Continuous nisin production with bioengineered Lactococcus lactis strains

Nisin production in continuous cultures of bioengineered Lactococcus lactis strains that incorporate additional immunity and regulation genes was studied. Highest nisin activities were observed at 0.2 h(-1) dilution rate and 12.5 g l(-1) fructose concentration for all strains. Recombinant strains were able to produce greater amounts of nisin at dilution rates below 0.3 h(-1) compared to the control strain. However, this significant difference disappeared at dilution rates of 0.4 and 0.5 h(-1). For the strains LL27, LAC338, LAC339, and LAC340, optimum conditions for nisin production were determined to be at 0.29, 0.26, 0.27, and 0.27 h(-1) dilution rates and 11.95, 12.01, 11.63, and 12.50 g l(-1) fructose concentrations, respectively. The highest nisin productivity, 496 IU ml(-1) h(-1), was achieved with LAC339. The results of this study suggest that low dilution rates stabilize the high specific nisin productivity of the bioengineered strains in continuous fermentation. Moreover, response surface methodology analysis showed that regulation genes yielded high nisin productivity at wide ranges of dilution rates and fructose concentrations.