BACTERIOLOGICAL TECHNIQUE. 215 



Fermentation broth is made usually by adding 1 per 

 cent, of glucose to the above. For accurate work the 

 meat sugars are first extracted by allowing the colon 

 bacillus to grow in the broth over night. The bouillon 

 is then sterilized and the peptone and salt added, and 

 the process already given gone through with. 



Fermentation bouillon is usually placed in a tube of 

 special construction, known as a fermentation tube (see 

 Fig. 14, p. 82). This is essentially a tube 1.5 cm. in 

 diameter, bent at an acute angle, closed at one end, and 

 provided with a bulb at the other end, which latter 

 should be large enough to receive all the fluid in the 

 closed branch should gas in any considerable quantity 

 collect there. The tube also serves a most important end 

 in giving information as to the aerobic and anaerobic 

 growth of the species under consideration, for the con- 

 necting tube being constricted serves to prevent, to a 

 great degree, the entrance of oxygen of the air into the 

 closed branch, and the free oxygen in the medium is 

 driven out by the heat during sterilization; from which 

 it may be seen that growth in the bulb is aerobic and 

 growth in the closed branch is anaerobic. For the study 

 of fermentation alone small tubes may be inverted into 

 larger ones or tubes may be bent on themselves. 



Nutrient Gelatin. To the bouillon already prepared 

 as described add 10 per cent, of sheet gelatin and neu- 

 tralize. Add the whites of two eggs for each litre and 

 boil for a few minutes. Filter, place in tubes or flasks, 

 and sterilize. Instead of adding gelatin to bouillon 

 already prepared, it may be added to the meat infusion 

 at the same time the peptone and salt were added in 

 preparing nutrient bouillon as just described. 



Nutrient Agar. This is prepared by adding to stock 



