Susceptibility of Escherichia coli to kanamycin and seven other aminoglycosides has been found to be strongly potentiated by salicylate. At pH 7.5, in the presence of 15 mM salicylate and 0.5 micrograms of kanamycin per ml, the efficiency of plating of the bacteria was 2 x 10(-5), whereas there was no significant killing in the presence of kanamycin or salicylate alone. With 0.75 micrograms of kanamycin per ml, the addition of 2.5 mM salicylate was sufficient to reduce the efficiency of plating by more than 10(4)-fold. Synergistic effects were found also at pHs 6.5 and 8.5. To determine whether the action of salicylate resulted from its behavior as a weak acid or its salicyl structure, similar experiments were carried out with acetate and salicyl alcohol. Acetate, a membrane-permeating weak acid, showed a synergistic effect on kanamycin susceptibility at pH 6.5 that was comparable to the effect seen with salicylate at pH 6.5. However, acetate had no synergistic effect with kanamycin at pH 7.5 or 8.5. This is consistent with the ability of acetate to increase the membrane potential of cells and the dependence of susceptibility to kanamycin and other aminoglycosides on the membrane potential. Salicyl alcohol, which has a hydroxyl group in the place of the carboxyl group that is present in salicylate, was an effective synergist with kanamycin. It was equally effective at pHs 6.5 and 7.5 and somewhat more effective at pH 8.5. These results support the hypothesis that two effects are involved in the synergy between aminoglycosides and salicylate: a weak acid effect, possibly to increase the membrane potential, and an uncharacterized effect related to the salicyl structure.
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