Animation showing the mechanism of action of the antibiotic drug penicillin. Penicillin is derived from the mould Penicillium sp., which is seen at lower right (yellow). The drug is active against Gram-positive bacteria, and the outer layers of such a bacterium are seen at centre. The bacterium has a plasma membrane made of a lipid bilayer (yellow) containing surface proteins, as do all cells. However it also has a thick protective layer of peptidoglycan (blue), which is a polymer constructed by cross-linking individual units (rectangles). Penicillin (red molecule) interferes with the construction of this wall, meaning the wall cannot form correctly when the bacterium divides, leading to the bacterium's death. The antibiotic nature of penicillin was discovered by Alexander Fleming (top left) in 1928.