GENERAL CONSIDERATIONS 3 



to study bacterial contamination mainly from cow manure or 

 excreta-laden dust. Although the objects of the bacteriological 

 examination of air are somewhat different it is in general true 

 to state that excreta (human and animal) and sewage are the 

 most important forms of pollution which the Public Health 

 Bacteriologist has to detect and estimate in connection with 

 his work. To do this the bacteriologist has to determine the 

 organisms which are prevalent in sewage and excreta and which 

 can be used by him to estimate the extent of the contamination 

 by these substances. 



These indicator organisms are of great importance. Those 

 which have been most widely used for this purpose are the 

 B. coli group, streptococci, and B. enteritidis sporogenes and 

 closely allied anaerobic bacteria. It will be convenient to 

 consider first the general and special characters of these groups 

 of bacteria and subsequently to deal with the grounds on which 

 they have been selected as suitable to measure excretal and 

 sewage contamination. 



B. coli group. 



A study of the characters and distribution of this group is of 

 the utmost importance to the bacteriologist who has to deal with 

 the examination of water, milk and other foods. Difficulty is 

 met with at the outset in defining the characters of the group 

 and it is easy to err on the one hand by adopting too compre- 

 hensive a definition, including organisms essentially different 

 and with different distributions in nature, and on the other, by 

 limiting the group by too narrow a characterisation, excluding 

 organisms of material importance and significance. 



All the members of the group are small non-sporing bacilli, 

 decolorised by Gram's method of staining, which grow well at 

 both 37 C. and 20 C, which ferment both glucose and lactose, 

 with the production of acid and gas, and which fail to liquefy 

 gelatine (within a reasonable period). Motility is present or can 

 be demonstrated by the use of proper media and technique but 

 it is not always exhibited and its absence in ordinary examina- 

 tions cannot be taken as excluding organisms from the group. 



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