Pure Cultures 9 



liquid may be separated from one another. He inocu- 

 lated liquefied portions of sterile gelatine with slight 

 quantities of material containing the bacteria which he 

 wished to study, distributed the latter uniformly by 

 shaking the liquid gelatine, and spread it out on a plate 

 where it was solidified by cooling. 



The germs fixed in isolated spots in the sheet of solid 

 gelatine could multiply only in those particular spots 

 until their numbers became so great as to form a little 

 heap or colony visible to the naked eye. These colonies, 

 each the offspring of a single cell, could furnish only one 

 species of bacteria. When transferred with a sterile 

 platinum needle to some liquid suitable for their de- 

 velopment, the organisms furnished a so-called pure 

 culture, that is, a growth of only one kind of bacteria, 

 uncontaminated by other organisms. When doubt still 

 existed as to the purity of the culture, new plates were 

 prepared from the purified material and the work re- 

 peated several times until, at last, the growth could 

 safely be regarded as pure. 



In the course of time, new culture media were de- 

 vised for organisms that would not grow on gelatine, — 

 culture media that made possible the isolation of soil 

 and water bacteria, bacteria of milk and of other food 

 products, and bacteria causing disease in plants. The 

 use of aniline dyes, proposed by Wygert and adopted 

 by Koch, made possible a differentiation of the cell 

 structure of the organisms, while the inoculation of 

 mixtures containing disease bacteria into experimental 

 animals, like rabbits or guinea pigs, offered a new means 

 for the^ identification and purification of disease germs. 



