THE RELATION OF MICROORGANISMS TO MILK. 311' 



Milk is usually curdled, although some members of the group do not 

 produce enough acid to cause curdling. Depending on the amount of 

 gas formed, the curd may be almost perfectly homogeneous or it may be 

 very spongy. In all cases the curd shrinks to a greater or less extent and 

 thus becomes so firm that it is difficult or impossible to emulsify it again. 

 The odor of the fermented milk is offensive and the taste disagreeable and 

 sharp. The organisms of this group are to be classed as undesirable and 

 the fermentation produced by them cannot correctly be called a lactic 

 fermentation. 



The representatives of these two great groups of acid-forming bacteria 

 are to be found in every sample of market milk in varying proportions. 

 Both find in milk favorable conditions for growth, and the normal souring 

 is produced conjointly by them, each producing its own specific products, 

 the relative amount of which is largely dependent on the number of each 

 group that were originally introduced into the milk. The value of milk 

 for butter and cheese is determined by the relative amount of the prod- 

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



The difference in taste and odor between milk fermented by pure 

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

 emphasizes the difference in the products of the fermentations pro- 

 duced by the two groups of acid-forming bacteria. 



Characteristics of theBact. 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 /* to 3 JJL long and 0.5 /* to 0.75 fj. 

 wide. There is a tendency to form long threads. They are Gram- 

 positive and 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 bacteria. 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 andthe term acidophilous has been applied 

 to the group. They grow slowly in milk, even at the optimum tempera- 

 ture, and curdling may not occur for several days; the curd is homogeneous 



* Prepared by E. G. Hastings. 



