Polyamines increase Escherichia coli tolerance to netilmicin

Polyamine effect on E. coli tolerance to aminoglycoside netilmicin was studied. In batch cultures of E. coli polyamine-proficient strains the number of persister cells tolerant to netilmicin was increased with increasing the cell density upon transition from exponential growth to stationary phase. In the control un-supplemented culture of polyamine-deficient E. coli strain the number of persister cells fluctuated over the same range throughout the cultivation. However, polyamine supplementation resulted in a concentration-dependent increase in the number of tolerant cells as the culture entered the stationary phase. Efficiency of polyamine action decreased in the following order: spermidine - putrescine - cadaverine. The threshold concentration of netilmicin leading to killing of all cells was determined as 1.0 ng/ml. The cultures treated with lower concentrations of the antibiotic displayed biphasic time-killing curves when sensitive cells were killed and subpopulations of tolerant cells survived after 5 hour exposure to netilmicin. When cells were treated by various concentrations of netilmicin, putrescine supplementation produced either full elimination of negative drug effect or 10-50-fold decrease in the number of viable cells depending on aminoglycoside concentration. Putrescine effect we have shown to be maximal when it was added to cells before the drug challenge.