Ampicillin resistance

All bacteria are encapsulated by cell walls. This sphere of cross-linked peptidoglycan polymers defines the shape of the bacterium and keeps the plasma membrane in place. Without a cell wall, the high osmotic pressure inside the cell would cause it to swell and burst. An enzyme called transpeptidase is constantly rebuilding the cell wall (image A). The antibiotic ampicillin fits exactly into the active site of the transpeptidase enzyme and irreversibly inhibits its function. The inactivated enzyme cannot rebuild the cell wall; consequently the cell wall will fall apart and cause the bacterium cell to burst (image B).

Spontaneous mutation in the transpeptidase enzyme may alter the three dimensional shape of its active site. This mutation might render itself less efficient in building the cell wall; however, due to the altered shape of the active site, the transpeptidase enzyme is resistant to the ampicillin (image C). In a normal environment, this mutation would be disadvantageous. Yet, in the presence of ampicillin, only cells with the mutated transpeptidase will be able to survive.