53 



bacteria in each preparation and make a drawing of a few 

 of them. 



Note the appearance of the cultures inoculated for the 

 study of the gas production. 



Read carefully in the text-books the methods for culti- 

 vating anaerobic bacteria. 



EXERCISE XXII. 

 STUDY OF THE GAS PRODUCTION BY BACTERIA. 



§82. Explanatory note. The knowledge of the powers 

 of a given species of bacteria to produce gas when grown in 

 a medium containing .sugar is quite important. It is desir- 

 able to determine both the quantity of gas and its relative 

 composition. Chemical analyses have shown that, in all 

 cases tested, the gas con.sists of a mixture of hydrogen (H) 

 and carbonic acid gas (CO.^) with mere traces of other gases. 

 It is importart to know also the quantity of gas produced 

 with the various sugars especially glucose, lactose and sac- 

 charose. To determine simply whether an organism will 

 produce gas it is onl}' nece.ssary to inoculate a tube of liquid 

 agar containing the sugars with it, but if the quantity of gas 

 is to be determined the fermentation tube is the most con- 

 venient apparatus to use. In some cases the gas formation 

 is one of the most striking differential properties as will be 

 seen in the study of hog-cholera and typhoid bacilli. (For 

 a discussion of the gas production and use of the fermenta- 

 tion tube .see article by Dr. Theobald Smith, Wilder Quarter- 

 century book p. 187). 



§ 83. General directions. Examine and describe the 

 cultures in the glucose and plain agar inoculated in Exercise 

 XX. Note the approximate size and number of gas bubbles 

 in the glucose agar, and explain the cause of difference in 

 the number of bubbles in the two agar cultures. 



