Purpose: To determine in vitro antimicrobial activities (minimum inhibitory concentration [MIC] and minimum bactericidal concentrations [MBC]) of 4 pure essential oils (EO; thyme, achillea, salvia and rosemary) against seven E. coliserotypes (O26, O45, O103, O111, O121, O145 and O157).
Methods: Antimicrobial activities were determined by micro-dilution techniques using 96-well microplates, Muller-Hinton broth and p-Iodonitrotetrazolium chloride to indicate bacterial growth. After initial determination of MIC and MBC, the obtained concentrations were tested at different pH (4 and 7) and temperatures (4 and 25°C); in the following treatments: T1 = pH 4 at 4°C; T2 = pH 4 at 25°C; T3 = pH 7 at 4°C; and T4 = pH 7 at 25°C by quadruplicates. Each experiment was conducted twice (n = 224).
Results: Thyme, achillea, and rosemary showed MIC activity at lower concentrations (2 mg/ml, 5 - 10 mg/ml, and 2 - 5 mg/ml; respectively); while for salvia, the MIC was determined at higher concentrations (12 - 25 mg/ml). The MBC was as follows: thyme (2 mg/ml), achillea (5 to 10 mg/ml), rosemary (2.5 to 5 mg/ml) and salvia (12 to 25 mg/ml). When EO’s MIC and MBC were tested in T1, T2, T3 and T4, no effects were observed. When lower concentrations of EOs (1 - 5 mg/ml) were tested in T1 and T2, thyme, achillea and rosemary showed bactericidal effects, evidencing a synergistic effect with pH and temperature.
Significance: Data shows that EOs, especially thyme and rosemary, can be effective against the 7 VTEC serotypes tested and suggests that these EOs might be effective against VTEC in acidic foods and at refrigerated temperatures.