V49-18 



MANUAL OF METHODS FOR PURE CULTURE STUDY 



After some experience a bacteriologist can usually devise some method for recording 

 on the Chart, by a system of numerals or + signs, the strength of reaction observed 

 with each indicator employed; such a system often proves practical for comparative 

 purposes, l)ut gives no very definite information as to final H-ion concentration. 



Gas production in liquid media 

 is ordinarily measured in percent- 

 age of gas in the closed arm of the 

 Smith or the Durham fermenta- 

 tion tube. The Durham tube 

 consists of small test tube (e. g. 75 x 

 10 mm.) inverted in a large tube 

 (e. g. 150 X 18 mm.). In the case 

 of solid media it is recorded as 

 present or absent according to 

 whether or not bubbles or cracks 

 are present in the agar. This test 

 is especially valuable if the organ- 

 ism is tested in a shake culture; 

 but the presence of gas can usually 

 be detected in an ordinary agar 

 slant. These tests for gas produc- 

 tion are chiefly useful if the organ- 

 ism produces primarily hydrogen; 

 if the gas is all carbon dioxide little 

 or none will accumulate in the fer- 

 mentation tube because of the 

 great solubility and rapid diffu- 

 sion into the air. A convenient, simple method that has been pro- 

 posed for the accurate determination of carbon dioxide is that of 

 Eldredge and Rogers (1914). (See Leaflet VI.) 



Interpretation of Results. In case an organism produces gas or con- 

 siderable increase in acidity in either broth or beef extract peptone 

 agar in the presence of some fermentable substance, and this does not 

 occur in the basal medium without the addition of the fermentable 

 substance, it may safely be concluded that cleavage of this sub- 

 stance has occurred. Very often for routine diagnostic purposes 

 such information is enough. To understand the true action of the 

 organism on any carbon compound, however, much more investiga- 

 tion must be made as explained elsewhere. (See Leaflet VI) . This is par- 

 ticularly necessary in the case of organisms that produce a small amount 

 of acid in some tubes but not in others containing the same carbon 

 source, and in cases where the addition of some carbon source results 

 in a distinctly improved growth without the appearance of demon- 



FlG. 1. 



The Smith Fermentation 



Tube 



