Penicillin is an antibiotic that destroys Bacteria by destroying the cell wall of the microorganism. It does this by inactivating an enzyme necessary for the cross linking of bacterial cell walls. The enzyme is known as transpeptidase. It accepts the penicillin as a substrate, it then alkanolates a nucleophilic oxygen of the enzyme, rendering it inactive. Cell wall construction stops and the bacteria soon die. The antibiotic nature of the penicillin is due to the strained b-lactam ring, on opening the ring strain is relieved this makes penicillin more reactive than ordinary amides

 The structure of a bacteria's cell wall

 These images show how Escherichia coli is destroyed by penicillin, the bacteria lengthens but cannot reproduce and eventually die (final frame).