ESMOND E. SNELL 



441 



metabolites, which by competing with the toxic metabolite will prevent 

 its toxic action within the cell and thus permit growth. 



We have seen that an organism may synthesize a growth factor in 

 one medium which it requires preformed in another. It is also true that 



100 



20 40 60 80 



fyllCROGRAMS OLEIC ACID PER 10 ML. 



100 



Figure 4. The comparative growth-promoting properties of oleic acid for 

 Lactobacillus bulgaricus in the presence (top curve) and absence (lower curve) 

 of Tween 40 (cf. 41). 



the magnitude of the requirement for an essential growth factor may 

 vary tremendously, depending upon the composition of the medium, 

 and under conditions such that the variation in the requirement cannot 

 be ascribed to variations in the amount of the growth factor synthesized. 

 Such variations frequently reflect the physiological function of the 

 vitamin, and though more experimental work should be done to estabhsh 



