158 CULTURE MEDIA 



organism as discussed in the chapters on Physiological 

 Activities of Bacteria. 



The thorough study of a bacterium involves, therefore: 

 its isolation in pure culture, its study with the microscope 

 to determine morphological features and special staining 

 reactions; growth on culture media for determining its 

 physiological activities, as well as the morphological charac- 

 teristics of the growths themselves; animal inoculations and 

 special serum reactions. Since isolation in pure culture 

 requires material for growing the organism, the first subject 

 to be considered is culture media. 



A culture medium for a given bacterium must not only 

 contain the elements necessary for its food, insofar as these 

 may not be derived from the air, but must contain these 

 elements in a form readily available to the bacterium. 

 Further, the medium must not be too dry in order to furnish 

 sufficient moisture for growth and to prevent too high a con- 

 centration of the different ingredients. The reaction must 

 be carefully adjusted to suit the particular organism dealt 

 with. There must be no injurious substances present. The 

 above are the chief points to be borne in mind in preparing 

 culture media. 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 

 composition 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 

 obtaining bacteria for the study of morphology and eel! 

 groupings and for ascertaining many of the physiological 

 activities of the organisms. Solid media are useful for study- 

 ing some few of the physiological activities and especially 

 for determining characteristic appearances of the isolated 

 growths of bacteria. These isolated growths of bacteria on 



