Nisin Tolerance of DnaK-overexpressing Lactococcus lactis Strains at 40°C

The effect of different concentrations of nisin on DnaK-overproducing Lactococcus lactis strains while growing at 40°C was examined. The plasmid pNZ-8048 which has a nisin promoter was used as a vector for transformation and expression of heterologous or homologous DnaK into L. lactis NZ9000. The transformants were then induced with different concentration of nisin and allowed to grow in presence of nisin. It was found that nisin, the antimicrobial peptide used for induction of DnaK-overexpression, itself conferred a lower but significant stress to the strains. Escherichia coli DnaK (DnaK _Eco)-overexpressing cells showed better tolerance to nisin than L. lactis DnaK (DnaK _Lla) and T. halophilus DnaK (DnaK _Tha)-overexpressing cells. However, all DnaK-overexpressing strains showed better tolerance than the cells without DnaK overexpression. These findings suggest the possibility for a relationship of DnaK protein and tolerance to different antimicrobial peptides.