Antibacterial activity and mechanism of combination natural essential oils from oregano and cinnamon against enterotoxigenic Escherichia coli K99

Essential oils from oregano (OEO) and cinnamon (CEO) have been reported an excellent antibacterial activity, but their synergistic antibacterial effect is rarely studied. In this study, the chemical compositions of OEO, CEO and mixed OEO with CEO (MEO) and their antibacterial activities were analyzed, the mode of MEO action against enterotoxigenic Escherichia coli K99 (ETEC K99) were investigated. Results showed that MEO had the synergistic antibacterial effect, which was proven by fractional inhibitory concentration index of 0.375. The inhibition zone diameter measured varying with volume ratio of OEO and CEO showed that the best synergistic effect was at a ratio 2:8. Meanwhile, the antibacterial mechanism of MEO demonstrated that the MEO changed the morphology of ETEC K99 by scanning and transmission electron microscopy, and disrupted the cell membrane integrity that were proved by the leakage of intracellular nucleic acid and lower alkaline phosphatase activity of ETEC K99. In addition, the MEO also inhibited DNA biosynthesis, biofilm formation and scavenged mature biofilm of ETEC K99. These results confirm the synergistic effect on improvement of antibacterial activity and elucidate a variety of mechanisms through which the ETEC K99 was inactivated, and support the application of MEO to against ETEC K99. • The mixture of EOs (MEO) exerted a synergistic antibacterial effect on ETEC K99. • MEO had the best synergistic antibacterial activity at a ratio of 2:8 (OEO: CEO). • There were multiple mechanisms for MEO reducing the pathogenicity of ETEC K99. • MEO changed the morphology and inhibited DNA biosynthesis of ETEC K99. • MEO inhibited biofilm formation and scavenged mature biofilm of ETEC K99.