148 MANUAL OF BACTERIOLOGY. 



midis albus, and not seldom the staphylococcus pyogenes 

 aureus, under normal conditions. 



Of the different pathogenic bacteria liable to furnish a 

 source of danger in milk, the most important is the bacillus 

 tuberculosis. Tuberculosis is a disease to which cattle are 

 exceedingly prone. There is good reason to believe that 

 infants acquire tuberculosis through taking as food the milk 

 of tuberculous cows, although the danger from this source 

 has probably been overestimated. The milk of tubercu- 

 lous cows may contain tubercle bacilli when there is no 

 tuberculous disease of the udder. 1 The frequency of tuber- 

 culosis among milch cows sometimes becomes as high as 25 

 per cent., or even more. Butter derived from the milk of 

 such cows may contain tubercle bacilli. The proper man- 

 ner for the States to deal with this problem, for it is one 

 that doubtless will fall to the individual States, has not yet 

 been determined. The cost of killing such a large number 

 of valuable cows would be very great. Furthermore, it is 

 by no means certain that this procedure would eradicate 

 the disease. The flesh of cattle also is capable of trans- 

 mitting tuberculosis, but is a smaller source of danger on 

 account of the universal practice in the United States of 

 thoroughly cooking beef. 



" Ripening " of cream and cheese is due to the growth of 

 bacteria which produce agreeable flavors in the butter and 

 cheese. Molds are also important in the ripening of some 

 kinds of cheese. 2 



In examining milk for bacteria the number may be esti- 

 mated by precisely the same technique as is used for the 

 estimation of bacteria in water, except that the milk must 

 be diluted; otherwise the plates are rendered opaque by 



1 See Mohler, " Infectiveness of Milk of Cows which have Reacted 

 to the Tuber nlin Test," U. S. Dept. Agriculture, Bureau Animal In- 

 dustry, Bull. No. 44, 1903. 



2 Conn, " Agricultural Bacteriology." 



