SUMMARY: Isotherms of the absorption of polymyxin E by and after 20 min. contact with the antibiotic were determined. Tested on seven different bacteria, the maximum amounts of polymyxin E absorbed showed that the polymyxin-sensitive bacteria were capable of absorbing much greater quantities of the antibiotic than resistant organisms. When the bacteria were suspended in buffer solutions, the ultraviolet spectra of their supernatant fluids showeda maximum absorption at 260 mp.; purines and pyrimidines contribute to this maximum. Addition of polymyxin E to suspensions of sensitive bacteria caused an immediate increase in the height of this maximum. Similar treatment of resistant organisms caused very little immediate increase in the amount of 260mp. absorbing material released. On prolonged incubation of untreated and polymyxin E treated cells there was a steady increase in the amount of 260mμ. absorbing cell solutes released into the suspension media. Within the pH range 4.4-7.6 no optimum pH value was found for the polymyxin E absorption process or for the cellular release mechanism. The absorption isotherms of polymyxin E obtained with cell-wall preparations derived from sensitive and resistant bacteria showed that cell walk derived from sensitive organisms bound more polymyxin E than cell-wall preparations from resistant organisms. On treatment of sensitive bacteria with concentrations of polymyxin E less than those required for 99 yo killing in 20 min. at 25O°, there was a linear relationship between the amounts of 260 mp. absorbing material released and the percentage of cells killed. Complete saturation of sensitive organisms with the antibiotic was not necessary to kill the cells. The amounts of polymyxh E. required for 99.5 % killing corresponded closely to those suflicient for the formation of a closely packed monolayer within the bacterial cell wall. It is suggested that, i t these bactericidal concentrations, polymyxin E combines with and thereby disorganizes the structures responsible for the maintenance of osmotic equilibrium within the cell wall.


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