220 



BACTERIOLOGY 



FIG. 37 



the first sterilization the small tube will be found almost 

 filled with fluid, over which a small air-bubble lies. After 

 the second or third sterilization this 

 air-bubble is completely expelled, and 

 the small tube contains nothing but 

 the liquid. 



The medium that Durham employs 

 for the fermentation test is a 1 per 

 cent, solution of Witte's peptone in 

 distilled water, to which have been 

 added known amounts of some such 

 fermentable sugar as glucose, saccha- 

 rose, lactose, mannite, etc., as the case 

 may demand. He prefers peptone to 

 meat-infusion bouillon for the reason 

 that the latter often contains traces 

 of muscle-sugar, and is thereby likely 

 to complicate the results. He prefers 

 neutralization with organic acids rather 

 than mineral acids, and uses citric acid 

 by preference, the reason for this being 

 that where sugars such as those men- 

 tioned are acted upon by mineral 

 acids under the influence of heat their 

 composition is apt to be altered. 



Durham's fermentation 

 tube. 



NOTE. Prepare two fermentation 

 tubes as follows: Fill one with 1 per 

 cent, watery solution of peptone to 

 which 2 per cent, of glucose has been 

 added; fill the other with a similar peptone solution, but 

 to which only 0.3 per cent, of glucose has been added. 



