The Effects of Indole on Intracellular Polyamine Content and Antibiotic Susceptibility of Escherichia coli

Abstract— Small regulatory molecules, such as indole and polyamines, are involved in the regulation of a variety of processes in bacteria, including protective response to antibiotics. It is known that small regulatory molecules can affect each other. However, there is still no data on the mutual effects of indole and polyamines. It is reported here that micromolar concentrations of indole in the culture medium cause a decrease in Escherichia coli susceptibility to fluoroquinolones, beta-lactams, and aminoglycosides. The decrease in the antibiotic susceptibility was more pronounced in the E. coli strain capable of synthesizing biogenic polyamines than in the putrescine- and spermidine-deficient strain. Exogenous indole increased the intracellular concentration of putrescine and spermidine by 2 and 2.5 times, respectively, and had no effect on cadaverine content. The presence of bacteria-synthesized putrescine, cadaverine, and spermidine polyamines in the medium had no effect on the amounts of indole produced by E. coli. The addition of spermine, which is synthesized mainly by eukaryotes, increased indole content in the medium (by not more than 20%). The obtained data indicate that indole reduces E. coli susceptibility to antibiotics with different mechanisms of action. One of the mechanisms by which indole affects the antibiotic susceptibility of bacteria is the stimulation of biogenic polyamines putrescine and spermidine accumulation in the cells.