72 THE STORY OF GERM LIFE. 



work tends to show that the majority of cases of 

 spontaneous souring of milk are produced by 

 bacteria which, though somewhat variable, prob- 

 ably constitute a single species, and are identical 

 with the Bacillus acidi lactici (Fig. 19). This spe- 

 cies, found common in the dairies of Europe, ac- 

 cording to recent investigations occurs in this 

 country as well. We may say, then, that while 

 there are many species of bacteria infesting the 

 dairy which can sour the milk, there is one which 

 is more common and more universally found than 

 others, and this is the ordinary cause of milk 

 souring. 



When we study more carefully the effect upon 

 the milk of the different species of bacteria found 

 in the dairy, we find that there is a great variety 

 of changes which they produce when they are al- 

 lowed to grow in milk. The dairyman expe- 

 riences many troubles with his milk. It sometimes 

 curdles without becoming acid. Sometimes it 

 becomes bitter, or acquires an unpleasant " tainted" 

 taste, or, again, a ''soapy" taste. Occasionally a 

 dairyman finds his milk becoming slimy, instead of 

 souring and curdling in the normal fashion. At 

 such times, after a number of hours, the milk be- 

 comes so slimy that it can be drawn into long 

 threads. Such an infection proves very trouble- 

 some, for many a time it persists in spite of all 

 attempts made to remedy it. Again, in other 

 cases the milk will turn blue, acquiring about the 

 time it becomes sour a beautiful sky-blue colour. 

 Or it may become red, or occasionally yellow. All 

 of these troubles the dairyman owes to the pres- 

 ence in his milk of unusual species of bacteria 

 which grow there abundantly. 



Bacteriologists have been able to make out 



