Evaluating the efficacy of an essential oil extract of thyme (Thymus vulgaris) against methicillin-sensitive and methicillin-resistant strains of Staphylococci

Abstract

An essential oil extract of thyme (Thymus vulgaris) was evaluated against MSSA and MRSA strains of Staphylococci. Minimum Inhibitory Concentrations (MICs) were determined using cation-adjusted Mueller-Hinton broth with 2% sodium chloride and modified with 0.2 % agar for suspension of Thyme oil. Oxacillin and vancomycin were used as positive controls. MICs were determined by visual inspection and optical density (OD). S. aureus ATCC 29213 showed MICs of 0.125 µg/ml in media with agar and 0.25 µg/ml in media without agar by visual inspection for oxacillin, but an MIC of 0.25 µg/ml was determined by OD for oxacillin in both media. Vancomycin MICs were 1 µg/ml for S. aureus ATCC 29213 by visual inspection but 2 µg/ml using OD in both media with and without agar. S. aureus ATCC BAA-1720 showed MIC values of 1 µg/ml in media without agar but 2 µg/ml in media with agar both by visual inspection and OD. MICs determined by optical density (OD) allowed detection of small amounts of growth not apparent by visual inspection, allowing better detection of heteroresistant strains against the standard antibiotics oxacillin and vancomycin. The MIC for thymeoil was 0.3125 µl/ml against both strains of S. aureus by visual inspection as well as OD. Essential oils provide an important new mode of action against these strains.