Discussion 



137 



butanol again. Simultaneously the culture was inoculated into the media 

 with different doses of butanol, to show the degree of resistance developed 

 by it. 



The stock culture could not survive a butanol concentration of more 

 than 0-8 per cent, the adapted one survived a 2-5 per cent butanol 

 concentration and propagated normally at a 2 per cent butanol concen- 

 tration. The experiment took 200 days, which is equivalent to 4,300 



ID 20 20f I 



Sporosfromxy?. / /pin)re5fromgPu.med. 

 x^e. // xyeosQ 



Z- . 



20 30 AO 50 60 hr 



Fig. 1. Adaptation of CI. aceto-butylicum to 

 fermentation of xylose. 



successive cellular generations. Fig. 2 shows that the resistance to 

 butanol increased gradually together with the increase in its concen- 

 tration. No abrupt increase was observed, which might be expected had 

 spontaneous mutations appeared in the population. At the same time 

 the conditions of the experiment were extremely favourable for the 

 selection of resistant mutants. A very simple calculation shows that 

 had there been only one mutant per million cells, the coefficients of 

 propagation of which would exceed that of original cells by a mere 0-1, 

 then after 200 generations, i.e. in approximately 8-9 days, the number 



