Inhibition of Beef Isolates of E. coli O157:H7 by Orange Oil at Various Temperatures

Plant essential oils have previously been shown to exhibit antimicrobial activities against various microorganisms. In this study, cold pressed terpeneless Valencia orange oil (CPTVO) was examined at various temperatures (37, 10, and 4 degrees C) to determine its antimicrobial activity against 3 strains of E. coli O157:H7 recovered from beef products. A micro broth dilution method using 96 well microtiter plates was used with trypticase soy broth with 0.15% agar and 2,3,5 tetrazolium chloride as a growth indicator as the medium. Serial dilutions of CPTVO were made, resulting in final concentrations of oil ranging from 0.2% to 25% or 0.1% to 10%. Plates were incubated statically at 4, 10, or 37 degrees C, and sampled hourly. After 6 h at 37 degrees C, all strains were inhibited at concentrations ranging from 0.2% to 0.6%, with a mean of 0.4 +/- 0.01%. At 10 degrees C, all strains were inhibited at concentrations ranging from 0.8% to 6.3%, with a mean of 1.1% +/- 0.2%, after 6 h. At 4 degrees C, all strains were inhibited after 6 h at concentrations ranging from 2.3% to 4.6%, with a mean of 3.5% +/- 2.1%. After 24 h at 4 degrees C the strains were inhibited at concentrations ranging from 0.7% to 1% with a mean of 0.8% +/- 0.3%. The ranges appear to be the result of effects from the variable nature of a complex media and an antimicrobial that presents potential multiple mechanisms for inhibition. It appears CPTVO is a viable option to inhibit E. coli O157:H7 growth at refrigeration temperatures.