208 OUTLINES OF BACTERIOLOGY 



The acetic-acid bacteria are very strongly aerobic, and form on the 

 surface of the liquid in which they are growing characteristic white or 

 grey films, consisting of agglutinated masses of these bacteria. The 

 so-called vinegar plant is made up of large numbers of rod-shaped 

 individuals of Bacterium xylinum. It grows on the surface of the 

 culture fluid, and effects the transformation of alcohol into acetic acid. 



III. Butyric acid fermentation. Butyric acid is a common ingredient 

 of stale milk and rancid butter, and is the chief cause of their un- 

 pleasant smell and taste. The acid is formed by the fermentation of 

 many substances, the best known being lactic acid, glycerine, starch, 

 mannite, and sugar. 



When sugars are broken down by lactic-acid bacteria, as, for 

 example, in the souring of milk, the resulting lactic acid is converted 

 by butyric-acid bacteria into butyric acid, carbonic acid, and hydrogen. 

 The best known of these organisms is one discovered by Pasteur, and 

 called by him Vibrion butyrique. 1 This organism is now known as 

 Bacillus butyricus (Pasteur), and is interesting, because it was whilst 

 studying its life-history that Pasteur alighted on the fact that there 

 were organisms which could live without oxygen. This bacillus is 

 widely distributed in nature, and grows best at a temperature of 40 C. 

 Normally it forms spores, the spore-containing individuals having the 

 characteristic appearance shown in Fig. 43. The substances which it 

 decomposes are lactic acid and substances like sugar, which can readily 

 give rise to lactic acid, also tartaric, citric, and malic acids, as well as 

 a number of other substances. In addition, it can digest cellulose, 

 and, indeed, very probably plays an important part in the digestion of 

 this substance inside the bodies of the higher animals. 



Other butyric-acid bacteria have been isolated. One of these is 

 Bacillus butylicus, which decomposes glycerine, the chief products of 

 the decomposition being butyric and lactic acids, butyl alcohol, carbon 

 dioxide, and hydrogen. This differs from the preceding in being a 

 facultative, not an obligative, anaerobe. Another is Bacillus acidi 

 butyrici, an anaerobe which was isolated from mixtures of sugar 

 solution and bad cheese or rancid cream-butter. It forms spores, and, 

 unlike the others, can liquefy gelatine. A fourth member of this 

 group is Bacillus ethylicus, which can ferment thin starch paste in the 

 presence of powdered chalk. It first of all secretes a diastatic ferment, 



1 This organism was named Cloatridium Inityricum by Prazmowski, but it has 

 since been demonstrated that the bacteria designated by this name consisted .of a 

 number of closely allied but distinct species ; that is to say, Prazmo\vski did not 

 work with pure cultures. 



