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L-640,876, 7-beta(1-benzylpyridinium-4-yl)amino-3-[( (1-methyl-1 H-tetrazol-5-yl)thio]methyl)-ceph-3-em-4-carboxylate, is a potent representative of a new family of beta-lactam antibiotics which are similar in some respects to mecillinam. When L-640,876 and mecillinam were compared for effects on growth and morphology of Escherichia coli, it was observed that both drugs caused the formation of lemon-shaped cells during the first 30 minutes of exposure and during this period the culture turbidity increased without an appreciable change in culture viability. Unlike mecillinam, after 60 minutes of exposure to L-640,876 the majority of the lemon-shaped cells transformed into spindle-shaped cells and in the continuing presence of the drug formed osmotically fragile spheroplasts. Membrane binding studies indicated that, like mecillinam, L-640,876 was bound to the PBP-2 of E. coli and Proteus morganii; however, some binding of L-640,876 to the PBP-3 of E. coli was detected. In Staphylococcus aureus binding differences were more evident as L-640,876 was more rapidly bound to PBP-1 and 2 whereas mecillinam was rapidly bound to PBP-3. The reversal of inhibition of certain strains of Gram-negative bacteria by high ionic strength media could not be directly attributed to a reversal of antibiotic binding to the PBPs. Permeability studies indicated that the superior potency of L-640,876 in E. coli was partly due to its higher concentration in the periplasm which was unaffected by the simultaneous addition of drug and NaCl, however, in cells cultured in high ionic strength medium there was a marked reduction in penetration rate of all beta-lactams tested.

Quaternary heterocyclylamino .BETA.-lactams. III. The mode of action of L-640,876 and the effect of NaCl on membrane permeability and binding.