86 METHODS OF CULTIVATION OF BACTERIA. 



ture. If the bacterium used is a gas-forming one, then, as 

 growth occurs, little bubbles appear round the colonies. 

 These frequently coalesce to form bubbles of a larger size, 

 and those which are superficial in process of time diffuse 

 out of the medium. This method is very frequently used 

 for studying gas formation by B. coli. 



(2) Durham's Tubes (Fig. 35 b\ The plug of a tube 

 which contains about one-third more than usual of a liquid 

 medium is taken, and a small test-tube is slipped into it 

 mouth downwards. The plug is replaced and the tube 

 sterilised thrice at 100 C. The air remaining in the 

 smaller tube is thereby expelled. The tube is then inocu- 

 lated with the bacterium to be tested. Any gas developed 

 collects in the upper part of the inner tube. 



(3) The Fermentation Tube (Fig. 35 c\ This consists 

 of a tube of the form shown, and the figure also indicates 

 the extent to which it ought to be filled. It is inoculated 

 in the bend with the gas -forming organism, and when 

 growth occurs the gas collects in the upper part of 

 the closed limit, the medium being displaced into the 

 bulb. 



The composition of the medium is, of course, of great 

 importance, and in testing the effect of a bacterium on a 

 given sugar it is essential that this sugar alone be present. 

 The first method is usually used with ordinary gelatine, and 

 the gas-formation in most cases results from fermentation 

 of the glucose naturally present in the medium from trans- 

 formation of the glycogen of muscle. (It is only a more 

 delicate method of demonstrating what sometimes occurs 

 along the line of growth in an ordinary gelatine stab-culture.) 

 The amount of glucose naturally present, however, varies 

 much, and therefore glucose should be added to the 

 medium if the effects on this sugar are to be observed. 

 When other sugars lactose, mannite, etc. are to be 

 tested, these should be added either to a simple peptone 

 solution as Durham recommends, or to bouillon previously 

 freed from dextrose as described below fermentation being 

 observed by either of the methods (2) or (3). 



