Screening of Plants for Antibacterial Properties: Growth Inhibition of Staphylococcus aureus by Artemisia Tridentata

Drug-resistant pathogenic and opportunistic bacteria are increasing in occurrence and prevalence, and pose a dangerous threat to human health. In the search for novel antibiotics with which to combat this threat, plants, specifically those used in traditional medicine with ascribed antibacterial properties, offer a promising and potentially vast source of such therapeutic compounds. The purpose of this study was therefore to screen chemical extracts created from various plant species for antibacterial properties versus pathogenic bacterial species. In the course of these antibacterial assays, we successfully identified a methanol extract derived from Artemisia tridentata tridentata plant material as capable of inhibiting the growth of the opportunistic pathogen Staphylococcus aureus. Three sub-fractions were created using hexane, ethyl acetate and water solvents. Each of these extracts displayed significant antibacterial activity versus a wild-type strain over a period of six hours, at concentrations as low as 62.5 µg/ml. The extracts also demonstrated an enhancement of antibiotic effects when combined with ampicillin, G418 sulfate or amikacin, for a period of up to twelve hours. Though the efficacy of the extracts was lessened when tested against an ampicillin-resistant strain, significant enhancement of the efficacy of this antibiotic was still observed. Gas chromatography-mass spectrometry analysis of these three extracts revealed the sesquiterpene lactone achillin as present in each. Column chromatography of the hexane extract resulted in a fraction retaining its antibacterial activity, and still containing this compound, further implicating it as responsible for the antibacterial activity of this plant. The results of serial dilution and plating of extract-treated samples, along with those of ethidium bromide assays and transmission electron microscopy analysis, indicated a bacteriostatic mechanism of action involving disruption of the bacterial membrane, which is in agreement with the literature on the antibacterial properties of this plant, and those of sesquiterpene lactones, respectively. We therefore conclude that achillin, likely produced as a secondary metabolite by Artemisia tridentata tridentata, possesses growth inhibitory properties versus Staphylococcus aureus, and should be isolated and studied further for the purposes of evaluating its potential use, either as a stand-alone antibiotic, or as an adjunctive therapeutic, in the treatment of drug-resistant bacterial pathogens