[A study on the bactericidal action of aspoxicillin against Escherichia coli]

In an attempt to clarify the role of a side chain, N4-methyl-D-asparagine, of aspoxicillin (ASPC) in the antibacterial action, we examined the bactericidal activity of dehydroxyaspoxicillin (AB-ASPC) and its affinity for the penicillin-binding proteins (PBPs) of Escherichia coli using piperacillin (PIPC), mezlocillin (MZPC) and apalcillin (APPC) as the reference penicillins. ASPC and AB-ASPC showed high bactericidal activities against E. coli K-12 even when a large inoculum size (2 x 10(8) CFU/ml) was used. The observation of these cultures with a phase contrast microscope revealed that E. coli cells lysed after the formation of spheroplast-like or bulged structures. On the other hand, PIPC, MZPC and APPC converted the cells to long filaments, but did not show lytic action in the range of the concentrations used. These morphological changes were also observed with a scanning electron microscope. Superior bacteriolytic activities of ASPC and AB-ASPC were further shown by measuring 'triggering' autolytic activity by the penicillins. The release of labeled murein from E. coli chi 1776 after the exposure to ASPC or AB-ASPC was clearly greater than those caused by the reference penicillins. ASPC showed affinity for PBPs of E. coli K-12, 1A, 1Bs, 2 and 3, and its affinity pattern resembled the one obtained with ampicillin (ABPC). AB-ASPC behaved in a fashion similar to ASPC, although its affinities for PBP 1A and 1Bs were lower and that for PBP 3 was slightly higher. These observations suggest that the highest bactericidal activity of ASPC against E. coli with lysis among the acyl-ureidopenicillins tested is due to N4-methyl-D-asparagine in the side chain of ASPC.