- Effects of amino acid alterations in penicillin-binding proteins (PBPs) 1a, 2b, and 2x on PBP affinities of penicillin, ampicillin, amoxicillin, cefditoren, cefuroxime, cefprozil, and cefaclor in 18 clinical isolates of penicillin-susceptible, -intermediate, and -resistant pneumococci. Nagai, K., Davies, T.A., Jacobs, M.R., Appelbaum, P.C. Antimicrob. Agents Chemother. (2002)