BACTERIOPHAGE MUTATION 



35 



Apart from the change in type of the liberated phage with ageing if the cul- 

 ture, the strain SF/C behaved as regards lysogenesis in the same fashion as any 

 classical lysogenic strain, e.g. the enteritidis strain BTM described by Burnet and 

 McKie (1929). Lysis of the strain by an unrelated phage failed to liberate any 

 detectably greater amount of phage. In these experiments phage Aul was used 

 (Burnet and Lush 1935), and after lysis titration of the C or C phage present was 

 carried out in the presence of excess of antiphage serum specific for phage Aul. 

 The strain also resembled BTM in that when it was rendered resistant to another 

 phage (in this case phage B) it still retained its characteristic lysogenicity, nor 

 could the lysogenic quality be removed by successive subculture of SP/C in the 

 presence of an antiphage serum highly active against C and C. 



Some differences were observed in the time taken for phage C to appear from 

 different SF/C cultures, and with one the change in type failed to occur. A num- 

 ber of experiments were made in an attempt to control the rate of appearance of 

 the variant phage, but no significant results were obtained. 



The transformation of the incorporated C phage into C which occurs as the 

 culture ages is a phenomenon of great interest, but very difficult to interpret. A 

 clue is perhaps provided by the fact that in the later platings one frequently en- 

 counters plaques which show both C and C characteristics. In every case such 

 double plaques indicate that from what was originally a C plaque a C variant 

 has appeared and given rise to a sector of C clearing in the plaque. Change from 

 C to C does not occur. It is reasonable to assume that the C — » C change in this 

 instance is via the resistant SF/C form which is developing in the centre of the 

 growing plaque. The phage particle which initiates the appearance of such a 

 composite plaque is clearly different from those present in stock phage C filtrates 

 or liberated from young cultures of SF/C in virtue of this greater ease with which 

 the C — > C transformation occurs. In some sense at least it must be regarded as 

 an intermediate unstable form. 



There are indications from two different directions that the liberation of phage 

 C from ageing cultures of SF/C is always associated with the destruction of the 

 coccus from which it is derived. Unless this were the case, platings from unfiltered 

 cultures on the susceptible strain SF would almost certainly show some C plaques 

 in which there was a distinct single colony of SF/C. If we consider a single coccus 

 lodging on the plate and within its first 3 or 4 generations giving rise to phage C 

 the appearance resulting would be a typical C plaque with a single central colony, 

 not the irregular accumulation of secondary colonies visible with a lens in the 

 central region of a C plaque. Such plaques were carefully looked for but never 

 seen. It is probable therefore that when the phage within a given coccus has de- 

 veloped the potentiality of giving rise to a C plaque, it destroys the coccus by 

 lysis as soon as the latter is placed in or on fresh media in which growth can occur. 



In the second place if we could convert the lysogenic strain SF/C by some 



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