A couple of key points when doing this test:
We keep the oxidase reagent either frozen or unopened in dropper bottles until
needed. If old reagent is sitting out on the bench and is PURPLE, ask for a new
tube from the instructor
Use a young culture, preferably less than 24 hrs old.
Use a culture growing on an agar plate or agar slant.
Use FRESH reagent, less than a couple of hours old.
Pick your inoculum, not with a metal loop (reagent may react with the metal), but
with a wooden stick.
Read the reaction within 20 seconds (NOT after), usually it will change in less
than 15 seconds. The oxygen will change the reagent color as time passes, so it
must be read quickly.