Evaluation of the antibacterial activities of selected medicinal plants and determination of their phenolic constituents

The purpose of this investigation was to determine the antibacterial activities of five medicinal plants, against both Gram-positive and Gram-negative bacteria. The levels of phenolic constituents in these medicinal plants were also quantified and compared. Minimum inhibitory concentrations (MIC) were determined colorimetrically using 96-well sterile microtitre plates and treatment with p-iodonitrotetrazolium chloride (violet). Concentrations of selected phenolic constituents were determined using HPLC methods, by comparison to standard plots prepared using catechin, caffeic acid, gallic acid, and quercetin standards. MIC tests indicated that Callicarpa formosana and Melastoma candidum possessed the strongest bacterial inhibitory activities, with MIC values ranging between 12.5 and 37.5 mg/ml and 0.80-8.3 mg/ml, respectively. M. candidum also demonstrated inhibitory activities against Gram-negative bacteria, with MIC value of 8.3 mg/ml. Quercetin was detected in all medicinal plants tested, with concentrations ranging between 0.25 and 0.47 mu g per mg of dried sample. Caffeic acid, catechin and gallic acid were detected in only some of the medicinal plants. Our results suggested that C. formosana and M. candidum could potentially be used for the isolation of potent antibacterial compounds.