The three dimensional structure of ampicillin.

Ampicillin is a beta-lactam antibiotic that has been used extensively to treat bacterial infections since 1961. It is often used as a selective agent in molecular biology to select for and to confirm the uptake of genes (e.g., of plasmids) by bacteria (e.g., E. coli). A gene that is to be inserted into a bacterium is coupled to a gene coding for an ampicillin resistance (in E. coli, usually the bla (TEM-1) gene, coding for beta-lactamase). The treated bacteria are then grown in a medium containing ampicillin (typically 50–100 mg/L). Only the bacteria that successfully take up the desired genes become ampicillin resistant, and therefore contain the other desired gene as well. As a powder, ampicillin is white with slight yellow cast and is soluble in water (150 mg/ml).