SUMMARY. The morphological responses of to N-formimidoyl thienamycin, azthreonam, ceftazidime and cefoperazone were studied by transmission electron microscopy. They were correlated with the results of viable counts and continuous turbidimetric monitoring. N-formimidoyl thienamycin initially caused the formation of abnormally shaped cells which developed into spheroplasts. The other antibiotics caused filamentation of the bacteria, which subsequently underwent lysis. The degree and mechanism of lysis varied between the antibiotics. Exposure to azthreonam also resulted in the deposition of electron-dense intracellular material. As judged by conventional minimum-inhibitory-concentration tests, all the agents exhibited similar activity against the strains of tested, with ceftazidime displaying the highest activity. However, N-formimidoyl thienamycin caused the most extensive morphological damage and resulted in the most rapid fall in viable count.


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