14 Dairy Bacteriology. 



noted. Some species, like leprosy, grow with only the 

 greatest difficulty outside of their proper host. Such as 

 this may be said to be an obligatory parasite. Then 

 again, other forms like the colon or feces bacillus are 

 normally saprophytes, but under certain conditions, as- 

 sume a semi-parasitic mode of life, becoming therefore, 

 facultative parasites, i. e., they possess at times the fac- 

 ulty of developing under parasitic conditions. 



23. Fermentation. Most of the saprophytic bacteria 

 are concerned in the breaking down of organic matter, 

 and consequently, they are often associated with the 

 many different phases that this process assumes as seen 

 in putrefaction, fermentation, decomposition or decay. 

 These changes are of a complex character and vary much 

 from one instance to another. 



The changes embraced under the special term, fermen- 

 tation, are distinguished by such a prominent character- 

 istic that they may well be considered separately. In 

 fermentation, complex substances are transformed by 

 regular steps into simpler compounds. 



In this class are to be included a large number of fer- 

 mentative changes that occur in milk, such as the produc- 

 tion of lactic acid in the souring of milk, the formation of 

 butyric acid, and others that will be mentioned later. In 

 fermentation, there are often large quantities of the fer- 

 mentable substances changed, and, while a certain amount 

 of the energy released in the breaking down of this ma- 

 terial may be utilized by the bacterial cells, yet the dis- 

 ruptive changes set in motion by the living germs are 

 out of all proportion to the results that are seen in the 

 end. 



Fermentation, although one of the best known pro- 

 cesses that occurs in nature is even yet a partial mystery. 

 Our knowledge of the changes that fermentable solutions 



