Direct and indirect antimicrobial activity of the root extract of Onosma visianii Clem

Introduction/Objective. The escalating resistance of numerous pathogens to currently available therapeutic agents has sparked a renewed interest in the search of novel antimicrobial compounds. Plants have become a potentially valuable source of these compounds. This study aimed to assess the direct and indirect antimicrobial effects of Onosma visianii Clem root extract on reference and clinical bacterial strains using broth microdilution and a modified ethidium bromide/acridine orange (EB/AO) fluorescence assay. Methods. The ethanolic extract obtained from the dried root of Onosma visianii Clem was used to determine the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) by broth microdilution, as well as the half maximal inhibitory concentration ( \mathrm{IC_{50}} ) values for reference and clinical bacterial strains. The EB/AO fluorescence method was employed to assess the effect on efflux pumps in methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VRE), using 50% of the MIC value of the extract. Results. The MIC, MBC, and \mathrm{IC_{50}} values for Gram-positive bacteria were all below 15 μg/mL. The extract demonstrated strong antibacterial activity against VRE and, in particularly, against MRSA isolates (MIC = 7.81 μg/mL and MBC = 7.81 μg/mL). Furthermore, treatment with 50% of the MIC concentration produced a significant inhibitory effect on efflux pumps in Gram-positive bacteria, ranging from 18% to 26% compared with untreated cells. Conclusion. The root extract of Onosma visianii Clem exhibited antimicrobial activity against both Grampositive and Gram-negative bacteria, with particularly strong effects against VRE and MRSA. In addition, the observed inhibition of bacterial efflux pumps underscores its promise as a candidate for pharmacological evaluation of the antibiotic properties of the Onosma visianii Clem root extract.