What does penicillin do in the body?

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 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

Penicillin operates by dissolving the cell wall of bacteria, dispersing its cytoplasm and other cell systems. One essential component of the bacterial cell wall is transpeptidase, which accepts molecules of penicillin as a substrate attachment. The penicillin activates, preventing peptidoglycan reactions that strengthen links in the cell wall. This leads inevitably to cytolysis and cell death.