Keywords: Efflux pump inhibitor (EPI) evaluation assay; MIC assay; methicillin-resistant Staphylococcus aureus (MRSA); Psidium guajava.

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is reputedly known as one of the most difficult bacteria to treat in patients and to eradicate in a hospital environment. Limited choice and capability of available drugs necessitates the quest for alternative cure. In this study, four compounds isolated from Psidium guajava leaves were evaluated for their potential inhibitory and resistancemodifying abilities against MRSA clinical isolates. Ursolic acid and 2α-hydroxyursolic acid showed inhibitory activities with minimum inhibitory concentration (MIC) values below 1 mg/mL while quercetin exhibited potential efflux inhibitory properties as compared to reserpine, a common plantbased efflux pump inhibitor (EPI). The presence of a hydroxyl group in 2α-hydroxyursolic acid may be responsible for the higher MIC value since ursolic acid did not have the additional functional group. In addition, the presence of an arabinose sugar group in guaijaeverin may lessen its efflux potentials as compared to quercetin, which is more hydrophobic. Both ursolic acid and 2α-hydroxyursolic acid showed good and moderate inhibitory activities, respectively. Additionally, this is the first communication to report quercetin’s potential as an EPI against a MRSA isolate. Furthermore, this study shows an example and usability of the simplified EPI assay to evaluate plant compounds against clinical MRSA isolates using compounds that exhibited weak antimicrobial activities.