FOOD OF MICROORGANISMS. 91 



compound that cannot be decomposed under certain conditions by certain 

 microorganisms. Even strong poisons like formaldehyde, and insoluble 

 compounds like paraffin, are known to serve as microbial food. 



NITROGENOUS FOOD COMPOUNDS. Nitrogen is an absolutely neces- 

 sary constituent of the protoplasm, and therefore indispensable to the 

 life of the cell. The amount of nitrogen compounds required for cell 

 construction is necessarily very small, because of the small size of the cell. 

 Green plants assimilate nitrogen only in the form of nitrates or ammonia; 

 animals require proteins, or peptones, and may occasionally live on 

 amido-acids. Nothing can be said about microorganisms in general, 

 since some require proteins, while others can feed on ammonia, nitrates 

 and even on free nitrogen. 



The few microorganisms assimilating the free nitrogen of the air are 

 very important since they increase the nitrogen content of the soil. They 

 must have some source of energy to form their protoplasm, and this is 

 supplied in the soil in the form of carbohydrates, organic acids, and 

 similar organic compounds. Unless some such compounds are available, 

 the nitrogen-fixing bacteria cannot grow. The amount of growth and 

 amount of nitrogen fixed depend mainly upon the amount of available food. 

 Nitrates are more commonly used by microorganisms as a source of nitro- 

 gen. They cannot serve as a source of energy, however, since they cannot 

 be decomposed in any way so as to yield energy. Many molds and some 

 bacteria can use nitrates. Ammonium salts are more readily used than 

 nitrates, and a large number of bacteria and yeasts and almost all molds 

 can use them in the formation of protoplasm. As a source of energy, 

 ammonium salts can be used only by the nitrate forming bacteria which 

 oxidize them to nitrates. The strictly organic nitrogen compounds may 

 be used as a nitrogen source and as an energy source at the same time. 

 This is true with the urea and with amido-acids. Many microorganisms 

 will thrive in a solution of asparagin [(CO 2 H.CH 2 .CH(NH 2 ).CO(NH 2 )]. 



With the amido-acids, we leave the chemically well-known compounds 

 and come to the very complex and chemically unknown peptones. 

 Peptones are a very good source of nitrogen and can be used as a source 

 of energy also by most organisms. Proteins are not quite so generally 

 used because many of them are insoluble. Many kinds of bacteria that 

 will grow in peptone solution do not liquefy gelatin. But a large num- 

 ber of microorganisms have very strong proteolytic (protein-dissolving) 

 qualities and use the protein to great advantage. Most molds and 



