Combined antibacterial activities of essential oil compounds against Escherichia coli O157:H7 and their application potential on fresh-cut lettuce

This study evaluated the antimicrobial activities of combined use of essential oil (EO) compounds against E. coli O157:H7, using the Fractional Inhibitory Concentration (FIC) index and time-kill assay. The mechanisms of enhanced antimicrobial efficacies in EO combinations were evaluated using flow cytometry, and their application potentials as decontamination rinse were assessed using fresh-cut lettuce. Thymol (Thy), carvacrol (Car), trans-cinnamaldehyde (TC), eugenol (Eug) and vanillin (Van) displayed minimum inhibitory concentrations (MICs) in a range of 0.63-2.5 mg/mL, whereas citral (Cit) and linalool (Lin) were ineffective (MIC > 10 mg/mL). FIC indices (1.00-2.00) suggested additive interactions of all EO combinations, and time kill assay showed enhanced bactericidal activities in Thy/Eug, Car/Eug and TC/Eug combinations. The stronger antimicrobial efficacy in EO combination was attributed to increased membrane damages. Application of Thy/Eug and Car/Eug on fresh-cut lettuce as decontamination rinse caused a significant (P < 0.05) decrease in bacterial count, compared to water rinse, but resulted in color-darkening and texture-softening in treated lettuce.