34 GROWTH OF BACTERIA 



phosphates, nitrates, and other inorganic salts of these 

 metals. The sulphates and phosphates provide sulphur 

 and phosphorus respectively, and both elements are also 

 derived in part from proteins and other organic compounds. 



Carbon can be obtained from a number of substances. 

 The nitrifying organisms are able to procure what they 

 require from the carbon dioxide of the atmosphere, and 

 it is probable that certain sulphur bacteria use this gas 

 as a source of carbon also. Many organic carbon com- 

 pounds, such as lactic, acetic, tartaric, citric, and other 

 acids or their salts, as well as sugars, alcohols, and their 

 allies, provide carbon ; additional sources are proteoses, 

 peptones, and other proteins. 



The element nitrogen is obtained by Clostridium 

 pasteurianum and species of Azotobacter from the free 

 nitrogen of the air, and the nodule organisms associated 

 with the roots of leguminous plants can also make use 

 of free uncombined nitrogen. Bs. subtilis, Bact.fluorescens, 

 and many other kinds can obtain it from nitrates, 

 especially if the carbon which they need is supplied in 

 the form of sugar. Some species use ammonium com- 

 pounds, while the majority procure their necessary 

 nitrogen from more complex nitrogenous organic com- 

 pounds such as the amino-acids and proteins. 



For the artificial nutrition of bacteria various liquid 

 media are employed, such as milk, whey, beef-broth, 

 blood-serum, and decoctions prepared from various 

 animal and vegetable substances. Solid jelly-like media 

 are also extensively used ; these consist of the liquids 

 just mentioned, or solutions of peptone, sugar, and other 

 nutrient materials, to which is added a certain amount 

 of gelatine or agar-agar. The preparation and use of 

 culture media are described in Chapter IV, 



