Nisin is a natural additive for conservation of food, pharmaceutical, and dental products and can be used as a therapeutic agent. This study was performed to optimize nisin production in co-culture system aiming at low-costs process and stimulating its utilization. The impact of co-culture of L. lactis (Nis+) with indicator bacteria namely, two lactic acid bacteria [Lb. plantarum and L. lactis(Nis-)] and two pathogense (B. cereus and L. monocytogenes) on nisin production were investigated by microbiological methods. Sterile supernatants derived from sin/co-culture of lactis(Nis+) with aforementioned indicator bacteria were examined by well diffusion and microtiter plate assays, which were extended by gene expression comparison of nisA by Real-time RT-PCR. Total RNAs were extracted and the selected mRNAs were converted to cDNA and Real-time PCR were performed using SYBR green I dye according to standard protocols. According to the results, the sterile supernatants derived from the co-culture systems, were able to induce a substantial increase in zone of inhibition and an outstanding decrease in growth turbidity of pathogens (B. cereus and L. monocytogenes) conducted by well diffusion and microtiter plate assays, respectively. On the other hand, absolute quantification of nisA gene expression in co-cultures of L. lactis (Nis+) with Lb. plantarrum; and L. lactis (Nis+) with L. lactis (Nis-) examined by Real-time RT-PCR analysis showed a significant fold increase up to 75.7 and 9.1 fold respectively Furthermore, co-cultures of L. lactis (Nis+) with L. monocytogenes and L. lactis (Nis+) with B. cereus examined by the same method, also showed 42.6 and 6.9 fold increase in nisA gene expression, respectively. According to finding by this investigation over-expression of nisA gene in co-cultures of L. lactis (Nis+) with mentioned bacteria is confirmed by microbiological and molecular quantification approaches.

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