Review Note

Penicillin does not harm human cells because it blocks new bacterial cell wall synthesis and we do not make cell walls, thank you very much! Three steps are needed for penicillin to work effectively:
1. Bacteria must be {{c1::growing}}. In order for the bacteria to expand and grow, the confining, rigid wall must be 'cut' in places by hydrolases (autolysin enzymes made by the bacteria when they are growing). As the wall is cut and stretched it must then be repaired to maintain wall integrity.
2. Penicillins & cephalosporins block the '{{c1::spot-welding.}}' (I.e., {{c1::transpeptidation}}, of new wall girders between the cut areas). Penicillin does this by covalently binding to the transpeptidase enzyme, permanently inactivating the enzyme. If the bacterial hydrolases keep cutting up wall, then the wall eventually falls apart.
3. In the presence of {{c1::an osmotic imbalance}} (the normal situation, with bacteria in tissue fluids or water), the now fragile cell swells from osmotic pressure and bursts.

Penicillin is an example of a {{c2::bactericidal}} antibiotic, which can kill bacteria directly under normal hypotonic conditions. {{c2::Bacteriostatic}} antibiotics inhibit the proliferation of bacteria, but do not generally kill them directly, and require immunity to eliminate the bacteria.