This Petri dish culture plate contained an unknown growth medium to which a light inoculum of Staphylococcus sp. bacteria had been applied. Atop the growth medium, antibiotic-impregnated disks had been laid, which gave rise to this example of an antibiotic sensitivity test. The medium was kept at a pH of 7.2. The size of the inhibition zones surrounding the antibiotic-impregnated paper disks, depends on the sensitivity of the bacteria present in the agar to the antibiotic, the antibiotic's ability to diffuse through the culture medium, and the overall pH of the medium.