Biochem/physiol Actions A β-lactam antibiotic with an amino group attached to the penicillin structure. Inhibits bacterial cell-wall synthesis (peptidoglycan cross-linking) by inactivating transpeptidases on the inner surface of the bacterial cell membrane. Bactericidal only to growing Escherichia coli. Mode of resistance: Cleavage of β-lactam ring of ampicillin by β-lactamase. Antimicrobial spectrum: Gram-negative and Gram-positive bacteria. Inhibits cell wall synthesis in Escherichia coli1; In the purification of penicillin acylase2; Enhances luminol chemiluminescence.3