48 LABORATORY BACTERIOLOGY 



EXERCISE XII 



THE EXAMINATION OF CULTURES ON SPECIAL MEDIA, 

 WITH A STUDY OF THE GAS PRODUCTIO*N 



74. As certain of these media are used to determine the 

 effect of the bacteria upon them, it is important to observe 

 very carefully not only the appearance of the growth of the 

 bacteria but also their effect, if any, upon the medium on 

 or in which they are growing. This is especially noticeable 

 in the milk, litmus milk, and sugar bouillon cultures. The 

 changes here are largely due to the action of the bacteria on 

 the sugars or their power to produce alkali. 



The knowledge of the powers of a given species of bacteria 

 to produce gas when grown in a medium containing sugar is 

 also quite important. It is desirable to determine both the 

 quantity of gas and its relative composition. Chemical analy- 

 ses have shown that in all cases tested the gas resulting 

 from the fermentation of the sugar consists of a mixture of 

 hydrogen (H) and carbonic acid gas (CO 2 ), with mere traces 

 of other gases. It is important to know also the quantity of 

 gas produced with the various sugars, especially glucose, lac- 

 tose, and saccharose. To determine simply whether an organ- 

 ism will produce gas, it is only necessary to inoculate it into 

 tubes of liquid agar containing the various sugars ; but if the 

 quantity of gas is to be determined, the fermentation tube 

 is the most convenient apparatus to use. In some cases the 

 gas formation is one of the .most striking differential proper- 

 ties, as will be seen in the study of hog-cholera and typhoid 

 bacilli. 



REFERENCES. For a discussion of the gas production and use 

 of the fermentation tube, see Smith, Wilder Quarter-Century Book, 

 J 893, P- *87 f r the chemical formulae, see Novy, Laboratory 

 Work in Bacteriology, 1899. 



