370 Bacteria in Relation to Country Life 



Bacterium lactis acidi (B. Guntheri), a universally dis- 

 tributed organism which, more than any other, is respon- 

 sible for the souring of milk at ordinary temperatures. 



In all, the number of lactic-acid bacteria is considera- 

 ble, some of them producing large quantities of other 

 products besides the lactic acid. Mention may be made 

 here of Bacillus coli communis, a common inhabitant of 

 the intestinal tract of animals, and Bacillus aerogenes 

 {B. lactis aerogenes), another organism widely distributed 

 in nature. Bacillus coli communis most readily gains 

 entrance into the milk with particles of manure; while 

 Bacillus aerogenes may come from the manure, bed- 

 ding, and food. The two organisms may prove a source 

 of great annoyance in the dairy, especially Bacillua 

 aerogenes, which occasionally causes much loss in cheese- 

 making. 



The second group includes organisms that cannot 

 cause the souring of milk, but possess the ability to 

 curdle it by means of a rennet-like enzyme secreted by 

 them. Many of these possess the ability, likewise, of 

 digesting the coagulated casein by means of another 

 pepsin-like enzyme, and of transforming the milk into 

 a clear transparent fluid. This group includes, also, 

 organisms that do not curdle the milk, but digest its 

 casein slowly until the liquid becomes clear and trans- 

 parent. We see, therefore, that the organisms of this 

 group produce either one or both of the two enzymes. 

 When the curdling is caused by them, the milk does not 

 become sour, but shows an alkahne reaction, the process 

 being designated as sweet curdling. 



The members of this group are rather numerous and 



