36 F. M. BURNET and DOHA LUSH 



means not involving the use of phage C into ;i variant showing the same type of 

 resistance as is shown by the natural non-lysogenic resistant strain SF/C, one 

 might expect to annul the production of phage C if its liberation were dependent 

 on actual lysis of the bacterium by internally appearing phage C. Reference to 

 Table 1 will show that by the action of phage B on SF (' a variant having SF (" 

 characters can be obtained. Two such variants SF/C/B were prepared and broth 

 cultures tested over a period of 8 days. In neither instance did any C type plaques 

 appear. On one occasion a single plaque of a new type was found. Tins showed 

 a clear centre, then a very narrow ring of secondary growth, then the usual clear 

 outer zone. Phage derived from this plaque bred true to type, consistently giving 

 plaques of similar character. In general this strain C" more closely resembled C 

 than ( 1/ , giving resistant variants of the SF/C type which were lysogenic, produc- 

 ing phage of C" type. The point to be stressed is that in the doubly resistant strain 

 produced by the action of phage B on SF/C, no mutation of phage C to C was 

 observed. It seems as if the resistance induced by B on SF/C (or the almost simi- 

 lar resistance of the non-lysogenic SF/C" obtained by the direct action of C on 

 the original SF culture) is of a more fundamental character than the resistance to 

 C shown by SF/C. The most reasonable interpretation is that SF (' resists (" 

 because of a surface change preventing effective attachment of the phage, but is 

 incapable of preventing its lytic action should it develop within the bacterium from 

 the incorporated phage. With the more resistant variants SF/C/B there is an 

 intrinsic resistance to C irrespective of whether attack" is from without or within 

 or alternatively the change to C is completely inhibited. 



DISCUSSION. 



This investigation arose out of a desire to elucidate the striking difference in 

 appearance between the plaques of phage and its mutant C'. and has led us 

 considerably further than we had anticipated. The actual form taken by the 

 phenomena is almost unique as far as can be judged from the literature and our 

 own experience, and any deductions are not immediately applicable to any other 

 types of bacteriophage activity. In the course of the investigation, however, a 

 number of points were established which may be of sufficient general interest to 

 workers with bacteriophages to warrant publication. 



In the first place the extraordinary ease with which phage produced resist- 

 ant variants suggested strongly that if any phages were capable of multiplication 

 without lysis of the sensitive bacterium this should be one of them. The experiment 

 illustrated by Table 4a shows conclusively that like every other phage that has been 

 lested, phage C multiplies explosively, 80 or more fresh phage particles being libe- 

 rated from a disrupted bacterium from 60 to 90 minutes after phage was added to 

 the culture. This provides a fresh confirmation of the correctness of d'llerelle's 

 original view of the nature of phage multiplication. 



219 



