38 WATER 



the presence of any one variety of streptococcus, but upon the 

 group as a whole. 



Neutral red broth is preferred to plain nutrient broth because 

 the streptococcus chains seem to be more readily detected micro- 

 scopically in it. 



In cases in which it is doubtful whether streptococci in chains 

 are actually present a definite opinion may often be arrived at 

 by centrifugalizing the fluid, and microscopically examining a 

 little of the deposit stained by methylene blue. 



Examination for B. enteritidis sporogenes. 



This examination is carried out by some bacteriologists and 

 is included here although not recommended. 



Even in waters from contaminated sources this organism 

 may be present in only small numbers, so that it is usually 

 necessary to examine a considerable volume of the water. Con- 

 venient amounts to examine are 10, 100, 500 and 1000 c.c. 



To deal with these large volumes of water the sample is 

 concentrated by filtration through porcelain as described in 

 Chapter II. 



If one litre is concentrated into 10 c.c., then 10, 100 and 

 500 c.c. of the original water will be represented by o'l, I and 

 5 c.c. respectively of the concentrated filtrate. 



The presence of the spores of this bacillus are determined by 

 inoculating milk tubes as described in Chapter II. 



B. The examination for specific disease-producing bacilli. 



Some diseases, such as diphtheria or scarlet fever, appear to- 

 be never spread through water. The only bacilli associated with 

 definite diseases for which it is ever necessary to examine water 

 are the typhoid bacillus, the cholera vibrio, the organisms of 

 dysentery in the tropics, and, under very exceptional circum- 

 stances, members of the Gaertner group in suspected outbreaks 

 of food poisoning or paratyphoid fever. The examination 

 for Gaertner group organisms is described in Chapter VIII 

 and modifications for the examination of water will readily 

 suggest themselves. Only the examination for the bacteria of 



