Carbenicillin disodium is a member of the carboxypenicillin β-lactam antibiotic family. Inhibits bacterial cell-wall synthesis by inactivating transpeptidases on the inner surface of the bacterial cell membrane. Unlike most β-lactams, carbenicillin disodium is limited to primarily gram negative bacteria including Pseudomonas aeruginosa and common enteric species. Carbenicillin is routinely used to select for mammalian cells containing the pET28 resistance plasmid in the Rosetta 2 cell line at an effective concentration of 50 µg/ml.