Enzyme inhibitors are substances that lower the rate of enzyme-catalyzed reactions. An irreversible inhibitor is a type of enzyme inhibitor that binds to the enzyme very tightly through strong covalent bonds and does not dissociate easily from it. Penicillin is the most common example of an irreversible inhibitor. It works by binding to and inhibiting the activity of transpeptidase, the enzyme responsible for forming bacterial cell walls. The absence of a cell wall allows water to get into the bacterial cell, causing it to swell, burst and die. Aspirin and nerve gas are other examples of irreversible inhibitors.