BACTERIOPHAGE MUTATION 



33 



that all the published experimental data are compatible with the hyp thesis that 

 increase in amount of phage results only from the disruption of infect d bacteria. 

 A priori it seemed likely that if there were any exceptions to this rule t\e action of 

 phage C on SF would be one of them. Phage C on the other hand was obviously 

 a typical bacteriophage — producing only about one resistant colony per million 

 bacteria lysed — and was known f rOm previous experiments to multiply in normal 

 fashion. A series of experiments was therefore carried out on the f unctbnal activi- 

 ties of phage C, mostly in comparison with C, in order to discover whether it 

 showed any fundamental peculiarities of behaviour. 



In their general intrinsic qualities the two phages are practically identical ; 

 serologically they are indistinguishable. Both are relatively resistant to the photo- 

 dynamic action of methylene blue and to inactivation by strong urea solution, and 

 both are unaffected by citrate in the medium. Functionally, too, there is extremely 

 little difference between the two phages, apart from the greater readiness of C to 

 produce resistant variants. On agar, plaques of each type can just be distinguished 

 about 2 hours after spreading. The two types develop at the same rate, and the 

 central bacterial growth of C is only clearly visible after about 5 hours. When 

 lysis by the two phages is studied by direct microscopic observation on the agar 

 surface, in each case first lysis is observed about 1J-1^ hours after mixing. With 

 C, many more bacteria give rise to microcolonies than with C, but typical disap- 

 pearance of the bacterium by lysis is readily seen, and appears to be quite similar 

 to that induced by C. 



Multiplication of phage C in broth follows the ordinary course, with, however, 

 a longer initial lag than is found with dysentery phages. 



Experiment 2. A tube containing a growing broth culture of SF was seeded with sufficient 

 phage to give 2 or 3 plaques when 0-02 c.c. was plated. It was maintained at 37 °C. and at suit- 

 able intervals eight such volumes were plated on agar. The plaque counts were as tabulated 

 (Table IV), each figure being the average of eight counts. 



Table 4. 

 Multiplication of Phage C in Broth at 37" C. 



Immediately. 30 min. 

 Average plaque count 2-0 1-75 



45 min. 

 2-4 



U min. 

 2-25 



75 min. 90 min. 

 23-0 160 



Using the method of single particle multiplication (Burnet 1934) the same 

 explosive increase occurred as is characteristic of all phages which have been 



EXPLANATION OF TEXT FIGURES. 



Fig. 1. Plate inoculated with SF and spread with a dilute mixture of phages C and C. 

 The plaques produced by phage C show central resistant growth while those of phage C have 

 a clear centre. 



Figs. 2, 3, and 4. Plates spread with 0-1 c.c. of 24 hour broth culture of SF diluted 1 : 100. 

 After drying, 02 c.c. of the phages shown was spread over the central area. Fig. 2. Phage C 

 undiluted stock filtrate. Fig 3. Phage C undiluted stock filtrate. Fig. 4. A mixture of equal 

 parts of phages C and C. 



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