Microdase Test (Modified Oxidase)
An original disk bench visual for reading modified oxidase activity to rapidly differentiate Micrococcus from Staphylococcus among catalase-positive Gram-positive cocci.
Microdase disks A-B
Read the disk at exactly 2 minutes. Blue to purple-blue is positive; no color change is negative.
Development of blue to purple-blue color within 2 minutes; oxidase enzyme reacted with cytochrome C to form indophenol.
No color change on the disk after 2 minutes; oxidase is absent.
Development of blue to purple-blue color within 2 minutes; oxidase enzyme reacted with cytochrome C to form indophenol.
No color change on the disk after 2 minutes; oxidase is absent.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Blue to purple-blue disk | Oxidase enzyme oxidized the reagent with cytochrome C to form indophenol | Positive — Micrococcus |
| No color change | Oxidase enzyme is absent; reagent remains uncolored | Negative — Staphylococcus |
| Read at 2 minutes | Strictly timed endpoint prevents false-positive results from air exposure | Do not read after 2 minutes |
| Colony smear on disk | Rub several colonies from blood agar onto the dry disk surface | Do not rehydrate the disk before use |
Microdase study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Positive | Blue to purple-blue color within 2 minutes | Presumptive Micrococcus species; correlate with Gram stain (GPC in tetrads) and colony morphology. |
| Negative | No color change within 2 minutes | Presumptive Staphylococcus species; continue workup with coagulase, novobiocin, or PYR as appropriate. |
| Limitations | S. sciuri, S. lentus, S. vitulus may be positive | Confirm species-level identification with additional tests when clinical context requires. |
- Microdase (modified oxidase) rapidly separates Micrococcus (positive) from Staphylococcus (negative).
- Both genera are catalase-positive Gram-positive cocci, so catalase does not separate them.
- The 2-minute read window is mandatory; late color development should not be reported.
Best mental model: Micrococcus has cytochrome oxidase and turns the disk purple; Staphylococcus lacks it and the disk stays white.