BUTYRIC ACID BACTERIA. 127 



According to Pasteur's experiments, the butyric acid 

 ferment can perform its functions without access to free 

 atmospheric oxygen. The usual spontaneous butyric acid 

 fermentations proceed most vigorously when oxygen is ex- 

 cluded. It has, however, been shown by recent experiments 

 that there are many butyric acid bacteria which behave 

 otherwise regarding free oxygen, for some are incapable of 

 growth in presence of oxygen anaerobic species whilst 

 others multiply and induce butyric acid fermentation when 

 they have access to oxygen aerobic species. In the course 

 of years a very large number of butyric acid bacteria have 

 been described. By the study of this mass of material, it 

 has been shown that they are divisible into two groups first, 

 the true butyric acid bacteria, being those that produce 

 butyric acid as the chief product of fermentation by decom- 

 position of carbohydrates or calcium lactate ; and, secondly, 

 there remain many species which form butyric acid along with 

 other products by the breaking down of albuminoids. This 

 applies particularly to putrefactive bacteria, many of which 

 only produce minute quantities of butyric acid. Thorough 

 chemical investigations have been carried out by Fitz, and 

 more recently by Perdrix, as well as by Schattenfroh and 

 Grassberger, who investigated the action of a number of 

 species upon starch, the sugars, glycerine, cellulose, and the 

 albuminoids, and determined the products of fermentation. 



One of the first species to be minutely described is Praz- 

 mowski's Clostridium butyricum (Bac. butyricus, Fig. 24). It 

 occurs in the form of short and long threads and rods, which 

 may be either straight or somewhat curved. The rods are in 

 brisk movement, and under a strong magnifying power they 

 are seen to be covered with a large number of cilia (Fig. 25). 

 Before the formation of spores in the rods, the latter swell 

 and form peculiar spindle and lemon-shaped, elliptical, or 

 club-like forms, as shown in the diagram ; at the same time 

 they are coloured blue by iodine. The spores can withstand 

 boiling for five minutes. On germination the spores burst 

 their outer envelope, and the germ filament grows in the same 

 direction as the longitudinal axis of the spore. Clostridium 

 butyricum grows most vigorously at a temperature of about 



