CHAPTER IX 



METHODS OF DETEEMINING BIOLOGICAL ACTIVITIES OF BACTERIA 

 ANIMAL EXPERIMENTATION 



Gas Formation. Bacteria of many varieties produce gas from 

 the protein and the carbohydrate constituents of their environment. 



Gas formation can be observed in a very simple manner by mak- 

 ing stab cultures in gelatin or agar containing the fermentable 

 nutrient substances. In such cultures bubbles of gas will form along 

 the track of the inoculation, or, in the case of such semisolid media 

 as the tube medium of Hiss, will spread throughout the tube. In 

 the case of some anaerobes gas formation in stab cultures will occur 

 to such an extent that the medium will split and break. It should 

 be borne in mind in carrying out such methods that air is readily 

 carried into the medium with the inoculating needle or loop by 

 splitting of the medium, also that media which have been stored 

 in the cold may absorb air. Expansion of the air in such tubes 

 may simulate small amounts of gas formation and lead to error. 

 It is advisable, therefore, whenever making stab inoculations with 

 the above purpose, to heat the media and rapidly cool them before 

 use. 



A more accurate method of gas determination is by the use of 

 fermentation tubes, such as those devised by Smith. The gas which 

 is formed collects in the closed arm of the fermentation tube and 

 may be quantitatively estimated. The fermentation, with gas 

 production, of certain substances such as carbohydrates, may be 

 determined by adding these materials in a pure state to the media 

 before inoculation with organisms. 



In the case of carbohydrates this method has proved of great 

 differential value, since the power of splitting specific carbohydrates 

 with gas production is a species characteristic of great constancy 

 for many forms of bacteria. 



ANALYSIS OF GAS FORMED BY BACTERIA. Carbon Dioxide. Foi 

 the estimation both qualitatively and roughly quantitatively of car- 



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