30 F. M. BURNET and DORA LUSH 



Table 2a. 

 Type of Resistant Colonies Produced. 



Sources of colonies Number of Lytic action of phages. Type of 



(see Table II). colonies. B C C D resistant culture. 



Phage C 10-.'., excess C 11 + — — * + SF/C 



Phage C 10-- r >, excess C 9 — — — + SF/C 



Excess phage C alone 10 — — — + SF/C 



Table 2 shows the number of colonies which developed on each sector. In the 

 absence of phage C about 15 colonies resistant to C appeared, and we can assume 

 that on each of the other sectors about that number are also due directly to the 

 influence of 0', the remainder representing the number of organisms which were 

 rendered resistant to phage C before they came into effective contact with phage 

 C. This assumption could be tested directly as far as it applied to the sectors 

 spread with 10 -r ' dilution of C. Table 2a shows that of 20 colonies tested from 

 these sectors, approximately half were of the SF/C type, the others being SF/C. 

 In the absence of phage C all the colonies were as was to be expected of the SP/C 

 type. At a dilution of 10 - 5 then, phage C produces under these conditions 13 or 14 

 resistant bacteria. The same quantity of phage would give rise to 20-50 plaques 

 as judged by previous titrations, but this was not actually determined in this ex- 

 periment. 



Several other experiments were carried out to determine the ratio between the 

 number of plaques produced under normal conditions and the number of resistant 

 colonies obtained in the present type of experiment. Table 3 shows the results of 

 four experiments using the same batch of phage C, and essentially the same tech- 

 nique as in Experiment 1. Certain variations were made in the time and conditions 

 of contact between phage C and the sensitive culture before C was added, but 

 they seemed to have no significant effect, and will not be discussed. In general the 

 results show that allowing for the Dreyer and Campbell Renton effect the number 

 of resistant colonies is roughly proportional to the amount of phage present, and, 

 that as a rule, about 10-20 p.c. of phage particles capable of giving rise to a plaque 

 will, under these conditions, induce the appearance of a resistant colony. 



Table 3. 

 Relation between Plaque Count and Number of Resistant Colonies. 



Average number of colonies in each experiment. 

 Phage C Plaque Phage C being added 15 minutes after phage C. 



concentration. count. 12 3 4 



10— -i 158 320 185 ±450 



10-4 ±500 60 38 18 54 



10-'"» 78 14 5-5 2-0 11 



2 X 10-6 16 3-0 2-6 6-0 



Nil 4 1-25 0-5 3-25 



213 



