Effects of nisin on the expression of virulence genes of methicillin- resistant/sensitive Staphylococcus aureus

Background and Objectives: Few studies have considered potential benefits of probiotic bacteria and their derivatives on human and animal health. Nisin is an antimicrobial agent that is produced by lactobacilli and served as a preservative in foods. This study aims to investigate whether nisin suppresses ordecreases the genes involved in the pathogenicity of meth- icillin-susceptible and methicillin-resistant Staphylococcus aureus (MSSA and MRSA). 
Materials and Methods: MSSA and MRSA strains were cultured at the ¼, ½, and 1 × minimum inhibitory concentration (MIC) of nisin. Next, RNAextraction was performed at the mid-exponential stage of growth, and cDNA was synthesized. The expression of virulence factors was measured by qPCR, and the data were analyzed by the ΔΔCt formula. 
Results: Depending on the incubation times and the Lactobacillus species, the MIC of nisin on MRSA and MSSA observed in 800 and 1600 mg/l, respectively.The qPCR assay showed the expression level of the sea, agrA, and spa genes decreased and the level of the sae gene increased at the sub-MIC of nisin, and no antagonism was observed. Concerning MRSA, the maximum downregulation rate was observed in the sea gene (up to 5.9 folds) while in MSSA, the maximumdownregulation rate was noticed in the agrA gene (up to 10 folds). 
Conclusion: Due to the high inhibitory effect of the sub-MIC of nisin on the expression of virulence factor genes in MRSA and MSSA, this compound could potentially reduce the virulence of S. aureus. 
 