Saeed Mirdamadi, Shadi Agha Ghazvini
Biological Journal of Microorganism ,Vol. 3,No. 12, 79- 92

Introduction: In the present study, the inhibitory effect of nisin- producing Lactococcus lactis during co- culture and pure standard nisin were assessed against selected foodborne pathogenes in growth medium and Iranian UF Feta cheese. In comparison L lactis, not only proves flavor but also plays a better role in microbial quality of Iranian UF Feta cheese as a model of fermented dairy products. Materials and methods: L. lactis subsp. lactis as nisin producer strain, Listeria monocytogenes, Escherichia coli and Staphylococcus aureus as pathogenic strains were inoculated in Ultra- Filtered Feta cheese. Growth curve of bacterial strains were studied by colony count method in growth medium and UF Feta cheese separately and during coculture with L. lactis. Nisin production was determined by agar diffusion assay method against susceptible test strain and confirmed by RP- HPLC analysis method. Results: Counts of L. monocytogenes decreased in cheese sample containing L. lactis and standard nisin, to 103 CFU/g after 7 days and it reached to undetectable level within 2 weeks. S. aureus counts remained at its initial number, 105 CFU/g, after 7 days then decreased to 104 CFU/g on day 14 and it was not detectable on day 28. E. coli numbers increased in both treatments after 7 days and then decreased to 104 CFU/g after 28 days. Despite the increasing number of E. coli in growth medium containing nisin, due to the synergistic effect of nisin and other metabolites produced by Lactococcus lactis and starter cultures, the number of E. coli decreased with slow rate. Discussion and conclusion: The results showed, L. monocytogenes was inhibited by L. lactis before entering the logarithmic phase during co- culture. S. aureus was also inhibited during co- culture, but it showed less sensitivity in comparison with L. monocytogenes. However, the number of E. coli remained steady in co- culture with L. lactis. Also, we found that, in all cheese samples, E. coli decreased slowly after 28 days which may be due to the synergistic inhibitory effects of nisin and other metabolites produced by L. lactis and starter culture strains. These conditions are compatible to UF Feta cheese making processes. The usage of L. lactis is more effective in terms of pathogenic inhibitory in comparison with free nisin. Using L. lactis as an adjunct starter culture can assist microbial quality improvement and prevent important pathogens, which may survive during food processing, because of the production of beneficial metabolites.

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