Ampicillin is one of the most widely used antibiotic in molecular cloning, for the selection of transformants. It belongs to the penicillin group of antibiotics i.e. beta-lactam antibiotics. The only difference between penicillin and ampicillin is the presence of amino group in the latter.
Unlike penicillin which is effective against only gram positive bacteria, ampicillin is also effective against gram negative bacteria, e.g. E. coli. The amino group present in the side chain of the ampicillin renders it to penetrate the outer membrane of gram negative bacteria and hence enter into it. Therefore, it was one of the first broad spectrum penicillins to be used.