220 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



Bacteria which can utilize carbohydrates for their energy require- 

 ments usually produce acid; many types produce gas as well. The 

 acid, which is commonly lactic, together with small amounts of acetic 

 and other fatty acids may be estimated by titration with standard 

 alkali. A more accurate estimation is based upon the determination 

 of the hydrogen ion concentration. 1 The gases formed are usually 

 carbon dioxide and hydrogen. An approximate ratio of the proportion 

 H/CO2 is conveniently made in the Smith Fermentation Tube, 2 in the 

 following manner: 



The level of the gas in the closed arm is marked with a wax pencil. 

 The bulb of the fermentation tube is then completely filled with 

 a 1 per cent, solution of sodium solution, and the gas brought 

 into contact with the alkaline solution by inverting the tube several 

 times. The gas is then entirely run back into the closed arm, and the 

 volume again measured. The volume is diminished proportionately 

 to the absorption of CO 2 by the caustic alkali. 



Smith 3 has determined the ''gas ratio" for the principal aerogenic 

 bacteria as follows: 



Organism. Dextrose. Lactose. Saccharose 



H CO 2 H CO 2 H CO 2 



B. coli . . . . 

 Hog cholera 

 B. lactis aero genes . 

 Friedlander bacillus 

 B. edematis maligni 



63 37 63 37 63 37 



66 34 



65 35 62 38 80 20 



67 33 86 14 67 33 



B. proteus 72 28 67 33 



B. cloacae 70 30 37 63 58 42 



Bacteria which ferment sugars grow in the closed arm of the fer- 

 mentation tube; those organisms, with very -few exceptions, which 

 cannot utilize the carbohydrate of a fermentation medium fail to 

 grow in the closed arm where free oxygen is not available; growth 

 appears only in the open arm. 



Occasionally a very slight change in the stereo-isomeric formula of 

 a carbohydrate, or a very small change in its terminal groups will 

 determine its fermentability by various organisms. Thus dextrose, 

 mannose, and their respective alcohols, sorbite and mannite, according 

 to Emil Fischer, 4 have the following stereo-isomeric formulae: 



1 Clark, Jour. Inf. Dis., 1915, xvii, 109. 



2 Theobald Smith, The Fermentation Tube, Wilder Quarter Century Book, 1895, 

 187 et seq. 



3 Loc. cit. 



4 Untersuchungen iiber Kohlenhydrate und Fermente, 1884-1908, Berlin, 1902. 



