THE BACTERIA OF SOIL, AIR, WATER, ETC. 145 



manner of Esmarch. When the number of colonies is not 

 large this may serve very well. Counting may be assisted 

 by drawing lines with ink on the outer surface of the test-tube. 

 It is obvious that the character of the bacteria is of prime 

 importance; that pathogenic organisms may occasionally 

 be present, even when the number of bacteria to the cubic 

 centimeter is small. But knowing the number usually found 

 in a good water-supply, any sudden variation above that num- 

 ber is to be looked upon with suspicion as indicating a possible 

 contamination. 



The bacteriological examination should always be accom- 

 panied by a chemical examination, and by an inspection of 

 the surroundings. A large number is to be expected when 

 the water has been subjected to unusual agitation from winds 

 or currents which stir up the bacteria which have settled. 



The detection of pathogenic bacteria in water* involves 

 great difficulties, and our knowledge in this direction is very 

 meagre. Koch and several others have reported finding the 

 spirillum of Asiatic cholera in water. The examination of 

 water-supplies for this organism has disclosed the fact that 

 bacteria resembling the organism of cholera in many respects 

 are not uncommon in water. This adds to the difficulty of 

 detecting the cholera germ in water. 



The bacillus of typhoid fever has many times been described 

 as occurring in water-supplies suspected of being contaminated 

 with the excreta of cases of the disease. The interpretation 

 of these observations is at present doubtful. f It is now known 

 that several forms of bacteria exist which closely resemble the 

 bacillus of typhoid fever, and which make its recognition in an 

 unknown specimen very difficult. J 



It will at once be appreciated that the number of cholera and 



* See also articles in Part IV. on the bacillus of typhoid fever, bacillus coli 

 communis and spirillum of cholera. 



fConsult editorial. Journal American Medical Association. Dec. 5, 1903. 

 For methods of detection see under Typhoid Bacillus, p. 370. 



