34 F. M. BURNET and DORA LUSH 



studied by this technique. The best of three consistent experiments gave the 

 plaque counts shown in Table 4a with phage C. In all experiments the first in- 

 crease indicated that 80-150 phage particles were liberated from each infected bac- 

 terium, lysis occurring between 70 and 90 minutes after mixing. A suitable modi- 

 fication of this experiment showed that a similar explosive multiplication took 

 place on the surface of agar, although in this instance the first increase appeared 

 later, between 90 and 105 minutes. 



Lysogenesis of 8F/C. 



The resistant culture SF/C which is so readily induced by the action of phage 

 C is consistently lysogenic, producing in young culture phage of C type, but in 

 older cultures it gives rise to plaques of both C and C types. Both types like the 

 original ones breed true after isolation and passage on SF. 



A series of cultures of a strain of SF/C which had been re-isolated .three or 

 four times since its induction by phage C, was set up in broth (pH 8-0) and kept 

 at 37° C. Each day the lysogenicity was tested by filtering portion of a culture 

 through a gradocol membrane (A.P.D. 0-8/x) and plating both filtrate and unfil- 

 tered culture on plates spread with SF. Details of one such experiment are given 

 in Table 5. 



Table 5. 



Lysogenesis of SF/C Cultures of Different Ages. 



Time. Unfiltered SF/C. Gradocol membrane nitrate SF/C. 



ldav IX 100, all C type _ 



2 days 1 X 10 s , all C type 3 X 100, all C type 



4 days 2 X 10- r «, 1% C type 2 X 10- r », 2% C type 



5 days 5 X 10 7, 0-5% C tvpe — 



6 days 2 X 108, 50% C type 2 X 108, 50<% C type 



7 days 5 X 10?, 1% C type 



In the majority of experiments C appeared somewhat earlier, usually on the 

 third day, a maximum mutation being reached on the fourth or fifth day. The test 

 shown in Table 5 was carried over a longer period of time, and a marked drop in 

 the amount of C present was noted on the seventh day. 



217 



