Catalase test

From Ganfyd

Introduction

Catalase (also known as peroxidase) is an enzyme that catalyses the breakdown of hydrogen peroxide to oxygen and water. Most higher organisms produce catalase, but in bacteriology this test is usually used to differentiate staphylococci (Catalase positive) from streptococci (Catalase negative).

Chemical equation for the breakdown of hydrogen peroxide:2H₂O₂ → 2H₂O + O₂

Methods

Testing on an agar slope

• Culture the organism for 24 hours on a nutrient agar slope.
• 1ml hydrogen peroxide solution (10 vol) is carefully poured onto the slope.
• Vigorous production of gas bubbles indicates a positive test.

Testing on a suspension

• Emulsify a colony in 0.5ml Tween 80 in a bottle with a screw top.
• Add 0.5ml hydrogen peroxide solution (20 vol).
• Effervescence suggests a positive reaction.

Testing on a slide

• Place a drop of hydrogen peroxide on a slide.
• Smear a colony on a cover slip (preferably an impression smear).
• Place cover slip onto hydrogen peroxide (10 vol) drop.
• Appearance of gas bubbles indicates a positive test.
• NB: Some methods use a an equal mixture of hydrogen peroxide (20 vol) and methylene blue.

Pitfalls

• All methods carry a small risk of aerosol formation.
• Do not mix hydrogen peroxide and the colony on an open slide as this can lead to increased aerosol formation.
• The presence of catalase in the culture medium (e.g. Blood agar) may lead to false positive results, though these are often weak.
• Using an iron-containing loop may also give a false positive result.

Control organisms

• Positive control: Staphylococcus epidermidis
• Negative control: Enterococcus faecalis

Typical results

A summary of typical results seen with some commonly encountered Gram-positive organisms.