THE BACTERIA OF SOIL, AIR, WATER, ETC. 151 



of the udder of the cow, from the hands of the milker, from 

 dirty pails, through agitation of the air of the stable, and in 

 other ways. In view of these sources of contamination it is 

 not to be wondered at that the number of bacteria found ordi- 

 narily in milk is usually very large. In ordinary milk as fur- 

 nished by milkmen the number of bacteria to the cubic centi- 

 meter is usually many thousands, and may run up to many 

 millions. Milk as obtained from the grocers, where it is apt 

 not to be kept properly cooled, frequently contains hundreds 

 of thousands or many millions of bacteria to the cubic centi- 

 meter. Many or most of these bacteria must be little if at 

 all detrimental to health, otherwise milk would be entirely 

 unsuitable as fcod in the raw state; whereas this is not the case. 

 On the contrary, there are some bacteria which are useful in 

 the manufacture of dairy products. The ripening of cream 

 and of cheese is due to the growth of bacteria introduced in 

 cultures or, what amounts to the same thing, it is due to the 

 growth of bacteria introduced in some previously ripened 

 cream or cheese. This imparts the agreeable flavor. Moulds 

 are also used to give the peculiar flavor to some cheeses.* 



But aside from these more or less innocuous forms and 

 those which are useful, milk may at times contain pathogenic 

 bacteria. 



Tuberculosis is a disease to which cattle are exceedingly prone. 

 There is good reason to believe that infants acquire tuberculo- 

 sis through taking as feed the milk of tuberculous cows, al- 

 though the danger from this source has probably been over- 

 estimated. The milk of tuberculous cows may contain tubercle 

 bacilli when there is no tuberculous disease of the udder. t 

 The frequency of tuberculosis among milch cows sometimes 

 becomes as high as 25 per cent., or even more. Butter derived 



*Conn. Agricultural Bacteriology. 



fMohler. Infectiveness of milk of cows which have reacted to the tuberculin 

 test. U. S. Department of Agriculture, Bureau of Animal Industry. Bulle- 

 tin No. 44, 1903. 



