Plastic "mock-ups" of a microtitre plate complete with pseudo colonies of bacteria, and an agar plate, showing penicillin "inhibition zones", c. 1976

How well antibiotics perform against particular bacteria can be gauged using a disc test. The brown layer in the Petri dish model represents the jelly-like agar containing a bacterium. Samples of antibiotics are placed in the agar and labelled. The space around the antibiotics labelled CX5 and MT10 indicates that the organism cannot grow. These antibiotics could be used to treat infections caused by the organism. The antibiotic labelled PG4, penicillin, allows the bacteria to grow, so will be useless against that particular strain of the organism. Many bacteria are now becoming resistant to antibiotics, which is why tests like this are important.