METHODS OF STUDYING PHYSIOLOGIC CHARACTERS 99 



atmospheric oxygen can grow in its absence providing ni- 

 trates are present to furnish an available supply of oxygen 

 though not in the free form. Other bacteria will grow in 

 the absence of oxygen providing they have suitable carbo- 

 hydrates. For example, the organism known as Bacterium 

 coli will grow only in the open arm of a fermentation tube 

 if sugar is absent, but in the presence of a suitable sugar 

 such as dextrose it grows both in the open and in the 

 closed arm. 



The obligate anaerobes are those which will grow only 

 in the absence of free oxygen or are definitely injured by 

 its presence. It is evident that special cultural conditions 

 are necessary for their study. 



A few bacteria have been termed mi^roaerophiles be- 

 cause they tend to grow in a definite concentration of oxy- 

 gen. When mixed with melted agar in a test tube and the 

 agar allowed to solidify, they will grow in a definite zone 

 some distance below the surface of the medium. 



Determination of Gas Production. Bacteria which 

 require free oxygen for their development frequently pro- 

 duce considerable quantities of carbon dioxide. Inasmuch 

 as they are growing in contact with air, however, this gas 

 diffuses into the air and does not collect as gas bubbles in 

 the medium. When it is desired to study gas production 

 by such bacteria it is necessary to grow them in a closed 

 vessel to which air enters through a small opening and 

 leaves through a tube which then passes through a suitable 

 absorbing agent, for example, strong sodium hydrate, which 

 will collect the carbon dioxide. The amount of gas may 

 be calculated by the increased weight of the sodium hy- 

 drate. 



Most of the bacteria producing considerable amounts of 

 gas in laboratory media are anaerobes or facultative anae- 

 robes. The gases developed depend somewhat on the com- 

 position of the medium, and also upon the character of the 



