The present invention relates to bacteria of a genus Lactococcus havingbacteriological properties of: (1) fermentability which curdles a 10% (W/W)reconstituted skim milk medium when cultivated at a temperature of 25°Cto 37°C for 16hours； (2) Bifidobacterium longum growth-promoting properties which lead to aviablecount of Bifidobacterium longum of 5 x 10 8 CFU/g or more, when co-cultivatedwithBifidobacterium longum in the 10% (W/W) reconstituted skim milk medium untilthe pHthereof is 4.4 to 4.6； and (3) Bifidobacterium longum survivability-improvingpropertiesduring 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 skimmilk medium until the pH thereof is 4.4 to 4.6, rapid cooling, and two weeksstorage at10°C.
