CULTURE METHODS 43 



tube. The whole operation is carried out as quickly as 

 possible, so as to reduce the chance of outside contamina- 

 tion to a minimum. 



* Stab ' Cultures. A trace of the growth is picked up 

 on the tip of a platinum needle, which is then thrust into 

 the depth of a tube containing about 10 c.c. of nutrient 

 medium solidified in the upright position, care being 

 taken to introduce the wire in a central line and in a direc- 

 tion parallel with the sides of the tube. The tube may be 

 held and the manipulations carried out in the same manner 

 as described for a ' streak ' culture. 



* Stab ' and ' streak ' cultures may be made in the same 

 tube, if this be filled and ' slanted ' in such a manner that 

 the slant ends halfway down it. 



To show the production of gas, stab cultures in glucose 

 agar, shake cultures in gelatin, or cultures in Durham's 

 fermentation tubes (see p. 37), may be employed. 



' Shake. ' Cultures. A tube of gelatin is liquefied by 

 heating the tube in a beaker of water at 40 C. The 

 medium is then inoculated with the organism under 

 examination. The plug is replaced, and the contents of 

 the tube gently mixed to distribute the organisms evenly 

 through the medium, care being taken not to allow any of 

 the gelatin to touch the cotton-wool plug. The contents 

 of the tube are allowed to set in cold water. The presence 

 of organisms capable of producing gas in this medium is 

 shown by the formation of bubbles. At the same time, 

 another tube of the medium is inoculated with an organism 

 known to produce gas as a control. This prevents an 

 error of registering this attribute as negative when really 

 non-production of gas is due to the use of unsuitable 

 medium. The test sometimes fails with gelatin made with 

 meat extract. 



Anaerobic Cultures. Most anaerobes will grow in a 

 deep stab in glucose agar or gelatin. The tube, three- 

 quarters full of medium, is kept in boiling water for five 

 minutes and then cooled. Dissolved oxygen is thus ex- 

 pelled and the medium softened. After the stab is made, 

 the top portion of the medium is gently warmed to seal 

 the top of the needle track. 



Air may be excluded from a culture in a fluid medium 

 by running a layer of sterile vaseline or olive oil (about a 

 centimetre thick) on to the medium. 



