How is oxidase activity determined?
A nutrient medium is streaked with bacteria. After colonies have arisen, individual colonies are removed using a sterile, non-metallic instrument (pre-sterilized plastic loop or sterile wooden splint). The cells are rubbed into a moistened strip impregnated with oxidase reagent. This chemical takes the place of oxygen as a recipient for the electrons from the oxidase cytochrome. The additional electrons turn the oxidase reagent from colorless to purple. If oxidase is not present, no color change is observed.