IDENTIFICATION OF BACTERIA 61 



tube (Fig. 11). The tube is filled beyond the bend with 

 the colored sugar medium and sterilized by the discon- 

 tinuous method. After inoculation it is incubated, and 

 if the organism is capable of producing gas from the 

 sugar present the gas will be found to have accumulated 

 in the closed arm and the medium displaced into the 

 bulb. If after twenty-four to forty-eight hours the 

 column of gas no longer increases the tube may be 

 removed and the amount noted. The gas produced is 

 mainly carbon dioxide and hydrogen. A rough esti- 

 mate of the percentage of each may be made by first 

 marking the tube at the line of displaced medium, then 

 filling the bulb with a solution of caustic soda. The 

 cotton plug is replaced by a rubber stopper and the 

 tube is inverted several times ; on placing the tube in 

 an upright position the remaining gas will again collect 

 in the closed arm and that absorbed (CO 2 ) may be 

 roughly estimated. The gas still remaining will be 

 hydrogen; if the tube be inverted so that it is forced 

 into the bulb the fact may be ascertained by exploding 

 it with a lighted match. 



C. Animal Inoculation. Certain organisms produce char- 

 acteristic lesions in definite animal species, which are often of great 

 value in identifying the organisms in question. Moreover, the 

 inoculation of susceptible animals with contaminated material 

 often facilitates the recovery of pathogenic bacteria in pure cul- 

 ture from the lesions produced. 



The animals most frequently used for experimental purposes 

 are mice, rats, guinea pigs, and rabbits, the choice depending 

 mainly on the purpose to be served. Inoculations are made as a 

 rule by means of a sterile hypodermic syringe and needle. The 

 material used may be a discharge such as pus, the juice of organs, 

 or a culture of bacteria ; if the latter has been growing on solid 

 medium several loopfuls are emulsified in a small quantity of 

 sterile broth or physiological salt solution. 



