Influence of the harvesting seasons on the polyphenol composition and antimicrobial activity of stinging nettle (Urtica dioica L.) extracts

Abstract The aim of our research was to study the water and ethanol extractable polyphenols of stinging nettle ( Urtica dioica L.) harvested in different seasons and to determine their antimicrobial activity against certain human pathogenic and food spoiling bacteria and yeasts. Our results indicate that the spring leaf extracts had higher polyphenol contents than the root one; however, close to the end of the vegetation period these values decreased considerably in both leaves and roots. Detection and quantification of the most abundant phenolic compounds in the spring extracts by HPLC revealed the occurrence of 12 different phenol carboxylic acids and flavonoids. Flavonoid compounds were more abundant than phenol carboxylic acids in the leaves; however, their proportion was equal in the case of the roots. Nettle leaf extracts had remarkable antimicrobial activity, the spring extracts were more efficient than the autumn ones. Escherichia coli and Staphylococcus aureus were sensitive to every leaf extract, while Listeria monocytogenes and Pseudomonas aeruginosa had reduced but remarkable sensitivity patterns. Among the yeasts, Candida glabrata was strongly inhibited by the aqueous leaf extracts. Most of the strains were insensitive to the root extracts, although Enterococcus faecalis was inhibited by the root and not the leaf extracts.