Effects of homologous bacteriophage on growth of Pseudomonas fragi WY in milk.

Pseudomonas fragi strain WY and its homologous bacteriophage were added in varying concentrations to sterile skim milk which was stored at 7 C for 72 hr. When the initial concentration of the bacterial host was 100,000/ml, addition of as few as 10 plaque-forming units per ml of bacteriophage resulted in significantly lower counts in treated skim milk than in the controls which contained no phage. There was no significant effect, however, when the phage input was 1 in 10 ml and the bacterial count was 1,000 or 100,00/ml. No differences in bacterial counts occurred even when the phage concentration was 1,000/ml if the initial bacterial concentration was only 1,000/ml.