132 BACTERIA 



gradually loses its brightness and assumes a bad odour and 

 disagreeable taste. The bacillus is a facultative anaerobe. 

 A number of workers have separated organisms, having a 

 lactic acid effect, which diverge considerably from the 

 orthodox type of lactic acid bacillus. This is but further 

 evidence of a fact to which reference has been made : that 

 nomenclature restricted to one individual has now become 

 adapted to a family. 



4. Butyric Acid Fermentation. 



Cause, Bacillus butyricus and B. amylobacter ; medium, milk, butter, sugar 

 and starch solutions, glycerine ; result, butyric acid. 



When sugars are broken down by the Bacillus acidi lactici 

 the lactic acid resulting may, under the influence of the 

 butyric ferment, become converted into butyric acid, car- 

 bonic acid, and hydrogen. Neither butyric acid nor lactic 

 acid is as commonly used as alcohol or vinegar. Both, like 

 vinegar, can be manufactured chemically, but this is rarely 

 practised. Butyric acid is a common ingredient in old milk 

 and butter, and its production by bacteria is historically 

 one of the first bacterial fermentations understood. More- 

 over, in its investigation Pasteur first brought to light the 

 fact that certain organisms acted only in the absence of 

 oxygen. In studying a drop of butyric fermenting fluid, it 

 was observed that the organisms at the edge of the drop 

 were motionless and apparently dead, whilst in the central 

 portion of the drop the bacilli were executing those active 

 movements which are characteristic of their vitality. To 

 Pasteur's mind this at once suggested what he was able later 

 to demonstrate, namely, that these bacilli were paralysed by 

 contact with oxygen. When he passed a stream of air 

 through a flask containing a liquid in butyric fermentation, 

 he observed the process slacken and eventually cease. So 

 were discovered the anaerobic micro-organisms. The aerobic 



