446 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



and on grain. They are introduced into the milk with the dirt. The 

 cleaner the conditions of production, the smaller will be the number of 

 these two groups of organisms found in fresh milk. 



The manufacture of the leading type of butter and of all kinds of 

 cheese is dependent on the action of microorganisms, hence dairy manu- 

 facturing should be classed as a true fermentation industry. In all 

 such industries one of the factors determining the quality of the product 

 is the type of microorganism employed to produce the desired fermen- 

 tation, and the importance of insuring the presence of desirable organ- 

 isms, and the exclusion of harmful kinds is well recognized. 



The most important properties of organisms employed in the fermen- 

 tation industries are the physiological rather than the cultural or mor- 

 phological, since the quality of the product is dependent on the by- 

 products of the fermentation. Hence in characterizing the groups of 

 acid-forming bacteria, the biochemistry of each group will be empha- 

 sized rather than the cultural and morphological characteristics of the 

 members of the group. 



Characteristics of the Bact. Lactis Acidi Group.* The organisms of 

 this group are widely distributed in nature, as is shown by the constancy 

 with which milk undergoes the characteristic fermentation produced by 

 the members of the group. 



The cells are oval in form, about 0.6^1 to i/z in length, and 0.5^ in 

 diameter. The shorter cells appear nearly spherical, which, together 

 with the fact that chains of cells often occur, has led some to classify 

 them among the cocci and Kruse has applied the name Strept. lacticus to 

 a member of the group. In milk the cells are usually in twos, the outer 

 ends of the two cells being pointed. None of the group is motile; spores 

 are not formed and capsules are often noted. The members of the 

 group are Gram-positive. 



The optimum temperature for growth lies between 30 and 35, the 

 minimum growth temperature ranging from 10 to 12, while the maxi- 

 mum is 4 2. They are to be classed as facultative aerobes. The growth 

 on all culture media is marked by its meagerness; in the absence of a fer- 

 mentable carbohydrate, no growth usually occurs; peptone favors the 

 growth even in milk. In the case of freshly isolated cultures, the 

 growth is almost invisible, on slopes of sugar agar appearing as small 

 discrete colonies. On sugar agar plates the colonies are small, often 



* Prepared by E. G. Hastings. 



