POTENT ACTIVITY OF MEROPENEM AGAINST ESCHERICHIA-COLI ARISING FROM ITS SIMULTANEOUS BINDING TO PENICILLIN-BINDING PROTEIN-2 AND PROTEIN-3

A mutant strain of Escherichia coli with reduced susceptibility to imi penem, designated TL2740, was selected following serial passage of the parent strain, E. coli C600, in broth containing increasing concentra tions of the carbapenem; the MIC of imipenem for TL2740 was eight-fold greater than that of the parent strain. The mutant also exhibited red uced susceptibilities to panipenem and biapenem and high-level resista nce to mecillinam, but was as susceptible to meropenem, ceftazidime, p iperacillin and the other beta-lactams tested as strain C600. The affi nity of penicillin-binding protein (PBP) 2 of TL2740 for imipenem and meropenem was ten-fold less than that of C600, thereby providing an ex planation for the mutant's reduced susceptibility to some carbapenems and mecillinam. However, this theory was confounded by the observation that the in-vitro activities of meropenem against both parent and mut ant strains were virtually the same and by the fact that PBP 2 is the principal target of the antibiotic. Imipenem and aztreonam, which bind to PBP 2 and PBP 3 respectively, demonstrated synergic activity when tested in combination against both C600 and TL2740. These results sugg est that the potent activity of meropenem against the mutant strain mi ght also be due to a synergic effect resulting from simultaneous bindi ng to both PBP 2 and PBP 3 and that the variable activities of the car bapenems against TL2740 were related to their different PBP binding pr ofiles. Compared with C600, TL2740 appeared shorter on electron micros copy and had a longer generation time, discrepancies which are compati ble with defective PBP 2 activities in the mutant strain. We also iden tified three clinical isolates of E. coli with beta-lactam susceptibil ity profiles which resembled that of TL2740 i.e. high-level resistance to mecillinam and low-level resistance to carbapenems, with the excep tion of meropenem to which these strains were susceptible; in common w ith TL2740, the combination of imipenem and aztreonam was synergic aga inst these isolates. The genetic basis of resistance in all of the mec illinam-resistant strains, including TL2740, mapped close to lip at 15 ' on the E. coli chromosome with transductional analysis. The results strongly suggest that the reduced susceptibilities of the clinical iso lates to carbapenems were due to mutations in the genes encoding the P BP 2s of these strains which affected their affinities for beta-lactam antibiotics.