450 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



aerogenes group over that of the Bact. lactis acidi group. The value of 

 milk for butter and cheese is determined by the relative amounts of 

 the products of the desirable and the undesirable acid-forming bacteria. 



The difference in taste and odor between milk fermented by pure 

 cultures of Bact. lactis acidi, and that which has soured spontaneously, 

 emphasizes the difference in the products of the fermentations produced 

 by the two groups of acid-forming bacteria. 



Characteristics of the Bact. Bulgaricum Group* The organisms of 

 this group are to be classed as true lactic bacteria, since they produce 

 almost exclusively lactic acid from the sugar fermented and only small 

 quantities of other acids as formic, acetic, and propionic. They vary 

 widely in form and size; but are usually large rods, 2^1 to $fj. long and 

 0.5/1 to in wide. There is a tendency to form long threads. They 

 are Gram-positive and when stained with methylene blue often show 

 distinct granules in the cells; with Neisser's stain the appearance of 

 some cultures is similar to that of the diphtheria bacterium. They 

 are non-motile and do not form spores; capsules are seldom noted. The 

 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 has 

 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 laevo-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 liquefy gelatin, but the casein of milk 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. 



* Prepared by E. G. Hastings. 



