CULTURAL TECHNIQUE 35 



to contamination from the air and from the surrounding 

 objects. All operations must be carried out in such a way 

 as to permit no contamination to take place. In inoculating 

 a culture tube the cotton plug must be removed and held 

 in such a way that the portion which is inserted into the 

 tube shall not come in contact with any object. The tube 

 or tubes should be held as illustrated in Fig. 8. If any 

 cotton adheres to the mouth of the tube, it should be re- 

 moved by passing the tube through the flame of a Bunsen 

 burner. The tubes should be kept open no longer than neces- 

 sary, and the needle sterilized just before using. 



Streak cultures. Agar may be allowed to solidify with the 

 tubes in an inclined position, thus giving a large sloping sur- 

 face that is inoculated by drawing over it in a single stroke 

 the infected needle or loop. Care should be used not to cut 

 or break the surface of the medium. Potato, blood serum, and 

 egg are used in the same manner. Gelatin can be used in 

 this way for organisms that are unable to liquefy it. 



Study of test-tube cultures. The following points should 

 be noted in the tube cultures; 



Nutrient broth : conditions of fluid, whether clear or 

 turbid ; character of turbidity ; amount of sediment ; surface 

 pellicle; odor. 



Gelatin stab cultures : character of growth along line of in- 

 oculation ; surface growth ; extent of liquefaction ; shape of 

 liquefied area. 



Agar stab cultures : character of growth along line of in- 

 oculation ; surface growth. 



Streak cultures : form ; size ; surface elevation ; topography 

 of surface ; color ; consistency ; odor ; luster ; change hi medium. 



For list and definition of the terms employed in the de- 

 scription of the cultural characters of an organism, see Frost's 



