Nine species of selected Thai medicinal plants bearing quinonoid compounds were investigated in
vitro for their antimicrobial activities. The plant materials were sequentially extracted by maceration with
petroleum ether and ethanol respectively. Thirteen tested pathogenic microorganisms included 5 gram
positive bacteria, 6 gram negative bacteria and 2 fungi. The assays were performed by using agar well diffusion
method for determination of inhibition zone and broth microdilution method for determination of minimum
inhibitory concentrations (MIC) and minimum bactericidal concentration (MBC) or minimum fungicidal
concentration (MFC) with two fold dilution. The results showed that most of the extracts and the quinone
derivative compounds demonstrated a promising inhibitory effect against gram positive bacteria followed by
fungi and gram negative bacteria. The ethanol extract of Xyris indica flowering heads and the petroleum ether
extract of Eleutherine americana bulbs expressed broadest spectrum of antimicrobial activity. The petroleum
ether extract of Rhinacanthus nasutus roots demonstrated lowest MIC and MBC on tested gram positive
bacteria. The petroleum ether extract of Morinda citrifolia roots had promising antimicrobial potential against
Candida albicans. This study revealed the antimicrobial potentials among selected Thai medicinal plants
bearing quinonoid compounds which used as crude drugs in traditional Thai medicine.
Keywords: Quinonoid bearing plants, Antibacterial activity, Antifungal activity