48 LABORATORY BACTERIOLOGY 
EXERCISE XII 
THE EXAMINATION OF CULTURES ON SPECIAL MEDIA, 
WITH A STUDY OF THE GAS PRODUCTION 
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,), with mere traces 
of other gases. It is important to know also the quantity of 
gas produced with the various sugars, especially with glucose, 
lactose, and saccharose. To determine simply whether an 
organism 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 
properties, as will be seen in the study of B. suipestfer and 
B. typhosus. 
75. Work for this exercise. Examine and describe the 
cultures made on the special media in Exercise XI. 
Examine the bacteria on the potato culture microscopically 
(1) in the fresh condition (hanging-drop preparation) and (2) in 
