RESEARCH ARTICLE Antimicrobial Action of Oleanolic Acid on

This study investigated the antimicrobial action of oleanolic acid against Listeria monocyto-genes, Enterococcus faecium, and Enterococcus faecalis. To determine the cytotoxicity of oleanolic acid, HEp-2 cells were incubated with oleanolic acid at 37oC. MICs (minimal inhibi-tion concentrations) for L.monocytogenes, E. faecium, and E. faecalis were determined using two-fold microdilutions of oleanolic acid, and bacterial cell viability was then assessed by exposing the bacteria to oleanolic acid at 2 × MIC. To investigate the mode of antimicro-bial action of oleanolic acid, we measured leakage of compounds absorbing at 280 nm, along with propidium iodide uptake. Scanning electron microscope (SEM) images were also analysed. The viability of HEp-2 cells decreased (P< 0.05) at oleanolic acid concentra-tions greater than 128 μg mL-1. The MICs were 16-32 μg mL-1 for L.monocytogenes and 32-64 μg mL-1 for E. faecium and E. faecalis, and bacterial cell viability decreased (P< 0.05) about 3-4 log CFU mL-1 after exposure to 2 × MIC of oleanolic acid. Leakage of 280 nm absorbing materials and propidium iodide uptake was higher in oleanolic acid –treated cells than in the control. The cell membrane was damaged in oleanolic acid-treated cells, but the control group had intact cell membrane in SEM images. The results indicate that oleanolic acid can kill L.monocytogenes, E. faecium, and E. faecalis by destroying the bac-terial cell membrane