Action of bacteriophage on mixed strain cultures. IV. Domination among strains of lactic streptococci.

Mixed strain cultures, used extensively for the manufacture of certain dairy products, have generally been thought to give a measure of protection against possible serious retardation of acid development resulting from attack by bacteriophages. Yet little information is available as to the influence that continued propagation has on the individual strains of lactic streptococci. Nichols and Ineson (1947) reported that certain strains rapidly outgrow others and become responsible for most of the acid produced in mixtures. Domination occurred at 22 C and at 32 C, and wide differences in inocula did not determine ultimate strain dominance. This investigation of domination among laboratory strains of lactic streptococci was combined with an investigation of the extent of domination within certain commercial cultures that had produced acid subnormally in the manufacture of cottage cheese. Five laboratory strains of streptococci and 14 commercial cultures were studied.