-CONSIDERATIONS 



methods, "o'ftfen'iat.prcicsdtiifeVof-'no great trouble or complexity. 

 On the other hand their isolation from water, food, air, etc. is 

 frequently a matter of the utmost difficulty for reasons which 

 readily present themselves. In these substances any pathogenic 

 bacteria are frequently present in but small numbers compared 

 with the total number of bacteria present, while these competing 

 bacilli, by growing more readily and abundantly in the media 

 used for the isolation of the pathogenic bacteria, may materially 

 enhance the difficulties of isolation. Further, there is always the 

 possibility that any pathogenic bacteria originally present may 

 have been completely eliminated by the time attention is called 

 to the need for bacteriological examination. These pathogenic 

 bacteria in water, soil or even in milk are, or may be, in an 

 environment either in itself unsuitable to their multiplication 

 and sustained life or in one relatively unsuitable compared to 

 that enjoyed by the horde of naturally present, or added, 

 competing bacilli. This may cause their actual or relative 

 disappearance when the food or water is submitted to bacterio- 

 logical examination. For these reasons the isolation of specific 

 pathogenic bacteria plays but a small part in this class of 

 bacteriological analysis. 



The second object of a bacteriological examination to 

 measure undesirable bacterial contamination is by far the 

 most valuable part of the bacteriological examination of the 

 materials under consideration. By modern methods it is 

 possible not only to say whether such pollution exists but, 

 with considerable approach to accuracy, to determine the extent 

 of such pollution and to furnish data as to the degree of its 

 harmfulness. 



Although the substances to be examined are various while 

 the sources of pollution are many and not all of the same 

 character there is for most of them a broad relationship as to 

 the nature of the pollution to be detected and measured. For 

 example, for water supplies bacteriological analysis is mainly 

 concerned with the detection and estimation of excretal and 

 sewage pollution. For shellfish the degree of sewage pollution 

 is again the object of the examination. For milk, apart from 

 the detection of pathogenic bacteria, the examination is chiefly 



