158 CULTURE MEDIA 



The thorough study of a bacterium involves, therefore: 



1. Its isolation in pure culture. 



2. Its study with the microscope to determine morpho- 

 logical features and staining reactions. 



3. Growth on culture media for determining its physio- 

 logical activities as well as morphological characteristics of 

 the growths themselves. 



4. Animal inoculations in certain instances. 



5. Special serum reactions in some cases. 



Since isolation in pure culture requires material for grow- 

 ing the organism, the first subject to be considered is culture 

 media. 



A culture medium for a given bacterium should show the 

 following essentials : 



1. It must contain all the elements necessary for the 

 growth of the organism except those that may be obtained 

 from the surrounding atmosphere. 



2. These elements must be in a form available to the 

 organism. 



3. The medium must not be too dr}^ in order to furnish 

 sufficient moisture for growth and to prevent too great a 

 concentration of the difterent ingredients. 



4. The reaction must be adjusted to suit the particular 

 organism dealt with. 



5. There must be no injurious substances present in 

 concentration sufficient to inhibit the growth of the organ- 

 ism or to kill it. 



Ordinarily, more attention must be paid to the sources of 

 the two elements N and C than to the others, for in general 

 the substances used to furnish these two and the water 

 contain the other elements in sufficient amount. For very 

 exact work on the products of bacteria, synthetic media 

 containing definite amounts of chemicals of known com- 

 position have been prepared, but for most of the work 

 with bacteria pathogenic to animals such media are not 

 needed. 



Culture media may be either liquid or solid, or for certain 

 purposes may be liquid at higher temperatures and solid at 

 lower, as indicated later. Liquid media are of value for 



