Purpose: Therefore, the objective of this study was to elucidate the mechanism of antimicrobial action of oleanolic acid on Listeria monocytogenes, Enterococcus faecalis, and Enterococcus faecium.
Methods: The inocula of L. monocytogenes ATCC15313, E. faecalis ATCC35038, and E. faecium ATCC19434 were inoculated in 96-well microtiter plates containing oleanolic acid at 0 to 512 μg/ml. The plates were then incubated at 37oC for 24 h to determine minimal inhibition concentrations (MIC). To determine cell viability of the pathogens, bacterial cell suspension was subjected to 2×MIC of oleanolic acid, and viable cell numbers were enumerated on brain heart infusion agar plates without oleanolic acid. The action of oleanolic acid on cell membrane permeability of the pathogens was evaluatd by propidium iodide uptake assay using FACS analysis and measuring leakage of 280 nm absorbing material.
Results: The MICs were 8-16 μg/ml, 32 μg/ml, and 32 μg/ml for L. monocytogenes, E. faecalis, and E. faecium, respectively. Cell viabilities of the pathogens decreased by 3-4 log CFU/ml after oleanolic acid at 2×MIC was added. In addition, microbial membrane permeabilities of three pathogens increased as oleanolic acid concentration added at 2×MIC.
Significance: This result indicates that oleanolic acid should be useful in destroying foodborne pathogens by disrupting microbial membrane of foodborne pathogens.