The catalase test is a biochemical procedure utilized in microbiology laboratories to classify and identify different species of bacteria. It relies on a microorganism’s metabolic capabilities, specifically its ability to produce a particular protective enzyme. This test provides an initial result that acts as a step in the diagnostic process for bacterial identification.
Why Bacteria Need Catalase
Bacteria that thrive in oxygen-rich environments produce energy through aerobic respiration. This process generates toxic compounds, including hydrogen peroxide, which can severely damage a cell’s internal structures, including its DNA, proteins, and cell membranes.
To survive this self-inflicted oxidative stress, bacteria must possess a defense mechanism to neutralize the threat. Catalase is the enzyme that serves this protective role, allowing the organism to detoxify the hydrogen peroxide and continue its normal metabolic function. The presence of catalase is commonly associated with bacteria that are either strict aerobes (which absolutely require oxygen) or facultative anaerobes (which can use oxygen when it is available). Conversely, bacteria that are obligate anaerobes, which cannot tolerate oxygen, generally lack this specific enzyme because they do not utilize oxygen in their energy production pathway.
Performing the Test and Interpreting Results
The catalase test is performed by mixing a small amount of the bacterial culture with a solution of hydrogen peroxide on a glass slide or in a test tube. An isolated colony is transferred from a culture plate to the slide, and a drop of the reagent is added directly to the sample. The technician immediately observes the reaction for any visible signs of change.
The immediate and vigorous formation of bubbles, or effervescence, indicates a positive result, confirming the presence of the catalase enzyme. This bubbling is the manifestation of the chemical reaction where catalase rapidly breaks down the hydrogen peroxide into two harmless substances: water and oxygen gas. The oxygen gas creates the bubbles observed during a positive test.
If no bubbles are produced within a short timeframe, usually 20 seconds, the result is considered negative, meaning the organism lacks the catalase enzyme. This absence of bubbling signifies that the hydrogen peroxide remains intact because the bacterial cells cannot break it down into water and oxygen.
Identifying Microorganisms in the Lab
The catalase test in a diagnostic laboratory separates large, clinically significant groups of bacteria into smaller, more manageable categories. This test serves as a foundational step in a flow chart designed to narrow down the identity of an unknown microbe isolated from a patient sample. For example, the test is routinely used to separate Gram-positive cocci, which are spherical bacteria that retain the purple Gram stain color.
Among these cocci, the test clearly distinguishes the two major groups: Staphylococcus species, which are catalase-positive, from Streptococcus species, which are catalase-negative. This distinction directs the microbiologist toward the next appropriate, more specific tests needed for final identification of the pathogen. A positive catalase result immediately eliminates all Streptococcus and Enterococcus species from consideration, allowing the focus to shift toward identifying the specific Staphylococcus involved.
Beyond the cocci, the catalase test is also helpful in differentiating certain Gram-positive rods, such as separating the catalase-positive Bacillus species from the aerotolerant, yet catalase-negative, Clostridium species. The enzyme’s presence is also a factor in identifying some Gram-negative organisms, including a number of species within the Enterobacteriaceae family. By quickly determining the presence or absence of this enzyme, the catalase test provides a rapid and efficient means of initial classification.