44 



PHYSIOLOGY OF NUTRITION 



the simple structure of microorganisms (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 (Nitrobacter) 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 favor- 

 able 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 designated by Vinogradskii as the 

 method of "selective culture." A culture 

 is selective if it promotes only a certain 

 function, or if it promotes a function which 

 is as restricted as possible. The more 

 closely limited or exclusive are the con- 

 ditions, the more favorable will these con- 

 ditions be for one species possessing a 

 particular property or function, in con- 

 trast 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 microorganisms in their struggle for existence, we increase 

 their numbers in our cultures and 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 nutri- 

 ent 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, ioo g.; ammonium 

 tartrate, i 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. 



Fig. 20. — Various forms of bacteria. 



1 Pasteur, Louis, Memoire sur la fermentation alcoolique. Ann. chim et phys. Ill, 58: 323-426. 1 860. 



