Chapter III —63 — Morphology 



all the properties of phage. In the presence of young cultures of S. 

 grisetis, the phage de\eloped rapidly and brought about the lysis of the 

 culture. The plaque method was used for measuring the concentration 

 of the phage. Streptomycin production was partly or completely pre- 

 vented by the phage. Cultures of S. griseus resistant to the action of 

 the phage could easily be isolated. 



Woodruff and Foster (516) exposed to laboratory air for 24 hours 

 a submerged culture of S. grisezis in a stationary condition, with plugs 

 removed from the flask. The freshly formed pellicle showed evidence 

 of plaque formation. The same phenomenon was observed in a factory 

 500 miles away. Upon transfer of a filtered culture into a fresh culture 

 of S. grisetis, the phage multiplied. It was calculated that after six 

 transfers, each phage particle increased to 75 X lO-*^ particles. The 

 phage was active against all streptomycin-producing strains of S. griseus 

 but not against the non-streptomycin-producing strains. Phage-resist- 

 ant strains developed readily. They retained their capacity to pro- 

 duce streptomycin but were not absolutely free from phage. The 

 phage of S. grisexis had properties similar to those of bacterial phages, 

 as shown both by cultural characteristics and by appearance in photo- 

 graphs made by means of an electron microscope (Fig. 15). 



The following method can be employed (355) for assaying the 

 activity concentration of phage in a gi\'en preparation. A 3 to 5-day- 

 old shaken culture of a streptomycin-producing strain of S. griseus is 

 filtered aseptically through paper and inoculated on plates. The 

 phage preparation is obtained by inoculating with phage, young cul- 

 tures of S. griseus grown in a shaken condition, allowing the cultures 

 to incubate further for 24 to 72 hours, and filtering them through a 

 Seitz filter. Dilutions of phage, ranging from 1:10'^ to 1:10^-, are 

 added to 10-ml. portions of sterile nutrient agar, previously inoculated 

 with 0. 1-ml. portions of the paper-filtered culture. The agar is poured 

 into plates, which are incubated at 28 °C. for 2 davs. The plaque 

 counts are made, as shown in Fig. 16, and calculated for 1 ml. of the 

 preparation. Some preparations gave 4 X 10^** or more particles per 

 milliliter. 



The phage preparation is kept in the refrigerator and used as a 

 standard. To illustrate the effect of actinomyces inoculum upon the 

 phage count, three diff"erent concentrations of filtered 7-day-old shaken 

 culture of streptomvcin-producing S. griseus were added to nutrient 

 agar. The plates were inoculated with the same amount of the phage 

 and incubated at 28 °C. for 48 hours. The following results were 

 obtained. 



