Characterization of mesophilic mixed starter cultures used for the manufacture of aged cheddar cheese

Seventy-one different Lactococcus lactis subsp. cremoris strains were isola ted from seven mesophilic mixed starters used in the manufacture of aged Ch eddar cheese in Canada. Based on plasmid profiles and growth in milk (with or without glucose, Casamino Acids or both), two mixed starters were highly heterogeneous, containing at least 18 to 24 distinct L. lactis strains. Th ree mixed starters were comprised of seven to nine strains, whereas two sta rters were relatively homogeneous, containing two or three strains. Many st rains with similar plasmid profiles behaved differently during growth in mi lk, indicating variability in the phenotypes. Only 20% of the strains could grow in plain milk, whereas 30% could not grow in milk supplemented with g lucose and Casamino Acids. Twenty-five lactococcal bacteriophages were also isolated from whey samples with single strains as hosts. Eighteen phages b elonged to the 936 species and seven to the c2 species. Thirteen strains we re insensitive to all 25 phages. Almost all sensitive strains were phage sp ecies-specific. The 936-like phages had a broader host range.