l6 GENERAL CONSIDERATIONS 



been made and particularly to the introduction by MacConkey 

 of the use of bile salt, the isolation and identification of B. coli 

 has now been placed on a very satisfactory footing. 



In general it may be said that the B. coli group is by far 

 the most reliable indicator of excretal pollution which we 

 possess and its use is equally applicable for water, shellfish, 

 soil and other substances. The other two indicators are chiefly 

 of value for confirmatory purposes. 



These bacteria not only indicate excretal contamination, but 

 they can be used to measure its extent by means of careful 

 numerical estimation. This is a matter of extreme importance, 

 since excretal pollution is so widespread that to some degree 

 evidence of it must also be widespread, and measurement, not 

 mere detection, is required. 



CHAPTER II 



GENERAL METHODS FOR THE ISOLATION AND 

 IDENTIFICATION OF INDICATOR ORGANISMS 



While for the isolation and identification of indicator organ- 

 isms considerable variations are required in detail, according to 

 the material submitted for examination, the general procedures 

 are identical whatever the source. It is therefore convenient and 

 saves repetition to discuss them together in a single chapter. 



The methods used must be capable not only of detecting 

 the indicator organism but must be able to estimate the number 

 present in a given sample. In this they differ from procedures 

 used for the isolation of pathogenic bacilli such as B. typhosus 

 or Sp. cholerae, since for these detection is all that really 

 matters. 



In the previous chapter it has been explained that the three 

 most useful groups of organisms for the estimation of excretal 

 contamination are B. coli and allied organisms, streptococci and 

 B. enter itidis sporogenes.. 



