THE BACTERIA OF SOIL, AIR, WATER, ETC. 147 



is the addition of a solution of peptone to a large quantity of 

 the water to be examined with a view to assisting the develop- 

 ment 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; Jacksonj advocates the use of ox-bile for 

 the same purpose. The committee on Standard Methods of 

 Water Analysis,. Am. pub. Health Assn., J suggest the follow- 

 ing modifications of previous recommendations for the detec- 

 tion of B. coli in water: 



Omit determinations of motility: omit tests for coagulation 

 of milk in case cultures have been isolated in lactose agar 

 plates; omit determination of the amount of gas formed and 

 the gas ratio in both presumptive and in confirmatory tests; 

 allowing the use of bile-broth instead of dextrose-broth. 

 Suspected bacteria may be tested by inoculation into animals; 

 the possession of pathogenic properties is thought by some to 

 create a probability in favor of their having come from some 

 contamination with animal excreta. 



If it is not already apparent from what has been said, it must 

 be here emphasized that the difficulty of detecting the presence 

 of pathogenic bacteria in water is very great, and the length of 

 time necessarily consumed in making the tests greatly lessens 

 the value of the results when obtained. Added to this is the 

 further limitation of the value that a negative result, i. e., where 

 no pathogenic bacteria found, cannot be taken as proof that 

 the water-supply under examination may not be contaminated 



*Prescott. Report of American Public Health Association. Vol. XXIX., 

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



^Journ. Infect. Dis., Sup. No. 3. May, 1907. p. 30-32. 



^American Journ. oj Public Hygiene. Vol. XVII., No. 4, Nov., 1907, p. 367. 



Consult Vaughan. Journal American Medical Association. April 9, 1904. 

 For special methods of detecting the Bacillus coli communis see under this 

 bacillus, page 310. For general subject of Water Supply and Public Health 

 see Jordan. Journ. Am. Med. Assn., V. XLVIIL, May i8-June 8, 1907. 



