Inhibition of oregano essential oil and EDTA on Escherichia coli O157:H7

Oregano essential oil with and without the addition of EDTA was tested against Escherichia coli. The amount of essential oil, the addition of EDTA and the incubation temperature of the medium influenced the maximum growth rate (mu (max)) and the lag phase of Escherichia coli O157:H7. In particular, the presence of oregano essential oil at the time of inoculation caused an increase in the lag phase, a decrease in the maximum growth rate and a reduced final population size. The inhibitory effect on E. coli O157:H7 is probably associated only with the oregano essential oil since there was no antibacterial action in broth samples inoculated with E, coli O157:H7 and supplemented alone with EDTA. However, the combination of the two was found to increase the inhibition dramatically. The minimum bacteriocidal concentration of oregano essential oil on cells of E, coli O157:H7 with and without EDTA affected the dense cellular materials and coagulated the cytoplasmic constituents, while the membrane structure tended to be irregular and rough.