Inhibition of Peptidoglycan Cross-Linking in Growing Cells of Escherichia coli by Penicillins and Cephalosporins, and Its Prevention by R Factor-Mediated Beta-Lactamase

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The degree of peptidoglycan cross-linking has been studied in growing cells of a Dap− Lys− auxotroph of Escherichia coli K-12 by following the incorporation of [3H]diaminopimelic acid into the lysozyme digestion products of crude, isolated peptidoglycan. The percentage of inhibition of cross-linking increases with increasing concentrations of penicillin G, cephaloridine, and cefuroxime. When the R factor R1drd 19 was introduced into the strain by conjugation, it was found that the type IIIa, β-lactamase specified by the plasmid was able to protect the cross-linking target against inhibition by penicillin G but not against cephaloridine, even though the β-lactamase hydrolyzes this substrate 50% faster than penicillin G. Cefuroxime, which is completely resistant to hydrolysis by the type IIIa β-lactamase, inhibited the peptidoglycan cross-linking target in both the R+ and R− variants of the assay strain. A mutant plasmid, R1drd19amp2, which specified no type IIIa β-lactamase synthesis, could not provide protection of the cross-linking target against penicillin G. The significance of these results, in relation to the ability of the antibiotics to pass the permeability barrier of the bacterial envelope, is discussed.

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