Medicinal plant extracts can variously modify biofilm formation in Escherichia coli.

Low concentrations of black tea and water extracts from medicinal plants Arctostaphylos uva-ursi, Vaccinium vitis-idaea, Tilia cordata, Betula pendula and Zea mays stimulated biofilm formation in Escherichia coli BW25113 up to three times. Similar effect was observed for tannic acid and low concentrations of quercetin. In contrast, the extract from Urtica dioica reduced biofilm production. Pretreatment with plant extracts variously modified antibiotic effects on specific biofilm formation (SBF). Extract from V. vitis-idaea increased SBF, while the extracts from Achillea millefolium, Laminaria japonica and U. dioica considerably decreased SBF in the presence of ciprofloxacin, streptomycin and cefotaxime. Stimulatory effect of the extracts and pure polyphenols on biofilm formation was probably related to their prooxidant properties. The rpoS deletion did not affect SBF significantly, but stimulation of biofilm formation by the compounds tested was accompanied by inhibition of rpoS expression, suggesting that a RpoS-independent signal transduction pathway was apparently used.