Types 



Technique in isolation and demonstration 



The method used by Christison and associates (1938) for 

 investigating different types of phage consisted of plating decimal 

 dilutions of phage v.lth a culture of a sensitive strain and incubating 

 at 25° C. Discrete, well-separated plaques were "picked off" after 

 2U to AS hours' incubation, placed in tubes of broth, which were 

 inoculated with the same strain of B. salmonicida and filtered after 

 2A hours' incubation. After several strains of phage had been purified 

 by repeating the process of plating, picking off discrete plaques, etc. 

 several tiines, the final filtrate obtained was regarded as being 

 appro jdLmately a pure type. 



Resistant strains of the original sensitive culture of B. 

 salrnor-icida, were obtained to each of these final filtrates by inoculating 

 0.1 cc. dilutions of each filtrate with a sensitive culture and 

 incubating until growth occurred. The sensitive strain of B. salmonic ida 

 used was "U.S.A. 5", a strain which was acted on by every phage 

 preparation obtained, indicating that it probably was not resistant to 

 the types contained in these filtrates. It had the further advantages 

 that resistant strains were developed fairly easily from it, usually 

 within 4. to 7 daj-'s; it also did not produce a diffusible pigment, 

 Tfjhicli facilitated the reading of results. 



Each of the final purified phage filtrates was tested individually 

 against every resistant strain so obtained and the relationships between 

 them were determined. Thus, if two phage preparations acted in the same 

 way tovrards their respective resistant strains, they were considered 

 identical, and if they had been purified sufficiently', it was assumed 

 that they contained the same single type of bacteriophage until the 

 contrary vras proved. On the other hand, if they acted differently tovi-ards 

 the respective resistant strains, they were regarded as different tj'pes 

 of bacteriophage. 



Occurrence 



Christison and associates (1938) found that it proved very 

 difficult to isolate pure types of the bacteriophage, though a 

 considerable amount of time was spent in picking off well-isolated 

 plaques from successive generations of the same phages. Enough evidence 

 was procured, however, to shew that there 'vvere at least three different 

 types in the specimens collected, and possiblj^ more. One type was fairij- 

 readily separated, but tlie others showed som.e degree of overlapping. 



Origin 



The origin of this phage is obscure. Tlie propagation and presence 

 of this phage in water in which furunculosis of fish is absent and from 

 which B. salmonicida cannot be demonstrated is difficult to e:!q3lain. 



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