Evaluation of Selected Medicinal Plants Extracted in Different Ethanol Concentrations for Antibacterial Activity against Human Pathogens

Abstract

Medicinal plant extracts prepared with selected ethanol concentrations from eight species, Peumus boldus (boldo leaf), Agathosma betulina (buchu leaf), Echinacea angustifolia (echinacea root), Humulus lupulus (hops strobile), Glycyrrhiza glabra (licorice root), Mahonia aquifolium (Oregon grape root), Usnea barbata (usnea lichen), and Anemopsis californica (yerba mansa root), were screened for antibacterial activity against four Gram-positive and four Gram-negative pathogens. The antibacterial activity of the extracts (50, 70, and 90% ethanol) was evaluated using a standard well assay and microbroth dilution method. Minimum bactericidal concentrations (MBCs) were also determined for each extract. Plant extracts showed strong antibacterial action against Gram-positive bacteria, Staphylococcus aureus, methicillin resistant Staphylococcus aureus (MRSA), Staphylococcus epidermidis and Streptococcus pyogenes, while negligible to no inhibitory activity against Gram-negative bacteria; Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Salmonella enteritidis was observed. Among the plant extracts, the boldo, hops, licorice and yerba mansa exhibited a strong antibacterial action at all three ethanol concentrations. Of these four, hops showed the strongest activity at 90% ethanol. Echinacea angustifolia extracts did not show any considerable antibacterial activity, while usnea showed strong activity only at 90% against S. epidermis. Except Echinacea angustifolia and usnea, the plant extracts were strongly inhibitory towards the MRSA strain. Buchu, yerba mansa and Oregon grape showed higher activity at 50% or 70% on MRSA. MBCs varied from 1/4 to >1/256 dilution levels and were in agreement with well assay results. The results suggest that the extracts of boldo, hops, licorice and yerba mansa could be considered as potentially effective antibacterial agents against Gram-positive bacteria including MRSA. For hops, buchu, Oregon grape and usnea, the activity is dependent on the concentration of ethanol used in the extraction procedure. The ratio of ethanol/water mixture used for extraction of medicinal plants is an important factor to obtain optimum antibacterial activity.