WATER 39 



typhoid fever and cholera will therefore be dealt with in this 

 chapter. 



Examination for B. typhosus. 



The methods available for the examination of water for the 

 typhoid bacillus are still not very satisfactory, and this is 

 evidenced by the very large number which have been recom- 

 mended. Great improvements in procedure have taken place 

 in recent years resulting from the introduction of selective differ- 

 entiating media and of more satisfactory methods of concentrating 

 the bacilli. 



The problem is a difficult one, because a comparatively 

 delicate organism, with no very definite morphological or cultural 

 characters, has to be detected among a large number of other 

 organisms which for the most part thrive much better on all 

 media and at all temperatures than itself. 



Further, the organism may be present in very small numbers, 

 for it tends to rapidly die out in water, so that it is necessary to 

 examine a large bulk of water. 



Moreover, owing to the long incubation period of the disease, 

 attention is usually not directed to the water as a possible source 

 of the infection until several weeks after the specific contamina- 

 tion, and when all the typhoid bacilli may have died out. 



The identification of B. typhosus from water naturally divides 

 itself into three stages : 



1. Preliminary methods, whereby any typhoid bacilli present 

 are obtained in a quantity of fluid small enough to be directly 

 plated. 



2. The isolation of the organism in pure culture. 



3. The tests necessary to establish its identity. 



i. Concentration. One or other of the following preliminary 

 methods may be used : 



(a) Enrichment. By the addition of concentrated broth or 

 other substance to the water, it may be converted into a nutrient 

 medium in which any contained typhoid bacilli can multiply so 

 as to be, after incubation, numerous enough to be present in a 

 very small quantity of the fluid. If only plain nutrient broth is 



