Synergistic antibacterial activity of Curcuma longa L. and Urtica dioica L. extracts and preservatives

Abstract

Curcuma longa L. (turmeric) and Urtica dioica L. (common nettle) are well-known medicinal plants. In recent decades, due to its excellent biological activity and low toxicity, significant efforts have been made to research the potential application of these plants. Accordingly, the aim of this study was to investigate the combined effects of preservatives (potassium nitrite and sodium nitrite) and ethanol, acetone, and ethyl acetate extracts of C. longa and U. dioica emphasizing their join, synergistic effects. The eight isolates of foodborne pathogenic bacteria and two reference strains were used. The interactions were tested by the checkerboard method and expressed as fractional inhibitory concentration (FIC) index. FIC index was ranged in intervals from 0.25 to 2.0. The extracts of C. longa showed better synergistic effect with preservatives than U. dioica extracts. Synergism was observed in relation to Staphylococcus aureus ATCC 25923, S. aureus, Bacillus subtilis, and Salmonella typhimurium. In the presence of subinhibitory concentrations (1/4 MIC, 1/8 MIC) of the extracts, the MICs of preservatives have decreased up to 8-fold. In addition, the contents of phenolic compounds (total phenols, flavonoids, tannins, and proanthocyanidins) were determined spectrophotometrically. The higher concentration of phenolic compounds was detected in C. longa extracts than in U. dioica extracts.