BACTERIA. 69 



aerobic species. Gruber found associated under the name of 

 Clostridium butyricum three well-defined species, two of 

 which are exclusively anaerobic. One of these last-mentioned 

 species consists of straight or slightly-curved rods, which 

 become spindle- or barrel-shaped during the formation of 

 spores. In nutrient gelatine it forms colonies which, when 

 seen in transmitted light, appear blackish-brown or black. 

 The second species consists of strongly-curved vegetative 

 rods, in which the spores are formed at the end ; it forms 

 yellowish or yellowish-brown colonies. The third specie's is 

 also capable of growth and of causing fermentation in the 

 absence of oxygen ; its development is, however, decidedly 

 promoted by the presence of oxygen, and it is only then 

 able to produce spores. The vegetative rods are cylin- 

 drical ; with the formation of spores the rods become 

 spindle-shaped, and in the centre of the spindle the large 

 spore is formed. The colonies in nutritive gelatine are 

 of a yellow colour. All three species form butyric acid 

 and butyl- alcohol. 



According to Fitz the spores of butyric acid bacteria can 

 withstand a boiling temperature for a period of time which 

 is naturally dependent, as in all cases, on their condition and 

 on the nature of the substratum ; Fitz gives three to twenty 

 minutes as the limits. They can, however, also be killed by 

 a lower temperature, if continued long enough ; thus they 

 are killed by being heated for six hours at 90 C. in a solution 

 of grape-sugar ; but in glycerine, at the same temperature, 

 only after six to eleven hours. 



Thus the same holds good for butyric acid fermentation as 

 for lactic acid fermentation, namely, that it is not produced 

 exclusively by one species. When butyric acid fermentation 

 occurs in distilleries, breweries, and pressed-yeast factories, 

 bacteria are frequently found which are entirely different from 

 those described above. 



Clostridium butyricum, and various other species, are 

 capable of dissolving cellulose, and therefore play an im- 



