31.15: Catalase Test
- Page ID
- 165426
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The catalase test is used to detect the presence of the enzyme catalase in bacterial cells. This enzyme plays a critical role in protecting aerobic and facultatively anaerobic organisms from oxidative damage caused by reactive oxygen species. During aerobic respiration, bacteria that use oxygen inevitably produce toxic byproducts, such as superoxide radicals and hydrogen peroxide (H₂O₂). These molecules can damage cellular components, including proteins, DNA, and membrane lipids.
To survive in oxygen-rich environments, bacteria must have mechanisms to neutralize these toxic compounds. Superoxide radicals are first converted into hydrogen peroxide by the enzyme superoxide dismutase. Hydrogen peroxide, although less reactive, is still harmful and must be broken down further. Catalase facilitates the decomposition of hydrogen peroxide into water (H₂O) and oxygen gas (O₂), both of which are non-toxic to cells. The overall reaction catalyzed by the enzyme is: 2H₂O₂ → 2H₂O + O₂
The release of oxygen gas during this reaction forms visible bubbles, which serve as the basis for the catalase test. A positive result is indicated by the rapid production of bubbles when hydrogen peroxide is applied to a bacterial culture. The presence or absence of this reaction helps differentiate catalase-positive organisms, such as staphylococci, from catalase-negative organisms, such as streptococci.
It is important to note that this test should not be performed on cultures you need to preserve, as the addition of hydrogen peroxide can damage or destroy the cells. The test should also be performed on actively growing cultures, as older cells may lose enzyme activity and produce false-negative results.
Once H2O2 is added to the culture the culture is no longer viable.
Be sure to make a new slant if there are more tests to be done.
Procedure
- Add one to two drops of 3 percent hydrogen peroxide (H₂O₂) directly onto the bacterial smear
- Observe the reaction immediately for the formation of bubbles
Interpretation of Results
- Immediate and vigorous bubbling: positive result, indicating the presence of catalase
- No bubbling or only a few scattered bubbles: negative result, indicating the absence of detectable catalase activity