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Bacteriophage-Lysis of Streptococcus Diacetilactis and its Effect on Biacetyl Production in Mixed-Strain Starter Cultures¹

Department of Bacteriology, Oregon Agricultural Experiment Station, Corvallis

ABSTRACT

Bacteriophages specific for Streptococcus diacetilactis were isolated from cottage cheese whey sampled in several commercial dairy plants. Electron microscopy of one of the bacteriophages revealed that it was tadpole-shaped with an over-all length of about 180 mµ; growth studies indicated the virus had a latent period of 30 to 33 min. and an average burst size of 44. Assays revealed that S. diacetilactis produced an average of five times more biacetyl in single-strain milk culture than other bacteria commonly used in controlled dairy fermentations. This aroma-producing organism was combined with one or more strains of Streptococcus lactis, Streptococcus cremoris, and Leuconostoc organisms in the presence and absence of homologous phages for S. diacetilactis. Selective phage-lysis resulted, with consequent marked reduction in biacetyl synthesis, but no effect on the rate of acid production. It is suggested that selective bacteriophage-lysis of S. diacetilactis organisms may occur under commercial conditions. This might account for some instances of variation in flavor and aroma of cultured dairy products.

¹ Published as Technical Paper No. 1266 with the approval of the Director of the Oregon Agricultural Experiment Station.