INDUCED MUTATIONS IN E. COLI 



261 



resistant to Tl and sensitive to T2, and were there- 

 fore considered to be true B/r/1 mutants. Two 

 colonies were resistant to both phage strains, and 

 were regarded as contaminants (in both cases the 

 colony morphology alone rendered them suspect). 

 The third colony was sensitive to both Tl and T2, 



Table 3. Test for Differential Killing of Strain B/r 



and B/r/1 by 3-Hour Exposure to 5% 



Sodium Desoxycholate 



Control = mixed culture in distilled water 



Experimental mixed culture in 5% 



Sodium Desoxycholate 



tially the same methods as those described in the 

 sample experiment above. Wherever possible, a 

 single experimental tube was incubated and sampled 

 over the entire 10-hour period, since the use of 



and since this colony appeared on the edge of the 

 plate, it is likely that it arose from a sensitive 

 bacterium which escaped lysis due to faulty spread- 

 ing of the phage suspension. 



Relation between the number of induced muta- 

 tions and time of exposure to 5% desoxycholate. 

 Demerec and Latarjet (i Q 46), in their studies of 

 radiation-induced mutations to phage-resistance, 

 investigated the relation between dosage of radia- 

 tion and the number of mutations induced. They 

 found that the number of zero point mutations to 

 B/r/1 is directly proportional to X-ray dose, and 

 bears a more complicated exponential relation to 

 ultraviolet dose. In the present investigations, ex- 

 periments were conducted to determine the 

 analogous relation between the number of induced 

 mutations and time of exposure to a 5% solution 

 of sodium desoxycholate. 



Fig. 2 shows a survival curve obtained with 

 sodium desoxycholate, in which the percentage of 

 surviving bacteria is plotted against time of ex- 

 posure to the standard concentration. Fig. 3 shows 

 the number of zero point mutations, corrected for 

 the background number, as a function of time of 

 exposure to 5% desoxycholate. The data upon 

 which this curve is based were obtained by essen- 



2 4 6 8 10 



HOURS EXPOSED TO 5% 

 SODIUM DESOXYCHOLATE 



Fig. 2. Survival curve of resting bacteria exposed to 5% 

 Sodium Desoxycholate. 



separate tubes for each determination gave some- 

 what more variable results. The linear relation be- 

 tween induced mutations and exposure time can be 

 compared directly with the X-ray dosage-effect 



<£ X-RAY DOSE IN ROENTGEN UNITS XIO" 4 

 2.5 5 7.5 10 



2 4 6 8 10 12 



HOURS EXPOSED TO 5% SODIUM 

 DESOXYCHOLATE 



Fig. 3. Relation between the number of zero point muta- 

 tions induced and time of exposure to 5% sodium desoxy- 

 cholate. The upper abscissa represents X-ray dose in roent- 

 gen units, on a scale required to make this curve coincide 

 with the curve obtained by Demerec and Latarjet (1946) 

 for the relation between X-ray dose and number of zero 

 point mutations. 



124 



