CULTURAL METHODS 75 



the animal body. It is apparent, therefore, that media of 

 many different kinds will be found useful in the laboratory. 



Several steps are common to the preparation of all kinds 

 of media. First, there must be the right mixture of the 

 necessary nutrients. Second, these must be present in the 

 right concentration in water. Third, certain substances 

 may or may not be added to cause the medium to be solid or 

 liquid as required. Fourth, the reaction must be adjusted 

 when necessary. Fifth, the medium should be placed in 

 suitable containers. Sixth, it should be sterilized. 



Nutrient Substances Used in Preparation of Media. — A 

 few microorganisms, some of them of considerable economic 

 importance, do not require any organic material for food, 

 that is, they are able by oxidation of certain inorganic sub- 

 stances to secure the energy necessary for them to build up 

 complex organic substances from inorganic. In preparing 

 media for such organisms, therefore, it is necessary to use 

 certain inorganic salts. In most instances some salt of nitro- 

 gen, either ammonia or nitrous acid, is necessary, likewise 

 some source from which the carbon can be secured, usually 

 a carbonate or carbon dioxide. Most organisms are stimu- 

 lated in growth by the presence of phosphate. As an exam- 

 ple of such a simple nutrient medium may be cited one 

 useful in cultivating from the soil the bacteria capable of 

 oxidizing ammonia to nitrous acid. Such a medium con- 

 tains ammonium sulphate, calcium or magnesium carbonate, 

 usually a little phosphate and traces of certain other salts, 

 but no organic matter. 



Most of the common bacteria and those usually cultivated 

 in the laboratory require more or less organic material as 

 food. Some forms will utilize inorganic nitrogen providing 

 they have organic carbon compounds. They will, for exam- 

 ple, take nitrogen from ammonia or from nitrates providing 

 carbohydrates are furnished. Some may even utilize free 

 nitrogen from the air. Other bacteria require organic 



