Evaluation of antmicrobial effect of Zataria multiflora Boiss. essential oil on food-borne pathogens in minced beef combining real time-PCR and propidium monoazide

Recently, real-time PCR in combination with PMA has successfully been applied to discriminate between live Escherichia coli O157:H7 and dead bacteria killed by cumin, clove, oregano and cinnamon essential oils. In this study, antimicrobial effect of Zataria multiflora Boiss. essential oil on food-borne pathogens in minced beef combining real time-PCR and propidium monoazide has been evaluated.
Essential oil was analyzed by gas chromatography-mass spectrometry equipment. Initial experiments were performed in order to elucidate the minimum bactericidal concentration of Z. multiflora essential oil on E. coli O157:H7, Salmonella enterica and Listeria monocytogenes. Thereafter PMA-qPCR was applied in order to selectively quantify life cells within a bacterial population treated with Z. multiflora essential oil in minced beef.
The main component of Z. multiflora Boiss. essential oil was carvacrol (71.12%). Inactivation was obtained at essential oil concentrations of 0.02, 0.035, 0.045 for L. monocytogenes, E. coli O157:H7 and S. enterica, respectively . L. monocytogenes were totally killed in 30 min while it took 1 h 30 min for the gram negative pathogens. According to the results of PMA-qPCR quantification obtained for the different combinations of live and dead cells in artificially inoculated minced beef, endogenous bacterial counts of the minced beef were 3.27×104 cfu/g. For S. enterica and E. coli O157:H7 PMAqPCR quantification values correlated with plate counts of live cells, but for L. monocytogenes, live cells were overestimated in all cases and in the sample with 100% of dead cells PMA-qPCR still detected a 3.8% of live cells.
As a conclusion Z. multiflora essential oil has potential as natural food additive or biopreservative since it was able to irreversibly inactivate the three pathogens tested, at lower concentrations than other essential oils and short exposition times. In addition, the PMA-qPCR approach proved efficient to selectively detect live pathogenic bacteria in raw minced beef following inactivation with Z. multiflora essential oil.