42 MICRO-ORGANISMS AND FERMENTATION. 



For moulds, in addition to beer-wort, fruit decoctions 

 and sugar solutions containing tartaric acid and tartrates 

 are used, or, again, the complicated Raulin's liquid, which is 

 also applicable to bacteria, and consists of water 1,500 c.c., 

 sugar 70 grammes, tartaric acid 4 grammes, ammonium 

 nitrate 4 grammes, ammonium phosphate 0-6 gramme, potas- 

 sium carbonate 0-6 gramme, magnesium carbonate 0-4 gramme, 

 ammonium sulphate 0-25 gramme, zinc sulphate 0-07 gramme, 

 ferrous sulphate 0-07 gramme, potassium silicate 0-07 gramme. 



If solid nutrients are required, 5 to 10 per cent, of gelatine is 

 added, or, in the case of cultures which are to be developed 

 at or above 30 C., about 1 per cent, of agar-agar, a jelly 

 derived' from salt water algae. For the cultivation of ther- 

 mophilous bacteria at 60 to 70 C., Miquel uses carragheen 

 moss instead of agar, in the proportion of 2 to 3| per cent. 

 Slices of potato sterilised in an autoclave are often used as 

 a solid nutrient. Black bread makes an excellent solid sub- 

 stratum for moulds. For the cultivation of the nitrifying 

 bacteria Winogradsky and Omelianski used gelatinous silicic 

 acid. 



For plate cultures of acid-forming bacteria (lactic acid and 

 acetic acid bacteria) some litmus or, preferably, according to 

 Beijerinck, carbonate of lime (finely precipitated chalk) is 

 added. The gelatine thus acquires a motley appearance, 

 but the colonies of acid bacteria are surrounded by a clear 

 zone, because the acid dissolves the chalk. By the use of 

 zinc carbonate in plate-cultures, the acetic acid bacteria 

 form colonies and display clear zones, whereas the lactic acid 

 bacteria are relatively sensitive to this salt, and their growth 

 is inhibited. 



Pasteur used liquids exclusively for his work on the 

 ferments. Later, solid media became of great import- 

 ance, and Koch laid the foundation for their application. 



Plate-cultures are prepared by introducing the growth into 

 the liquefied gelatine, and then pouring the mixture into a 

 Petri dish. When the gelatine solidifies the individuals are 

 separated throughout the mass, and, on development, they 

 appear as colonies, visible to the naked eye. Streak-cultures 

 are those in which a minute portion of the growth is intro- 



