THE BACTERIA FOUND IN MILK 37 



importance. Pasteur, in 1857, was the first to describe a 

 definite organism as the cause of the souring. Since that 

 date our knowledge of lactic acid fermentation has steadily 

 increased, and it is now recognised that the production of 

 lactic acid from the sugars in milk is a property possessed 

 by a considerable number of bacteria. Some of the more 

 important organisms which are able to produce lactic acid 

 from milk are B. acidi lactici (Htieppe), B. lactis acidi (Leich- 

 mann), B. coli, B. lactis aerogenes, Streptococcus lacticus, B. 

 Indgaricus. Some other less important bacteria have also this 

 property, while there are some lactic acid producing yeasts. 

 The whole subject has been made very complicated by much 

 confusion of nomenclature and bacterial description. Lactic 

 acid producing bacilli have been described as one organism 

 which were really a mixture of bacilli, the same lactic 

 bacillus has been described again and again by different 

 observers as a new bacillus, and frequently under a fresh 

 name, while recently it has been contended that what has 

 hitherto been called a bacillus is in reality a streptococcus. 



It would seem, however, that ordinary lactic acid fermen- 

 tation in milk is mainly, if not entirely, due to two groups 

 of organisms : 



() B. lactis aerogenes and its allies. 



(&) The organisms which are either bacilli or streptococci, 

 and accordingly called B. acidi lactici or Streptococcus lacticus. 



B. lactis aerogenes closely resembles B. coli in most of its 

 characters, and has been included here under that group. 



Of the Bacterium acidi lactici type Conn, Ester, and 

 Stocking * give a good description in their report on dairy 

 bacteria, and the following is taken from their description : 



Bad. lactis acidi (type). 



A bacterium. Size, 0'7/x- 1'2/x x 0'5/x - 0'8/x. Sometimes so 

 short as to be described as a streptococcus, and the elements in 

 some cultures are very clearly cocci. No long chains. No 

 motility, no spores. Gram stain is positive. 



Gelatine colonies. Small points, rather opaque, not charac- 

 teristic. Almost wholly under the surface, and typical colonies 

 never grow on the surface. In litmus gelatine they are rather 



1 "Classification of Dairy Bacteria," Report of the Storrs (Connecticut) 

 Agricultural Experiment Station for 1906. 



