e. coli highly resistant to ampicillin and carbenicillin but not to cephalothin ( class i ) were found to have one of two levels of r factor-mediated , periplasmic-beta-lactamase which resembled r ( tem ) and was located behind a permeability barrier to penicillins but not to cephalosporins .
C0006976
carbenicillin
C0030842
penicillins