tarrand JJ, Gröschel DH. Rapid, modified oxidase test for oxidase-variable bacterial isolates. Journal of Clinical Microbiology. 1982;16(4):772-774.

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

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