Microbes Must Eat 81 



cannot be used in such a medium. Some bacteria, includinsf those 

 which Hve and thrive in such locations as hot springs, cannot be 

 cultivated unless the optimum temperature for the growth is pro- 

 vided. This may mean incubation of the cultures at 50° C. or 

 higher. With agar as a base, the medium can be relied upon to 

 remain solid even at these elevated temperatures, since agar 

 normally does not melt until the temperature approaches 90° C. 

 Gelatin has a narrow differential between the melting point and 

 the solidifying point, about 2 degrees, while agar has a 50-degree 

 spread, which is an advantage under some circumstances. 



When incubating cultures in special plates called fetri dishes, 

 such plates must be inverted if agar is the solidifying agent. The 

 water is not bound the way it is in gelatin media, and the heat of 

 the incubator allows moisture to escape from agar media and to 

 condense on the lid of the petri dish. When the drops of con- 

 densed moisture become large enough, they can drop onto the agar 

 surface and link up the growths from well-isolated colonies, 

 nullifying the otherwise effective isolation procedure. Gelatin does 

 not produce water of condensation, and hence it does not require 

 incubation in an inverted position. In fact, gelatin plates should 

 be kept upright in the incubator because of the liquefying ability 

 of some organisms. The slight opalescence imparted to solidified 

 agar does not interfere with its usefulness in distinguishing col- 

 onies of organisms growing either in or on the medium, while the 

 clarity of gelatin is completelv overshadowed by the serious dis- 

 advantages previouslv discussed. 



CLASSIFICATION OF MEDIA 



We can conveniently arrange the types of media under four 

 major headings: 



I. Natural Media— Substances occurring in nature. 

 1. Milk. 

 2 Eo-cTs 



3. Blood and other body fluids and tissues. 



4. Extracts of plant and animal tissues. 



