ASSIMILATION OF CARBON 43 



(more properly, just because of this very simplicity) they usually exhibit far- 

 reaching physiological peculiarities. Each one of these organisms carries out its 

 own little work, but it constitutes a very important link in the processes of nature. 

 For example, the presence of two kinds of bacteria appears to be requisite for the 

 oxidation into nitric acid of the ammonia present in the soil. One of these 

 (Nitrosomonas) carries the oxidation as far as nitrous acid, the other (Nitro- 

 bacter) oxidizes this to nitric acid. Ammonia is essential as nutrient material 

 for the first form and nitrous acid is a waste. But this by-product constitutes 

 an essential food substance for the other form. Is it possible, then, to conceive 

 of some nutrient medium that would be equally well suited for the nutrition of 

 both these bacteria? This question must receive a negative answer; a medium 

 must be used that is favorable only to the microorganism under investigation, 

 and that is especially adapted to its particular requirements. The use of such 

 media is highly important if pure cultures are desired. This use has been desig- 

 nated by Vinogradskii as the method of "selective culture." A culture is 

 selective if it promotes only a certain func- 

 tion, or if it promotes a function which is 

 as restricted as possible. The more closely 

 limited or exclusive are the conditions, the 

 more favorable will these conditions be for 

 one species possessing a particular property 

 or function, in contrast to others not so 

 endowed, and the growth of these latter 

 in a medium thus alien to them will be 

 quite impossible or at least very difficult. 

 In thus assisting the desired microorgan- 

 isms in their struggle for existence, we in- 

 crease their numbers in our cultures and Fig. 20.— Various forms of bacteria. 



thereby render their discovery easier. When a specific bacterium has once been 

 found, it is thus usually possible to discover also the method by which it may 

 be isolated in pure culture. On this general principle is based the now frequent 

 employment of many different kinds of nutrient substrata, both liquid and solid. 

 The first attempt to prepare an artificial nutrient medium for microorganisms, 

 was made by Pasteur, 1 whose solution for the culture of yeast had the following 

 composition: water, 100 g.; ammonium tartrate, 1 g.; saccharose, 10 g.; and 

 yeast ash, 0.075 g. 



Meat extract is used most commonly for the culture of bacteria (Fig. 20). 

 The addition of gelatine to peptone bouillon (10 per cent, of gelatine in winter 

 and 15 per cent, in summer) produces a solid substratum. Agar-agar may be 

 used instead of gelatine. Besides the various kinds of meat extracts, milk, 

 blood serum, yeast water, beer-wort and other similar materials may be used. 

 Among other things, cylinders cut from potato tubers may be employed as 

 solid media. 



'Pasteur, Louis, Memoire sur la fermentation alcoolique. Ann. chim. et phys. ///. 58: 323-426. 

 i860. 



