BACTERIA IN WATER 6 1 



cient to indicate sewage pollution, seeing that it is so widely 

 distributed. But in large numbers, and in company with the 

 other named species, it is almost certain evidence of sewage- 

 polluted water. 



It may occur to the general reader that, as the typhoid 

 bacillus is not extremely rare, drinking water may frequently 

 act as a vehicle to carry the disease to man. But, to 

 appreciate the position, it is desirable to bear in mind the 

 following facts : the typhoid bacillus is only found in the 

 human excrement of patients suffering from the disease ; 

 it is short-lived ; in ordinary waters there exist organisms 

 which can exert an influence in diminishing its vitality ; 

 exposure to direct sunlight destroys it ; and it has a ten- 

 dency to be carried down-stream, or in still waters settle 

 at the bottom by subsidence. Even when all the conditions 

 are fulfilled, it must not be forgotten that a certain definite 

 dose of the bacillus is required to be taken, and that by 

 a susceptible person. Into these latter questions of how 

 bacteria produce disease we shall have an opportunity of 

 inquiring at a later stage. 



We must now mention several of the special media and 

 tests used in the separation of Bacillus typhosus and B. coli. 



i. The Indol Reaction. Indol and skatol are amongst the 

 final products of digestion in the lower intestine. They are 

 formed by the growth, or fermentation set up by the growth, 

 of certain organisms. Indol may be recognised on account 

 of the fact that with nitrous acid it produces a dull red 

 colour. The method of testing is as follows. The suspected 

 organism is grown in pure culture in broth, and incubated for 

 forty-eight hours at 37 C. Two cc. of a 4 per cent, solution 

 of potassium nitrite are added to 100 cc. of distilled water, 

 and about I cc. of this is added to the test-tube of broth 

 culture. Now a few drops of concentrated sulphuric acid 

 (unless quite pure, hydrochloric should be used) are run 

 down the side of the tube. A pale pink to dull red colour 



