646 BA CTERJO LOGY. 



where present in the ground, cannot be isolated by the 

 ordinary methods, and will not appear in plates made 

 after either of the above plans. The special devices for 

 their cultivation are described in the chapter on Soil- 

 organisms. 



BACTERIOLOGICAL STUDY OF MILK. 



The possibility of milk serving as a vehicle in which 

 disease-producing bacteria may be disseminated through- 

 out a community has long been recognized, and epi- 

 demics of typhoid fever have been traced directly to 

 infected milk, while such diseases as diphtheria and 

 scarlet fever are also frequently regarded as being con- 

 veyed in the same manner. 



In recent years the detailed study of the milk of 

 individual cows has revealed the fact that streptococcus 

 mastitis is not an uncommon occurrence in herds, and 

 it has frequently been observed that milk rich in strep- 

 tococci may prove dangerous when fed to infants and 

 convalescents. 



Since milk is such a favorable medium for the growth 

 of a variety of bacteria it is not at all uncommon to 

 find market milk very rich in bacteria, especially if it 

 has been collected in a careless manner in dirty recep- 

 tacles, in unsanitary stables, and has been shipped long 

 distances at comparatively high temperatures. 



For these various reasons the bacteriological study 

 of milk has gained considerable prominence during the 

 past few years so much so that in some localities an 

 effort is being made to establish a bacterial standard for 

 market milk that is, milk containing more than a cer- 

 tain number of bacteria is not regarded as suitable for 

 use. Whether such a standard can be maintained or 



