70 EXPERIMENTAL DAIRY BACTERIOLOGY 



by the organism with the production of gas, the medium will 

 be rent by the gas. Tubes of broth can also be used for the 

 qualitative determination of gas by placing in the tube before 

 sterilization a small piece of glass tubing sealed at the upper 

 end. In the sterilization the air is completely driven out and 

 the small tube filled with the broth. If gas is formed in the 

 developing culture, bubbles will collect in the small tube. 



When it is desired to determine whether or not an organ- 

 ism is able to ferment a certain sugar, it is essential that the 

 medium to which the sugar is added be free from sugar. In 

 media prepared from meat or beef extract, a small amount of 

 muscle sugar (inosite) is present. This can be removed by 

 inoculating the meat infusion, after straining, with a culture 

 of an organism able to ferment inosite (B. coli commuriis), and 

 incubating for twelve to sixteen hours, or until all the sugar 

 is decomposed. The medium is then finished in the usual 

 manner. 



In the testing of sugars easily affected by heat, it is essen- 

 tial that the sterilization be as short as possible, otherwise 

 the sugar may be decomposed into its constituent sugars and 

 these be then fermented. Saccharose in slightly acid solu- 

 tions is easily inverted, lactose far less easily. 



Exercise. Inoculate fermentation tubes filled with lactose brotli 

 from the cultures furnished. Determine the amount and composition 

 of the gas produced (see p. 69). Also inoculate tubes of melted lac- 

 tose agar from the various cultures. 



Action on nitrates. Many organisms have the power of 

 reducing nitrates with the formation of nitrites, and some- 

 times ammonia and free nitrogen. Nitrate broth is prepared 

 by dissolving 1 gram of peptone in 1 liter of tap water and 

 adding 0.2 grams of nitrite-free potassium nitrate. Tube 

 and sterilize in the usual way. Inoculate and incubate for 



