Kinetic studies of ampicillin action were made on exponentially growing
Escherichia coli and on
E. coil-spheroplasts using a range of inhibitory and subinhibitory concentrations of ampicillin. For each concentration, the value (
ko-ka) representing the difference in generation rates of ampicillin-free culture (
ko) and the generation rate of the culture with ampicillin (
ka) was calculated and plotted against ampicillin concentration. A straight line relation was obtained with
E. coli cells, its intersection with the abscissa, where (k
o-k
a)=0, give the concentration of ampicillin which exerts no inhibitory action on the cells (0.25 μg/ml). When
ka was plotted against ampicillin concentration, the relation was also linear. Its intersection with the abscissa gives the minimum lethal concentration of ampicillin on the bacterial cells (1.05 μg/ml). With
E. coli-spheroplasts such plots were non-linear which means that a different order of reaction was involved. This difference is probably due to a different mechanism
of ampicillin action as revealed by the electroscanning microscopy.
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