Under stress conditions, polyamines decreased the permeability of the outer membrane of Escherichia coli. This effect is caused by at least three mechanisms providing for an increase in the resistance to antibiotics transported through porin channels (fluoroquinolones, beta-lactams): a positive modulation of the gene micF transcription (its product antisense RNA inhibits the synthesis of porin proteins on the translational level); a positive effect on the cell content of the multiple stress resistance factor sigma(S) (it is accompanied by a decrease in the porin transport because of suppression of ompF transcription and induction of cadaverine synthesis); a direct inhibition of the transport activity of porin channels. The production of cadaverine in E. coli cells significantly increased in response to various antibiotics, and this was likely to be a manifestation of oxidative stress.