In Kirby-Bauer
testing, bacteria are placed on a plate of solid growth medium and
antibiotics (white disks, shown) are added to the plate.
After allowing the
bacteria to grow overnight, areas of clear media surrounding the
disks indicate that the antibiotic inhibits bacterial
growth.
A blank disc is
act as the control. The purpose of the control is two-fold. First,
it helps to show what normal growth of bacteria looks like. Second,
it helps to show that the antibiotics, and not the paper, are
affecting the bacteria.