Ampicillin is a penicillin derivative that prevents bacterial cell wall synthesis. Ampicillin is effective against both Gram (+) and (-) bacteria. Resistance to ampicillin is conferred by the bla gene which encodes ß-lactamase that cleaves the ß-lactam bond of the antibiotic. Ampicillin is commonly used to select for ampicillin resistant plasmid bearing strains of bacteria.