Antibiotic test. Antibiotic drugs (discs) being tested on colonies of Staphylococcus aureus (yellow), which has been cultured on an agar growth medium in a petri dish. This is called an antibiogram. S. aureus is found on healthy human skin and mucous membranes, but causes illnesses ranging from minor skin infections and abscesses, to life-tretening diseases, such as pneumonia. Infection occurs when the bacteria enter the body via a hair follicle or a break in the skin.