Catalase Test- Principle, Procedure and Result Interpretation
Objective of Catalase Test
This test differentiates catalase-positive micrococcal and staphylococcal species from catalase-negative streptococcal species.
Principle of Catalase Test
Aerobic and facultative anaerobic organisms produce two toxins during normal metabolism, hydrogen peroxide (H2O2) and superoxide radical (O2−). These bacteria have two enzymes that detoxify the products of normal metabolism. One of these enzymes, catalase, is capable of converting hydrogen peroxide to water and oxygen. The presence of the enzyme in a bacterial isolate is evidenced when a small inoculum introduced into hydrogen peroxide (30% for the slide test) causes rapid elaboration of oxygen bubbles. The lack of catalase is evident by a lack of or weak bubble production.
Procedure of Catalase Test
1. Use a loop or sterile wooden stick to transfer a small amount of colony growth to the surface of a clean, dry glass slide.
2. Place a drop of 3% hydrogen peroxide (H2O2) onto the medium.
3. Observe for the evolution of oxygen bubbles.
Result Interpretation of Catalase Test
Positive: Copious bubbles are produced.
Negative: No or few bubbles are produced.
Limitations of Catalase Test
Some organisms (enterococci) produce a peroxidase that slowly catalyzes the breakdown of H2O2, and the test may appear weakly positive. This reaction is not a truly positive test. False positives may occur if the sample is contaminated with blood agar.
Positive: Staphylococcus aureus (ATCC25923)
Negative: Streptococcus pyogenes (ATCC19615)