BACTERIA IN WATER 8 1 



Diseases Conveyed by Water. There are a few preliminary 

 features to be noticed before we enter in detail upon the 

 characteristics of several of the chief pathogenic bacteria in 

 water. 



In sterilised water, and in very highly polluted water or 

 sewage, pathogenic bacteria do not flourish. In the former 

 case they die of starvation, although there are some experi- 

 ments on record which appear not to support this view ; in 

 the latter case they are killed by the enormous competition 

 of common bacteria. Even in ordinary water there is a 

 wide divergence of behaviour. Some bacteria are destroyed 

 in a few hours; others appear to flourish for weeks. In all 

 cases the spores are able to resist whatever injurious proper- 

 ties the water may have much more persistently than the 

 bacilli themselves. These changes in the vitality of bacteria 

 in water, partly due to the water and partly to the other 

 micro-organisms, bring about two characteristics which it is 

 important to remember, viz., that pathogenic germs in water 

 are, as a rule, scanty and intermittent. It is these features 

 in conjunction with the enormous quantities of common 

 water bacteria which make the search for the bacillus of 

 typhoid what Klein has called " searching for a needle in a 

 rick of hay." Not that it cannot be detected, but its de- 

 tection is one of the most difficult of investigations. We 

 shall refer to this matter again when Bacillus typhosus is 

 under consideration. 



In artificial cultivation water bacteria respond very readily 

 to external conditions. Increase of alkalinity (.01 grams of 

 sodium carbonate added to 10 cc. of ordinary gelatine) 

 causes the number of colonies to be five or six times greater 

 than that revealed by using ordinary gelatine ; on the other 

 hand, very slightly increasing the acidity of a medium as 

 markedly diminishes the number of bacteria. Advantage is 

 taken of this in culturing the bacillus of typhoid, which does 

 not object to an acid medium. 



Water may become polluted in a variety of ways, and it 



