Abstract
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 pgaA 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 recA mutant indicate involvement of SOS-response into reaction towards exposure with TA. Gallic acid-induced killing was more pronounced in the biofilms of csgA mutant revealing role of curli in protection against GA toxicity.
Highlights
Investigation of bacterial biofilms is a relevant topic both in fundamental and applied sciences
Earlier, using E. coli BW25113 we have shown that 1 mg/ml TA and 4 mg/ml GA acted as minimum inhibitory concentrations (MICs) on M9 media with glucose, no minimal bactericidal concentration (MBC) or minimal biofilm prevention concentration (MBPC) for TA was found, but for GA these were equal to 4 mg/ml [16]
We studied the effects of sub-MICs of TA and GA on colony-forming ability of biofilms and their counterpart planktonic cultures, mass and specific biofilm formation in wild-type E. coli BW25113 strain and mutants lacking genes involved in surface attachment, exopolysaccharide production, general stress response and SOS-response as well as estimated effects of TA and GA on changes in membrane potential (∆Ψ) and extracellular potassium (K+), expression of sulA::lacZ and katG::lacZ genes
Summary
Investigation of bacterial biofilms is a relevant topic both in fundamental and applied sciences. Production of biofilm matrix main exopolysaccharides poly-beta-1,6-N-acetyl-Dglucosamine (PGA) and colanic acid is controlled by pgaABCD and cps operons [8,9] Regulation of these processes requires global stress response regulator RpoS which increases levels of signal molecule known as bis-(3’-5’)-cyclic diguanylic acid (c-di-GMP), stimulating loss of flagella, expression of curli, cellulose and PGA synthesis [10]. We studied the effects of sub-MICs of TA and GA on colony-forming ability of biofilms and their counterpart planktonic cultures, mass and specific biofilm formation in wild-type E. coli BW25113 strain and mutants lacking genes involved in surface attachment (csgA, ydeH), exopolysaccharide production (pgaA, wcaM), general stress response and SOS-response (rpoS, recA) as well as estimated effects of TA and GA on changes in membrane potential (∆Ψ) and extracellular potassium (K+), expression of sulA::lacZ and katG::lacZ genes
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