Casein as a necessary factor in the production of stimulatory material for associative growth of lactic streptococci.

Strains of Streptococcus cremoris KH and HC produced material that was stimulatory for S. cremoris R(6) in milk and in the dialyzable fraction of milk, but not in the dialysate fraction of milk, lactic acid whey, or lactose broth. The addition of casein to these latter media permitted the production of this stimulatory material to occur. Tryptone, peptone, and yeast extract could not be substituted for casein in producing the stimulatory material or in initiating associative growth in the lactic acid whey. The minimum concentration of casein required appeared to be from 2.0 to 2.5%.