142 MANUAL OF BACTERIOLOGY. 



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



It has been said that a water-supply containing no more 

 than 500 bacteria per cubic centimeter is to be regarded as 

 safe, one having between 500 and 1000 is to be looked upon 

 with suspicion, and that where there are more than 1000 

 to the cubic centimeter the water is unfit for drinking pur- 

 poses. It is obvious, however, that the character of the 

 bacteria is of prime importance; that pathogenic organ- 

 isms 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 number is to be looked upon 

 with suspicion. An increase is to be expected when the 

 water has been subjected to unusual agitation from \vinds 

 or currents. 



The detection of pathogenic bacteria in water 1 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 diffi- 

 culty of detecting the cholera germ in water. 



The bacillus of typhoid fever has many times been de- 

 scribed 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. 

 It is now known that several forms of bacteria exist which 

 closely resemble the bacillus of typhoid fever, and which 

 would make its recognition in an unknown specimen very 

 difficult. 



It will at once be appreciated that the number of cholera 

 and typhoid organisms necessary to contaminate a consid- 



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

 coli communis and spirillum of cholera. 



