252 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
(c) To a determination of the products of fermentation; these are 
commonly lactic, acetic and formic acids; less commonly butyric or 
propionic. Alcohol is also frequently formed. 
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 of the true reaction is based upon 
the determination of the hydrogen-ion concentration. ^ The gases 
formed are usually carbon dioxide and hydrogen. An approximate 
ratio of the proportion H/CO2 is conveniently made in the Smith fer- 
mentation tube,' 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 
10 per cent solution of sodium hydrate 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 proportion- 
ately to the absorption of COo by the caustic alkali. 
Smith^ has determined the "gas ratio" for the principal aerogenic 
bacteria as follows: 
Organism. Glucose. Lactose. Saccharose. 
H CO2 H CO2 H CO2 
B. coli 63 37 63 37 63 37 
Hog cholera .... 66 34 — — — — 
B. lactis aerogenes ... 65 35 62 38 80 20 
Friedlander bacillus .67 33 86 14 67 33 
B. edematis maligni . . 67 33 ? ? 
B. proteus 72 28 — — 67 33 
B. cloacffi 70 30 37 63 58 42 
It must be borne in mind that these figures are only approximate. 
Elaborate apparatus must be employed for strictly quantitative deter- 
minations. 
The Voges-Proskauer^ Reaction.— Certain bacteria, grown in glucose 
media for several days, produce a substance from the fermentation of 
the glucose which gives a pink coloration with strong caustic alkali. 
The substance is said to be acetyl-methyl-carbinol (CH3.CHOH.CO.- 
CHs)^ which upon further oxidization yields diacetyl (CH3.CO.CO.- 
CH3). This in turn reacts with peptone to produce the pink coloration.^ 
The test is of considerable value in the final identification of certain 
intestinal bacteria. B. cloacee, B. aerogenes (lactis aerogenes)— but 
1 Clark: Jour. Infec. Dis., 1915, 17, 109. (See p. 213.) 
2 Theobald Smith: The Fermentation Tube, Wilder Quarterly Century Book, 1895, 
187 et seq. Tarozzi: Centralbl. f. BakteroL, orig., 1905, 38, 619. Herter and Kendall: 
Jour. Biol. Chem., 1908, 5, 283. Kendall: Ibid., 1909, 6, 257. 
' Am. Jour. Med. Sci., 1895, 110, 283. 
* Voges and Proskauer: Ztschr. f. Hyg., 1898, 28, 20. 
5 Harden and Wal pole: Proc. Roy. Soc, 1905-1906, 77, 399. Harden: Ibid., p. 424. 
^ See Levine for additional details, Jour. Bacterid., 1916, 1, 153. 
