Different bacteria produce varying spectra of enzymes. For example, some enzymes are necessary for the bacterium's individual metabolism, and some facilitate the bacterium's ability to compete with other bacteria or establish an infection. Tests that measure single bacterial enzymes are simple, rapid, and generally easy to interpret. They can be performed on organisms already grown in culture and often provide presumptive identification.

1. Catalase test: The enzyme catalase catalyzes the degradation of hydrogen peroxide to water and molecular oxygen (H2O2 → H2O + O2). Catalase-positive organisms rapidly produce bubbles when exposed to a solution containing hydrogen peroxide (Figure 4.8). The catalase test is key in differentiating between many gram-positive organisms. For example, staphylococci are catalase positive, whereas streptococci and enterococci are catalase negative. The production of catalase is an important virulence factor because H2O2 is antimicrobial, and its degradation decreases the ability of neutrophils to kill invading bacteria.

2. Oxidase test: The enzyme cytochrome c oxidase is part of electron transport and nitrate metabolism in some bacteria. The enzyme can accept electrons from artificial substrates (such as a phenylenediamine derivative), producing a dark, oxidized product (see Figure 4.8). This test assists in differentiating between groups of gram-negative bacteria. Pseudomonas aeruginosa, for example, is oxidase positive.

3. Urease: The enzyme urease hydrolyzes urea to ammonia and car- bon dioxide (NH2CONH2 + H2O→ 2NH3 + CO2). The ammonia produced can be detected with pH indicators that change color in response to the increased alkalinity (see Figure 4.8). The test helps to identify certain species of Enterobacteriaceae, Corynebacterium urealyticum, and Helicobacter pylori.

4. Coagulase test: Coagulase is an enzyme that causes a clot to form when bacteria are incubated with plasma (see Figure 4.8). The test is used to differentiate Staphylococcus aureus (coagulase positive) from coagulase-negative staphylococci.