Antimicrobial Activity of Oleanolic Acid on Listeria monocytogenes under Sublethal Stresses of NaCl and pH

The objective of this study was to evaluate the inhibition of Listeria monocytogenes growth by oleanolic acid under sublethal stresses of NaCl and pH. L. monocytogenes ATCC15313 (6 log CFU/mL) was inoculated in microplate wells containing brain heart infusion (BHI) broth supplemented with oleanolic acid in various amounts (0, 0.25, 0.5, 1.0, 1.5, 2.0, and $4.0\;{\mu}g/mL$), and different pHs (5 and 7) and NaCl concentrations (0, 3, and 6%), followed by incubation under accelerated storage condition ($37^{\circ}C$, 48 h). The optical density (OD) of the samples was measured at 0, 6, 12, 24, and 48 h at 600 nm. After the lag phase duration was observed at the early stage of incubation, the OD values of L. monocytogenes significantly increased (p<0.05) in BHI broth formulated with 0 and 3% of NaCl during accelerated storage at pH 5 and 7. However, the growth of L. monocytogenes in 6% NaCl and at less than $0.5\;{\mu}g/mL$ of oleanolic acid had no growth at pH 5 and only gradual growth at pH 7. Moreover, L. monocytogenes generally had lower OD values as the concentrations of oleanolic acid increased. As expected, the OD values of L. monocytogenes were generally higher (p<0.05) at pH 7 than at pH 5. These results indicate that oleanolic acid should be useful in inhibiting the growth of L. monocytogenes.