Evaluation of the antimicrobial activity of thyme essential oil

Enterococci are an important part of the microflora of foods of animal origin. Their effects may be either positive (probiotic action, production of flavour compounds during food ripening) or negative (production of biogenic amines, antibiotic resistance, biofilm formation). The aim of this work was to determine resistance to different concentrations of thyme essential oil and the antibiotic resistance of enterococci isolated from pork (n = 3) and poultry (n = 17). The antibiotic resistance of isolates was determined by the disc diffusion method and the antibacterial effect of thyme essential oil was examined by a microdilution method in 96-well microtitration plates after determination of absorbance at 630 nm (A630). Of the 20 enterococci tested, 85% were resistant to tetracycline, 35% to erythromycin, 15% to ampicillin and 5% to gentamicin. No resistance to vancomycin was detected. All the tested strains of enterococci were able to grow and reproduce at thyme essential oil concentrations of 0.033% and 0.066%. The inhibitory effect of thyme essential oil began at a concentration of 0.099%, though only for 10% of the strains tested. Even the highest tested concentration of thyme essential oil (0.166%) did not inhibit all the tested strains as 25% of enterococcal strains continued to grow. No correlation between antibiotic resistance and resistance to thyme essential oil was detected in the enterococci tested. Thyme essential oil has a potential in the food industry as an inhibitor of spoilage and pathogenic microorganisms, but its antimicrobial activity must be tested in more in vitro and in vivo experiments and its impact on the sensory properties of foods must also be evaluated.