Inhibition of verocytotoxigenic Escherichia coli in model broth and rumen systems by carvacrol and thymol

The antimicrobial activities of thymol and carvacrol were assessed against a selection of verocytotoxigenic Escherichia coli (VTEC) strains (n = 11) and other bacterial species and spoilage bacteria (n = 7) using a model broth system. The effects of pH, temperature, water activity, sodium chloride concentrations, inoculum size and the presence of competing microflora on the activities of thymol and carvacrol against E. coli O157:H7 strain 380-94 were also determined. The minimum inhibitory and bactericidal concentrations (MIC and MBC, respectively) and numbers of surviving E. coli O157:H7 were determined following incubation. The mean numbers of VTEC surviving exposure to thymol or carvacrol at concentrations of ≥ 500 μg/ml were between 2.0 and 7.8 log cfu/ml less than the numbers in the corresponding controls. The susceptibility of E. coli O157:H7 to carvacrol or thymol was found to increase with decreasing storage temperature, water activity, pH and E. coli O157:H7 inoculum size. Sodium chloride (0.5–2.5%) and the presence of a microflora cocktail did not significantly (p > 0.05) affect the antimicrobial activities of thymol or carvacrol against E. coli O157:H7. The antimicrobial activity of carvacrol against E. coli O157:H7 was also tested in a model rumen system. A MIC of 500 μg/ml carvacrol reduced E. coli O157:H7 inoculated at levels of 103 and 106 cfu/ml to undetectable levels in the system after 24 h incubation. This concentration of carvacrol significantly (p < 0.05) decreased the total gas production and volatile fatty acid concentrations in the model rumen assay.