80 



Microbes and You 



GELATIN VS. AGAR 



Of all the solidifying agents employed in microbiology, agar is 

 by far the most common. Robert Koch introduced gelatin as one 

 of the first substances, but because of undesirable characteristics 

 of this solidifying agent, it was replaced by agar. A brief com- 

 parison of gelatin and agar is presented in Table 7. 



Table 7 

 CHARACTERISTICS OF GELATIN AND AGAR 



1. Food value. 



2. Chemical composition. 



3. Melting point. 



4. Solidifying point. 



5. Other properties. 



GELATIN 



May be used by some 

 bacteria. 



An incomplete protein. 

 About 25° C. 

 About 23° C. 

 Forms no water of con- 

 densation. 

 Clear in appearance. 



AGAR 



No food value. 



A hemicellulose. 

 About 90° C. 

 About 40° C. 

 Produces water of con- 

 densation. 



Clear when melted, but 

 slightly opalescent upon 

 solidification. 



Agar is a complex sugar, a POLYSACCHAmDE, that for all practical 

 purposes is not attacked by organisms in their quest for food, but 

 gelatin is not very resistant to attack by microbial enzymes. If 

 gelatin is being employed as the solidifying agent in the attempted 

 isolation of bacterial colonies, and if the organisms are capable of 

 liquefying the gelatin through enzymatic activity, much of the 

 value of the gelatin has been sacrificed. It should be emphasized, 

 however, that nutrient gelatin serves as a useful medium in the 

 study of physiological reactions as part of the procedure in the 

 identification of organisms, since some organisms can attack gelatin 

 while others cannot. 



In the isolation, study, and identification of organisms derived 

 from warm-blooded animals, it is oftentimes imperative that the 

 cultures be incubated at body temperature or above. Since gelatin 

 becomes liquid without anv physiological activity when the temper- 

 ature approaches 25° C, it is obvious that this solidifying agent 



