190 AGRICULTURAL BACTERIOLOGY 



at 110 to 120 F. The visible change that they produce 

 in milk is identical with that produced by the lactic bac- 

 teria. The chemical change is also similar. These organ- 

 isms form an exception to bacteria in general, in that they 

 can grow in rather acid solutions. Owing to this property, 

 they continue to grow after the lactic and colon organisms 

 have ceased to grow. They can produce from 1 to 3 per 

 cent, of acid in milk. They play no part in the souring of 

 milk, since they grow very slowly at from 60 to 80 F. 



These organisms play an important role in the ripening 

 of certain varieties of cheese. They are also widely used 

 in the preparation of fermented milks, either alone or with 

 the lactic bacteria. Attention was first directed to them 

 in this connection by the investigations of Metchnikoff in 

 regard to the cause of senility. He believed it to be due 

 to the gradual change in the intestinal flora from one that 

 was primarily acid-forming to one in which putrefactive 

 bacteria predominated. His idea was that this change 

 could be prevented by the ingestion of the peculiar type of 

 fermented milk used by the people of Bulgaria. Metchni- 

 koff believed the Bulgars are characterized by a large num- 

 ber of people who attain an extreme age, a fact that he 

 ascribed to the use of yoghurt, the name given to the fer- 

 mented milk as it is prepared in Bulgaria. 



The organism was isolated, and cultures soon distributed 

 to all parts of the world. Their use is often recommended 

 by physicians for cases of digestive troubles. It is not at 

 all certain that this particular type of organism is more 

 valuable in this regard than is the true lactic organism. In 

 fact, most of the fermented milk, commercially prepared, 

 is made by employing the two groups of organisms, which 

 are grown separately, and then the fermented milks are 

 mixed in the desired proportions. Bad. Bulgaricum gives 

 to the fermented milk a smooth, creamy texture, which 



