156 



M. J. Thornley, J. Sinai and J. Yudkin 



B 



2345678 56789 789 



Time (days) 

 Fig. 8. Induction to proflavine resistance by 2 (i,g./ml. proflavine in a non- 

 dividing suspension of Esch. coli R^. Samples of the cells washed and resuspend- 

 ed in buffer and tap water without proflavine at different periods during 9 days 

 of incubation at 37°C. The resuspended cells incubated further at 37°C to 

 complete the 9 days incubation period. 



A — suspension incubated 1 day in presence of proflavine and 8 days in absence 

 B — suspension incubated 2 days in presence of proflavine and 7 days in absence 

 C — suspension incubated 5 days in presence of proflavine and 4 days in absence 

 D — suspension incubated 7 days in presence of proflavine and 2 days in absence 

 1 — total viable count 

 2 — cells resistant to 20 [i.g./ml. proflavine 

 3 — cells resistant to 50 [jtg./ml. proflavine 

 4 — cells resistant to 100 (Jig./ml. proflavine 



Time from beginning of incubation of the induction suspension. 



In similar experiments, we were also able to induce to strep- 

 tomycin resistance but not to chloramphenicol resistance. 



Demonstration and isolation of pre-existing mutants 



The fluctuation test of Luria and Delbriick (1943) was 

 performed on the sensitive strain of Esch. coli. We found a 

 significantly higher variance in the number of colonies on 

 proflavine plates arising from several small cultures than in 

 the number arising from one larger culture. This suggests 

 that there is a spontaneous mutation in the culture to pro- 

 flavine resistance. 



