Increase in activity of essential oil components carvacrol and thymol against Escherichia coli O157:H7 by addition of food stabilizers.

The major components of oregano and thyme essential oils that had previously been shown to inhibit Escherichia coli O157:H7 were determined by high-performance liquid chromatography with UV detection and liquid chromatographic tandem mass spectrometry. The MICs and MBCs of carvacrol, thymol, p-cymene, and gamma-terpinene against a strain of E. coli O157: H7 phage type 34 isolated from bovine feces were determined by microdilution assay. The constituents were then tested in checkerboard assays to detect possible interactions. Carvacrol and thymol displayed bacteriostatic and bactericidal properties with MICs of 1.2 mmol/liter and were additive in combination. p-Cymene and gamma-terpinene displayed no measurable antibacterial activity up to 50 mmol/liter, and neither influenced the activity of carvacrol or thymol. Growth curves in the presence of nonlethal concentrations of carvacrol with the addition of agar (0.05%, wt/vol) or carrageenan (0.125%, wt/vol) as stabilizer were produced by optical density measurement. The stabilizers agar and carrageenan both significantly improved the effectiveness of carvacrol in broth, possibly because of a delay in the separation of the hydrophobic substrate from the aqueous phase of the medium. When carvacrol was dissolved in ethanol before addition to broth, stabilizers were not needed. Carvacrol and thymol, particularly when used in combination with a stabilizer or in an ethanol solution, may be effective in reducing the number or preventing growth of E. coli O157:H7 in liquid foods.