Characterization of antioxidant and antimicrobial activities of nettle leaves (Urtica dioica L.)

Abstract

Samples of stinging nettle or common nettle (Urtica dioica L.) were collected from the area of Banja Luka. To measure and evaluate the content of chlorophyll (a and b), carotenoids, and soluble proteins, as well as peroxidase activity (POD, EC 1.11.1.7.), fresh nettle leaves of different developmental stages were used. Dried nettle leaves were used to obtain ethanol extract. The dry residue of ethanol extract was dissolved in methanol and the obtained solution was used to determine the content of total phenols, flavonoids, flavonols, as well as non-enzymatic antioxidant activity and antimicrobial activity. The non-enzymatic antioxidant activity was determined by different methods: FRAP, DPPH, and ABTS. The results were compared to those of standard substances like vitamin C, BHT, and BHA. Antimicrobial activity was screened by using macrodilution method. The obtained results showed insignificantly higher content of chlorophyll, carotenoids, and proteins in young nettle leaves as well as an increase in the soluble peroxidase activities. Native electrophoresis of the soluble fraction showed the presence of two peroxidase isophorms in the soluble protein fraction of nettle leaves. The total phenolic content in nettle extracts amounted to 208.37 mg GAE/gdw, the content of total flavonoids was 20.29 mg QE/gdw, and the content of total flavonols was 22.83 mg QE/gdw. The antioxidant activity determined by FRAP method was 7.50 mM Fe(II)/gdw, whereas the antioxidant activity measured by using DPPH and ABTS methods, with IC50 values, were 31.38 and 23.55 μg mL-1, respectively. These results showed the weak and moderate antioxidant capacity of stinging nettle. Extract of Urtica dioica L. was tested for antibacterial acivity against various Grampositive and Gram-negative bacteria: Bacillus subtilis IP 5832, Lactobacillus plantarum 299v (Lp299v), Pseudomonas aeruginosa, and Escherichia coli isolated from food and Escherichia coli isolated from urine samples. Ampicillin, erythromycin, ciprofloxacin, and gentamicin were used as positive control. The results showed that minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the extract ranged from 9.05 to more than 149.93 mg mL-1.