| Summary: | Tannic (TA) and gallic (GA) acids are known to have both anti- and prooxidant properties however recently they have been described as potential anti-biofilm agents although their mechanisms of action on bacterial cells remain obscure. The aim of our research was to elucidate the role of prooxidant actions of these plant phenolic compounds in bactericidal effects and biofilm formation. In our experiments, both compounds demonstrated strong oxidative properties that altered activity of stress regulons and contributed to decrease of CFU and ability of cells to maintain membrane potential. Stimulation of biofilm formation was observed in all the strains with the exception of the strains deficient in flagella synthesis. Both compounds demonstrated bactericidal effect which was weakened in biofilms. TA efficiently killed bacteria in the bioflms of <em>pgaA</em> mutant which pointed out an important role of poly-beta-1,6-N-acetyl-D-glucosamine (PGA) polysaccharide in matrix formation. Similar effects of TA in <em>recA</em> mutant indicate involvement of SOS-response into reaction towards exposure with TA. Gallic acid-induced killing was more pronounced in the biofilms of <em>csgA</em> mutant revealing role of curli in protection against GA toxicity.
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