Biosynthesis of the Peptidoglycan of Bacterial Cell Walls: XII. Inhibition of Cross-Linking by Penicillins and Cephalosporins: Studies in Staphylococcus Aureus in vivo

Abstract

Abstract An uncross-linked monomer (nascent peptidoglycan unit) accumulates in cells of Staphylococcus aureus treated with low concentrations of penicillin G or with ampicillin, methicillin, or cephalothin. The uncross-linked monomer has been isolated, and analyses indicate that it represents a prefabricated subunit of the wall bearing both of the d-alanine residues of its pentapeptide precursor as well as an open pentaglycine chain. Pulse labeling experiments indicate that this uncross-linked unit is a direct precursor of the cross-linked peptidoglycan and that its cross-linking is inhibited by penicillin G. At high concentrations of penicillin G, wall synthesis ceases abruptly and no accumulation of the nascent peptidoglycan units is observed. These data have been obtained in support of the hypothesis that penicillins are substrate analogues of the d-alanyl-d-alanine end of the nascent peptidoglycan units and that they acylate the transpeptidase which catalyzes the cross-linking reaction. The data may also provide an explanation of the paradoxical observation that the killing rate in S. aureus by penicillin G is higher at low concentrations than at high concentrations of the antibiotic. In the presence of low concentrations a weakened wall may be formed, thus rendering the organisms more susceptible to lysis than at high concentrations of penicillin where wall synthesis abruptly ceases.