6 A MANUAL OF BACTEEIOLOGY 



on which the orgEuaism is found to grow in nature. For the 

 study of pathogenic bacteria investigators make use of meat 

 extracts, blood serum, etc. Beef extract in some form is the 

 basis of nutrient media for many bacteria, since it furnishes car- 

 bon and nitrogen in such suitable combinations. By the addition 

 of agar-agar or of gelatin we obtain a solid medium which may 

 be easily liquefied by heat. 



Agar-agar is a colloidal substance prepared from seaweeds 

 growing on islands in the Pacific Ocean. Agar is liquefied at 

 about 90° C. and hardens at about 45° C. ; it can therefore be 

 incubated at 87.5° C. without danger of liquefaction. Agar media 

 are not usually affected by the peptonizing action of bacteria. 

 Few instances have been reported in which agar media were 

 liquefied by bacteria. 



Gelatin is a colloidal substance of animal origin. It consists 

 chiefly, of chondrin and gluten and is prepared from bone, ten- 

 don, and hide. Gelatin media are liquefied at 35° C. and harden 

 at about 24° C. 



Gelatin media are better suited for the growth of many bac- 

 teria than agar media, especially for bacteria which are natives 

 of the bodies of warm-blooded animals. 



The peptonizing enzymes produced by many bacteria and 

 fungi liquefy gelatin, each in its characteristic manner. The type 

 of liquefication caused by bacteria in stab cultures is used as a 

 diagnostic character. 



We are indebted to Robert Koch for the introduction of 

 gelatin and agar-agar into bacteriological work. 



Exercise 7. Preparation of Nutrient Agar 



1. Place in a clean beaker or granite-ware pitcher the follow- 

 ing substances, in the order named. Two students may cooperate 

 to make double the quantity named. 



500 cc. . Distilled water 



2 g Beef Extract (Liebig's) 



6 g Peptone (Witte) 



2 g Sodium chloride 



10 g Agar-agar 



