A β-lactam antibiotic structurally related to penicillin. Inhibits bacterial cell-wall synthesis by inactivating transpeptidases on the inner surface of the bacterial cell membrane. Ampicillin can be used as a selective agent in several types of isolation media. Ampicillin is routinely used to select for cells containing the pcDNA3.1 and pEAK10 resistance plasmids in cell line A904L at an effective concentration of 50 μg/ml. MW: 403.45.