Penicillin interferes with the synthesis of peptidoglycan in bacterial cell walls. Remember that peptidoglycan is a complex molecule made of sugars and polypeptides that forms a tough, strong lattice that surrounds bacterial cells. Peptidoglycan is a major component of most bacterial cell walls. The rigid peptidoglycan layer helps bacteria stay intact in the face of osmotic pressure, which is when water tends to flow into or out of a cell to balance out the concentration of solutes on either side of a membrane. Penicillin prevents peptidoglycan from cross-linking properly in the last stages of bacterial cell wall synthesis. This greatly weakens the cell wall and causes the bacterium to lyse, or burst open, because of osmotic pressure. Penicillin is bactericidal because it directly kills bacteria.