The aim of the present study was to investigate the antimicrobial mechanism of oregano essential oil (OEO) against Shewanella putrefaciens. Antimicrobial activity of OEO against Shewanella putrefaciens was investigated by the agar disc diffusion method. The change of growth curve, electric conductivity, the integrity of cell membrane, alkaline phosphatase (AKP), and lactate dehydrogenase (LDH) activity were measured to evaluate its antibacterial mechanism. The morphology of bacterial cells was observed by scanning electron microscopy (SEM). The interaction between OEO and Shewanella putrefaciens genomic DNA was measured by ultraviolet–visible (UV–Vis) spectroscopy, and DNA ethidium bromide adduct was analyzed by fluorescence. The results showed that the minimum inhibitory concentration of OEO against Shewanella putrefaciens was 0.09% (v/v), and OEO could inhibit the growth of Shewanella putrefacien with a dose-dependent manner. The cell membrane and cell wall of Shewanella putrefaciens were destroyed by OEO, which led to the leakage of nucleic acid, protein, and the release of AKP and LDH. The results of SEM confirmed the damaging effect of OEO on the bacterial morphology. The results of UV–Vis and fluorescence titration indicated that binding of the complexes to DNA was an intercalative mode.
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