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

Limitations

  • 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)

References

  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, PhD

Dr. Sagar Aryal is a microbiologist, researcher, and science communicator from Nepal. He is also co-founder and Research Head of the Department of Natural Products at the Kathmandu Research Institute for Biological Sciences (KRIBS), where he leads research on natural product discovery and microbial biodiversity. Dr. Aryal earned his Ph.D. in Microbiology from Tribhuvan University, Kathmandu, Nepal, in 2025, supported by a DAAD Research Grant and a University Grants Commission (UGC) Nepal Ph.D. Fellowship. His research interests include Actinomycetes, Myxobacteria, microbial taxonomy, and natural products, particularly the discovery of bioactive compounds with potential pharmaceutical applications. He isolated and identified Nocardia nepalensis, a novel bacterial species from Nepal, contributing to understanding microbial diversity in the region. Dr. Aryal has published more than 16 research articles in international journals and contributed three book chapters to established academic publications. He has presented scientific research findings and developed international research collaborations through visits to more than 18 countries across Asia, Europe, North America, and Oceania. From 2023 to 2025, he served as the American Society for Microbiology (ASM) Young Ambassador to Nepal. Since December 2025, he has served as an Editorial Board Member of Scientific Reports, a Nature Portfolio journal. In recognition of his scientific contributions, Dr. Aryal received the NAST Science and Technology Youth Award 2025 from the Nepal Academy of Science and Technology. He previously received the Frs. Locke and Stiller Research Award (LSRA) for the 2017-2018 cycle from St. Xavier's College, Kathmandu, and a Nepal Academy of Science and Technology (NAST) Research Grant for 2017-2018. Alongside his scientific career, Dr. Aryal has more than 15 years of experience in blogging, content writing, and search engine optimization (SEO).