Burdock (Arctium lappa) Leaf Extracts Increase the In Vitro Antimicrobial Efficacy of Common Antibiotics on Gram-positive and Gram-negative Bacteria

This work aimed to study the potential effects of four Arctii folium extracts, 5 mg gallic [GAE] acid equivalents per 1 mL sample, on six antibiotics (Ampicillin/AM, Tetracycline/TE, Ciprofloxacin/CIP, Sulfamethoxazole-Trimethoprim/ SXT, Chloramphenicol/C and Gentamicin/CN) tested on four Gram-positive (Staphylococcus aureus ATCC 6538, Staphylococcus aureus ATCC 25923, Enterococcus faecalis ATCC 29212, and Staphylococcus epidermidis ATCC 12228) and five Gram-negative (Proteus mirabilis ATCC 29245, Escherichia coli ATCC 35218, E.coli ATCC 11229, E.coli ATCC 8739, and Bacillus cereus ATCC 11778) bacteria. Arctii folium extracts were the whole ethanol extract/W and subsequent ethyl acetate/EA, aqueous/AQ, and chloroform/CHL fractions. Chemical qualitative analysis (HPTLC method) emphasized five main polyphenol compounds in Arctii folium polar extracts: chlorogenic acid (Rf approximate to 0.52/0.55) and its isomer, 1,5-di-O-caffeoylquinic acid (Rf approximate to 0.90/0.92), plus cynarin (Rf approximate to 0.77), hyperoside (Rf approximate to 0.68/0.64) and isoquercitrin (Rf approximate to 0.69/0.71). Microbiological screening indicated Arctii folium polar extracts (AQ and W) efficacy on S.epidermidis ATCC 12228; the MIC values were in the range of common antibiotics, being 32 and 128 mu g GAE per mL sample respectively. The unpredictable effects (stimulatory or inhibitory) of Arctii folium extracts in combination with typical antibiotics as well as a potential use of the whole ethanol extract/W for restoring the antimicrobial potency of susceptible antibiotics have also been evidenced.