Loss of the intrinsic plasmid-encoded eps genes in Lactococcus lactis subsp. cremoris FC grown at elevated temperature abolishes exopolysaccharide biosynthesis

Abstract

Strain FC of Lactococcus lactis subsp. cremoris is used to produce viscous yogurt because of its metabolite exopolysaccharide (EPS) and is typically grown at 25–30 °C. Herein, we isolated the EPS non-producing variant C4 by incubating strain FC at 37 °C and evaluated the genes involved in EPS biosynthesis. Southern blotting revealed that strain C4 lost the plasmid encoding epsX and epsL. Furthermore, specific PCR fragments for epsB, epsD, and orfY were not amplified. Therefore, strain FC cultured at 37 °C lost the plasmid harbouring the eps gene cluster, and EPS biosynthesis was thus abolished. Moreover, we analysed the physical properties of yogurts fermented with these strains. Yogurt fermented with strain FC was harder, stickier, and more cohesive than that fermented with strain C4. Furthermore, the volume of separated whey was less for strain FC. For the production of viscous yogurt, strain FC must be carefully maintained at its optimal growth temperature to prevent the loss of the eps-encoding plasmid.