Nowadays foods are preserved with using some methods to minimize contamination. One of these methods is food preservatives. Food preservatives with antagonistic effect usually interfere with cell membrane, enzyme action or genetic structure of microorganisms. In this investigation the effects of some preservatives were studied on the growth curve of Listeria monocytogenes that is isolated from dairy products. First the growth curve of bacteria was drawn when the preservatives were absent. Then the growth curve of bacteria was drawn when citric acid, which is a chemical preservative and nisin, which is a natural preservative, were present at sub-MIC concentrations. Next the growth curve of bacteria was drawn at the presence of both acetic acid and sodium citrate at different concentrations and in this period the changes of pH were measured. Listeria monocytogenes did not grow against citric acid at any sub-MIC concentrations which shows the sensitivity of bacteria to citric acid. Although this bacterium was sensitive to MIC concentrations of nisin, but it was significantly resistant to nisin after 18 hours. Also acetic acid and sodium citrate together can have more effect against this bacterium. This study showed that food isolated Listeria monocytogenes is resistant to sub-MIC concentrations of some preservatives, so it is suggested not to apply less concentration of MIC concentrations.

Keywords: Listeria monocytogenes, Nisin, Preservatives, MIC

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