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Nowadays plant extracts are viewed as promising agents in medicinal and pharmaceutical biotechnologies either to design novel antimicrobials or enhance efficiency of known antibiotics. Both pro-and antioxidant properties of certain herbs were shown to modulate bacterial susceptibility to antibiotics and are associated with their polyphenol content. The aim of the study was to investigate the effect of disturbance of iron homeostasis caused by chelating activity of the plant extracts and Fur deficiency on the susceptibility of mature Escherichia coli biofilms and planktonic cultures to ciprofloxacin and cefotaxime. Polyphenol-rich extracts of Arctostaphylos uva-ursi, Vaccinium vitis-idaea, green and black teas exerted strong bactericidal effects and stimulated biofilm production as well. Pretreatment with the extracts followed by antibiotic addition could lead to remarkable antagonistic interactions with ciprofloxacin and cefotaxime. Iron deregulation in the Δfur mutant provided synergism with ciprofloxacin in the biofilms. This elucidates prebiotic capacity of the extracts and prospect in development of preparations for stimulation and support of commensal microbiota biofilms during courses of antibiotic treatment. But in case of pathogenic strains antagonistic interactions with antibiotics induced by the tested extracts require additional research and careful adjustment of antibiotic therapy and herbal medicine combinations in order to exclude unwanted losses of antibiotic efficiency.