Illustration showing the antibiotic penicillin (red molecule) inhibiting bacterial cell wall (blue layer) growth. Penicillin was discovered by the Scottish microbiologist Alexander Fleming (1881-1955, top left) from the fungus Penicillium (yellow, right). The bacterial cell wall is made up of peptidoglycan, which forms a rigid mesh-like layer outside the plasma membrane (orange layer). To build the cell wall, peptidoglycan monomers (building blocks) are cross-linked together with the help of an enzyme called transpeptidase. This process is repeated to allow the cell to grow and replicate. Transpeptidase is also referred to as a penicillin-binding protein (PBP). When penicillin or other antibiotics in the beta-lactam family bind to transpeptidase, it inhibits its crosslinking activity, preventing new cell wall formation.