Abstract: The action mechanism of oregano essential oil (OEO) against Staphylococcus saprophyticus was investigated in this paper. The minimum inhibitory concentration (MIC) was determined by measuring inhibition zone diameter using the agar well diffusion method, and bacterial growth curve, electrical conductivity, alkaline phosphatase (AKP) activity, lactate dehydrogenase (LDH) activity, ultraviolet absorption and scanning electron microscopy (SEM) were used to evaluate the antimicrobial effect of OEO. The results showed that the MIC of OEO against Staphylococcus saprophyticus was 0.25%. When treated by OEO at MIC or 2 MIC, the logarithmic phase was delayed obviously, accompanied by the leakage of intracellular contents and increased significantly activities of AKP and LDH. The bacterial growth was concentration-dependently inhibited by OEO, with structural changes of the cell wall and membrane being observed. The SEM results revealed that the bacterial cells appeared morphologically deformed after being treated with OEO at MIC for 6 h, and OEO at 2 MIC caused cell surface disruption, outer membrane detachment and finally cell death. Our findings support that OEO exerts its bacteriostatic effect by destroying the integrity of the cell wall and membrane.

Key words: oregano essential oil, Staphylococcus saprophyticus, action mechanism