104 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Other insoluble carbohydrates may be substituted for 

 starch in culture media. Finely powdered cellulose or 

 hemicellulose, for example, may be used in agar to test the 

 ability of microorganisms to digest these substances. 



Digestion of Proteins. Many microorganisms have the 

 power of liquefying gelatin due to the production of an 

 enzyme called gelatinase. This is best observed in stab 

 cultures in test tubes. A few bacteria possess the power 

 of liquefying coagulated proteins, such as blood serum. 

 Some bacteria are able to digest casein in milk bringing 

 about a peptonization of this material. This latter phe- 

 nomenon may also be demonstrated by the use of a casein 

 agar, that is, nutrient agar to which casein has been added. 

 This medium is somewhat opaque in appearance. Those 

 organisms which digest the casein will clear the medium for 

 a certain distance around the individual colonies. 



Fixation of Atmospheric Nitrogen. Usually bacteria 

 which grow luxuriantly in a nutrient solution containing no 

 compounds of nitrogen are those which are capable of 

 securing their needed nitrogen from the atmosphere. 

 Quantitative tests for amount of nitrogen fixation can be 

 made only by chemical analyses of the culture medium 

 when the organism is well grown. In soil bacteriology it 

 is frequently customary to use the soil itself as a culture 

 medium, applying a suitable carbohydrate to supply growth 

 energy to the nitrogen fixing bacteria. Comparative analy- 

 ses of soils which have received a carbohydrate with others 

 kept under the same conditions but without this amend- 

 ment will indicate the amount of nitrogen fixed. 



Oxidation of Ammonia and of Nitrites. Certain soil 

 bacteria are capable of oxidizing ammonia to nitrites. 

 When they are grown in a suitable solution containing am- 

 monium sulphate without organic nitrogen, the develop- 

 ment of the nitrite may be observed by using the test above 

 indicated for the detection of nitrites. 



