Catalase

Catalase is an enzyme found in most organisms that reacts with oxidizing materials to render them inert and harmless to the organism. The typical reaction that occurs is between this protein and hydrogen peroxide. This can be seen when applying peroxide to an open wound. The same foaming effect is visible with bacteria and is indicative of a positive result. Almost any method to get the bacteria in contact with hydrogen peroxide will work for the test, though it's best to use small quantities of both to reduce potential messes/contamination.