384 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



optimum growth temperature is from 40° to 50° and the minimum is 

 asserted to be 25°, although for many members of the group it must be 

 much lower. 



The growth on all ordinary culture media is meager or is absent; 

 the colonies are often microscopic in size and show radiating threads; 

 Free acids do not inhibit development and the term acidophilous ha.s 

 been applied to the group. They grow slowly in milk, even at the 

 optimum temperature, and curdling may not occur for several days; 

 the curd is homogeneous and in litmus milk reduction occurs. The 

 maximum amount of acid varies from 1.25 to 4.0 per cent. Some 

 members of the group produce dextro-, others Isevo-acid, and racemic 

 acid is formed in some cases. The curd may be easily broken by agita- 

 tion, and through the solvent action of the acid is partially dissolved. 

 The organisms do not Uquefy gelatin, but the casein of mUk is partially 

 changed into soluble decomposition products, as was first shown by de 

 Freudenreich, and later confirmed by Hastings. 



It has been supposed by many that this group was confined to 

 and characteristic of certain of the fermented milks, especially those 

 of eastern Europe and western Asia, such as Yogurt and Matzoon. 

 Recent work has shown that this group is widely distributed in nature. 

 Representatives of this group are found constantly in milk and other 

 dairy products. Their presence in jnilk can be demonstrated by 

 placing a sample of milk in a corked bottle, and incubating at 37°. The 

 acidity of the milk increases rapidly at first, due to the growth of the 

 members of the ^wo previous groups. These ordinary acid-forming 

 organisms are soon inhibited by the appearance of free acid, but; the 

 acidity of the milk nevertheless continues to increase slowly, and 

 with this continued increase a change in flora is noted, the short, 

 plump bacilli ceasing to predominate and long slender rods constantly 

 increasing in numbers. The source of this group is undoubtedly 

 the alimentary tract of the animal. 



Characteristics of the Coccus Group* — This group is well represented 

 by the bacteria which form the characteristic flora of the udder. They 

 vary greatly in size and in other properties. They retain Gram's 

 stain; many are chromogenic, the color ranging from a white to a 

 deep orange. They grow slowly on all ordinary culture media, but 

 the growth is not necessarily meager. Generally they are aerobic, 



* Prepared by E. G. Hastings. 



