he appearance of a dark purple color within 20 seconds indicates that the organism is oxidase-positive. Oxidase-negative organisms produce either no color to a yellowish color (the original color of the colony) or change to a very light gray color within the 20-second test perio
one drop of H2O2 solution to the culture sample and watch for the production of bubbles in the suspension. These bubbles are the oxygen gas evolved as a result of catalase activity, as the enzyme catalyzes the breakdown of hydrogen peroxide. If no bubbles are produced, the test is negative for the presence of catalase.