CHAPTER I 



GENERAL CONSIDERATIONS 



Although water, food, air, etc. are not all examined bacterio- 

 logically with the same objects, broadly there is a similarity of 

 procedure and certain general considerations may be conveniently 

 considered together, in this way avoiding repetition in the 

 chapters dealing with the examination of the different substances. 



Bacteriological examinations of food and other substances 

 are undertaken essentially for one or other, or for all, of the 

 following purposes. 



(a) To examine for the presence of definite disease-pro- 

 ducing organisms. This is done either with the object of 

 detecting their presence or, on the negative side, to judge by 

 the failure to find them, whether they are absent or, if at one 

 time present, have been eliminated. 



(b) To measure the extent to which the substance under 

 examination has been polluted by material derived from 

 undesirable or harmful sources. 



(c) To assess the value and completeness of any purification 

 processes to which the substance under examination has been 

 subjected. 



The examination for specific disease-producing organisms is 

 a procedure of great importance but is of limited applicability 

 and practical utility in connection with the examination of water 

 and foods. The detection of pathogenic bacteria in the human 

 or animal body and in many of their excretions is, with modern 



s. w. i 



