132 MANUAL OF BACTERIOLOGY. 



made from samples of water will recognize the difficulty of 

 detecting one or a few colonies of the bacteria of cholera or 

 typhoid fever among a hundred or more colonies of ordinary 

 water-bacteria. The existence of contamination with animal 

 excreta might, however, be indicated by finding the bacillus 

 coli communis, whose detection offers a greater prospect of 

 success. It is not certain just how much importance is to be 

 attributed to the presence of small numbers of the colon bacillus 

 in water.* Until our knowledge is more complete any sus- 

 picious water should be discarded. 



At present investigators seem to agree that if, using several samples of a 

 water each i c.c. in volume, colon bacilli are found in a majority of the samples 

 the water is probably polluted; if the colon bacillus is only found when larger 

 volumes of water are examined, the results are suspicious though less signif- 

 icant. Some investigators hold that the presence of streptococci in water is 

 indicative of pollution. f 



Certain devices have been adopted to hasten the develop- 

 ment of the desired bacteria and to retard that of the ordinary 

 water-bacteria. Among these may be mentioned the influence 

 of the heat of the incubator, which will hasten the growth of 

 organisms derived from the human body, and which retards 

 the growth of water-bacteria. Another is the addition of a 

 solution of peptone to a large quantity of the water to be ex- 

 amined with a view to assisting the development of the desired 

 bacteria by furnishing them suitable food for growth. In 

 another method (Parietti's) small quantities of carbolic acid 

 are added to bouillon and mixed with the water, with a view to 

 retarding the development of all except typhoid | and colon 

 bacilli. Suspected bacteria may be tested by inoculation into 



* Jordan. Journal oj Hygiene. Vol.1. 1901. Savage. Journal oj Hygieve. 

 Vol. II. 1902. Winslow and Hunnewell. Journal Medical Research. Vol. 

 VIII. 1902. 



t Prescott and Baker. Journal oj Infectious Diseases. I. 193. 



X Prescott. Report of American PubUc Health Association. Vol. XXIX, 

 356. Clark and Gage. Ibid: 386. Bissell. Ibid. 360. 



