CHAPTER IV 



CULTIVATION AND STUDY OF BACTERIA, 

 YEASTS, AND MOLDS 



The studies of the botanist and the zoologist are more 

 larirrly confined to the structure of the individual than to 

 its physiology, to form rather than to function. The bac- 

 teriologist finds that little of practical importance can be 

 gained from such a study of the organisms in which he is 

 in it-rested. He must extend his observations to the part 

 they play in the decomposition of organic matter. The 

 bacteria, the yeasts, and the molds occur in nature in such 

 confusion and in such mixture of kinds that a study of them 

 in their native habitat is of no avail. Before one can learn 

 anything of the functional activities of a single kind, it 

 must be separated from all others, and it 'must be grown un- 

 der controlled conditions. This work involves the isolation 

 and the cultivation of pure cultures, that is, those that con- 

 tain but a single kind in which each cell is like every other 

 cell. 



Culture media. Almost any kind of organic matter will 

 serve .as food for some type of organism. Under laboratory 

 conditions it is desirable to use as few kinds of nutritive 

 materials as possible, and to have these supply conditions for 

 the growth of as many kinds of organisms as possible. 

 These nutritive materials, when used in the laboratory for 

 the growth and cultivation of microorganisms, are called 

 culture media (singular, medium). It is essential that all 

 or a portion of the material used be soluble in water, other- 

 wise it can not pass through the cell membrane and the 

 layer of cytoplasm lining the cell wall. It is essential that 



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