BACTERIOPHAGE MUTATION 29 



after 4 hours at 37° C. The percentage reduction in plaque count is shown in the 

 table. 



The essential point to be noted is that SF/C is also resistant to the action of 

 phage C. SF/C is resistant to both C and C but differs from SF/C in being also 

 resistant to phage B. These reactions are paralleled by the phage absorption re- 

 sults obtained with dead bacteria. Serologically no difference could be detected 

 between SF and SF/C by cross absorption tests, but SF/C is serologically dis- 

 tinguishable from SF by the same technique (Burnet and McKie 1929). 



Most of the experiments to be described depend on the fact that organisms of 

 SF/C type are never produced by phage C, but when present can multiply norm- 

 ally in the presence of excess of this phage. This makes it possible to study fairly 

 closely the influence of phage C in converting SF organisms into the heritable 

 modification SF/C. 



The Production of SF/C by Phage C. 



It is obvious from the appearance of its plaques that phage C provokes resist- 

 ance with the greatest of ease. This can be established quantitatively in a number 

 of ways. If one plates a series of dilutions of SF culture on normal agar plates and 

 on plates spread with concentrated phage C, the number of colonies is identical 

 within the limits of experimental error in the two series, but on the phage spread 

 plates the colonies are all of SF/C type. 



In another type of experiment we have plated serial dilutions of C with SF 

 as in a plaque titration, but, instead of allowing plaques to form, the areas are re- 

 spread with strong phage C within half an hour of mixing C and SF. After in- 

 cubation we find that the number of resistant colonies on each area is roughly pro- 

 portional to the concentration of phage C, and in some experiments approximates 

 to the number of plaques obtained in a normal phage titration. A typical protocol 

 is as follows : 



Experiment J. Plates of nutrient agar were spread with 0-1 c.c. of a 4 hour broth culture 

 of SF. When these had dried, 02 c.c. quantities of serial dilutions of phage C in broth were 

 spread on sectors according to the usual technique in phage titrations. Fifteen minutes later 

 each sector was spread with a drop of undiluted phage C to cover the whole of the area on which 

 the C phage dilution had been placed. The plates were again allowed to dry, and incubated over 

 night. 



Table 2. 



Relation between Concentration of Phage C and the Numbers of Resistant 

 Colonies induced. 



212 



