The present invention relates to bacteria of a genus Lactococcus having bacteriological properties of: (1) fermentability which curdles a 10% (W/W) reconstituted skim milk medium when cultivated at a temperature of 25°C to 37°C for 16 hours； (2) Bifidobacterium longum growth-promoting properties which lead to a viable count of Bifidobacterium longum of 5×10 8 CFU/g or more, when co-cultivated with Bifidobacterium longum in the 10% (W/W) reconstituted skimmilkmediumuntil the pH thereof is 4.4 to 4.6； and (3) Bifidobacterium longum survivability-improving properties during storage, which lead to a survival rate of Bifidobacterium longum of 30% or more, after co-cultivation with Bifidobacterium longum in the 10% (W/W) reconstituted skim milk medium until the pH thereof is 4.4 to 4.6, rapid cooling, and two weeks storage at 10°C.
