Antimicrobial activity of rosemary extracts (Rosmarinus officinalis L.) against different species of Listeria

Reduction or elimination of chemically synthesized additives from foods is a current demand in food industry. A new approach to prevent the proliferation of microorganisms or protect food from oxidation is the use of essential oils or plant extracts as natural additives in foods. We have studied antimicrobial activity of rosemary extracts (Rosmarinus officinalis L.) against different species of Listeria and against different strains of L. monocytogenes. We used two extracts of rosemary, VivOX 20 and VivOX 40 (Vitiva d.d., Slovenia) containing different levels of carnosic acid. We wanted to proof an antimicrobial activity of selected rosemary extracts with two most commonly used methods: disc diffusion method and broth dilution method. With the disc diffusion method we have obtained the inhibition zone and at the lowest concentrations, where no visible bacterial growth was recorded, were assumed as minimal inhibitory concentration values (MIC). We determined MIC values in the ranges from 625 μg extract/ml EtOH to 5000 μg extract/ml EtOH for VivOX 20 and from 312.5 μg extract/ml EtOH do 2500 μg extract/ml EtOH for VivOX 40 in the medium. We have established that the resistance of Listeria species against rosemary extracts depends on: selected extract, selected concentration, various species and strain of Listeria. With broth dilution method we have determined minimal bactericidal concentration (MBC), as the concentration giving 0.1 % bacterial survival. With this method we have tested two strains of L. monocytogenes and in determinate MBC values in the range from 15.63 μg/ml TSB to 98.5 μg/ml TSB for both tested extracts. Results have confirmed our assumption that resistance of Listeria against rosemary extracts depended on the selected strain.