BACTERIA OF MILK 



531 



the curd is clear. Being commonly concerned in the 

 souring of milk and producing principally lactic acid, 

 the organisms of this type are known as the true lactic 

 acid bacteria. The Streptococcus lacticus must not be 

 confused with the mastitis streptococci (Fig. 6), which 

 also ferment lactose and have other corresponding char- 

 acteristics. The latter organisms grow in long, inter- 

 twined chains, the individual members of which are rec- 

 tangular or oval in form, with the long axis at right 



FIG. 5. Preparation showing Streptococ- 

 ens lacticus or Bacterium lactis acidi. 



Fia. 6. Preparation from sediment 

 obtained by centrif ugalizing milk from a 

 cow a fleeted with catarrhal mastitis, show- 

 ing streptococci and leucocytes. 



angles to the length of the chain. On agar, they form 

 extremely minute, punctiform, brownish colonies. 



The Bacterium acidi lactici ( Hueppe) , also called the 

 Bacillus lactis aerogenes f is frequently concerned in the 

 spontaneous souring of milk, usually in association with 

 the Streptococcus lacticus. This is a short, plump, non- 

 motile bacterium which is closely related to the coli-aero- 

 genes group of bacteria and may be regarded as the most 

 active acid-forming member of that group. It grows 

 upon the surface of solid media, forming thin, partially 

 translucent, leaf-shaped colonies, or round semi-globular 



