486 BACTERIOLOGIC TECHNIC. 



2. The Employment of the Different Nutrient Media 

 Depends upon the Following Points of View : 



I. Fluids (Bouillon, Sugar-bouillon, Milk, Non-albuminous 

 Nutrient Media) 



are employed: 



1. To produce culture en masse. 



2. To obtain suspensions of bacteria in which the number can be 

 accurately determined (counting by means of plates). 



3. For the study of the formation of pellicles and sediments. 



4. For the study of the metabolic products (compare p. 58 and 

 what follows). 



II. Solid Nutrient Media. 



i. Gelatinous Nutrient Media. — The gelatinous, transparent nu- 

 trient media (agar and gelatin) are most extensively employed for the 

 following reasons: 



(a) They may be employed as fluid and solid nutrient media: as 

 fluids, allowing a separation of the bacteria; and as solid substances, 

 a fixing of the isolated germs and their separate growth into colonies. 



(b) On account of their transparency they allow a macroscopic as 

 well as microscopic observation of the cultures; they allow a thorough 

 differential diagnosis of varieties and an early recognition of any con- 

 taminations. 



They are especially used: (a) For plate cultures — i. e., for demon- 

 stration, for accurate separation and counting of the individuals and 

 varieties. 



(6) For obtaining characteristic, macroscopic cultures which serve 

 in differential diagnosis. 



(e) For permanent cultures, or collections of living bacteria. 



The special advantages of agar and gelatin are: 



(a) Gelatin. — Advantages: Easily prepared, readily made into 

 plates (at 25°) ; the property of being liquefied by many bacteria is of 

 great diagnostic value. Disadvantages : Since it melts at 25° it cannot 

 be used in hot weather nor at incubator temperature. 



(6) Agar. — Advantages: It may be used at incubator temperature 

 (i. e.j for the rapid growth of bacteria — spores — and especially thermo- 

 philic bacteria) . Disadvantages : Difficulty of preparation, more diffi- 

 cult to make plates from (the agar, melted at 80°, must be cooled 

 to 40° before being inoculated ) . Colonies are often not characteristic. 



2. Blood-serum, glycerin-agar, and glycerin-ascites-agar. — 

 Employed especially for growing pathogenic varieties, which thrive 

 poorly or not at all on other nutrient media. It is only possible to 

 make plate cultures with glycerin-agar and mixtures of agar and serum. 



3. Potato. — (a) To obtain macroscopically characteristic cultures 

 of great durability and for differential diagnosis. 



(b) Sometimes for spore-formation. 



