Catalase test

Principle

Catalase breaks hydrogen peroxide into water and oxygen, seen as bubbles.

Procedure

• Place a drop of 3% H2O2 on a clean slide (or in a tube).
• Add a colony with a wooden stick or plastic loop and watch for immediate bubbling.

Results

• Positive: brisk bubbles (e.g. Staphylococcus, Listeria)
• Negative: no bubbles (e.g. Streptococcus, Enterococcus)

Pitfalls

• Do not take colonies from blood agar with red cells: RBCs contain catalase.
• Avoid nickel/chrome loops, which can cause false positives.
• Controls: S. aureus (positive), Streptococcus pyogenes (negative).