DNA Hydrolysis (DNase Test Agar)
An original Learn Microbes bench guide for recognizing deoxyribonuclease (DNase) activity on methyl green agar to differentiate key staphylococci, enterics, and respiratory cocci.
DNase agar plates A-B
Look for a clear, colorless zone surrounding the streak of growth, indicating DNA-methyl green complex degradation.
A wide, colorless/clear zone surrounds the streak of growth because DNA hydrolysis has released methyl green from the complex.
No DNA hydrolysis occurs; the agar medium remains its original pale green-teal color right up to the edge of the growth streak.
A wide, colorless/clear zone surrounds the streak of growth because DNA hydrolysis has released methyl green from the complex.
No DNA hydrolysis occurs; the agar medium remains its original pale green-teal color right up to the edge of the growth streak.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Colorless cleared zone | DNA hydrolyzed, releasing methyl green indicator | Positive reaction (DNase present) |
| Unchanged teal-green agar | DNA-methyl green complex remains intact | Negative reaction (DNase absent) |
| Cream-colored growth streak | Bacterial multiplication and colony formation | Confirms strain viability and growth |
Coagulase study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Wide colorless zone | DNase Positive | Confirms Staphylococcus aureus (separates from CNS) or Serratia marcescens (separates from Enterobacter). |
| No colorless zone (agar stays teal) | DNase Negative | Confirms coagulase-negative staphylococcus (e.g. S. epidermidis) or Enterobacter aerogenes (non-degrading). |
| Moraxella vs Neisseria | Moraxella catarrhalis (+) | Moraxella catarrhalis is DNase positive, while pathogenic Neisseria species are DNase negative. |
- Differentiates DNase-producing Staphylococcus aureus and Serratia marcescens from non-producing relatives.
- Measures active enzymatic hydrolysis of deoxyribonucleic acid (DNA).
- Uses methyl green dye complexed with highly polymerized DNA as a color indicator at pH 7.5.
A clear colorless halo around the colony is positive. Light scattering under thick growth is not a zone.