BACTERIA IN WATER 1023 



of a 7.75 per cent solution of sodium hyposulphite and 20 c.c. of a 

 10 per cent solution of lead nitrate. When the precipitate has settled, 

 the clear supernatant fluid is decanted and the precipitate dissolved 

 in a saturated sodium hyposulphite solution. This clear solution is 

 then plated. Willson 13 has modified this method by adding to the 

 water 0.5 gm. of alum to each liter. The supernatant fluid is removed 

 and the precipitate plated. 



The isolation of the vibrio of cholera is less difficult than that 

 of B. typhosus, primarily because of the much greater numbers of 

 these microorganisms discharged into sewage. The number of 

 cholera spirilla in the excreta of cholera patients is enormously 

 higher than is that of B. typhosus in the stools of typhoid-fever 

 patients. It is not infrequent, therefore, that the source of a cholera 

 infection may be directly traced to the water supply. Koch, 1 * the 

 discoverer of the cholera vibrio, has indicated a method which has 

 frequently found successful application. 



To 100 c.c. of the infected water are added one per cent of 

 pepton and one per cent of salt. This mixture is then incubated at 

 37.5 C., and after ten, fifteen, and twenty hours, specimens from 

 the upper layers are examined microscopically and are plated. The 

 scum from the surface of such a medium may be plated on the 

 starch agar of Stokes and Haechtel, 15 on which colonies of intestinal 

 spirilla will appear pink and spreading. 



Because of the great difficulties outlined above in isolating specific 

 pathogenic germs from polluted waters, bacteriologists have at- 

 tempted to form an approximate estimation of pollution by the 

 detection of other microorganisms which form the predominating 

 flora of sewage. Chief among these is B. coli. The isolation and 

 numerical estimation of B. coli in polluted water has been for a 

 long time one of the criteria of pollution. This so-called colon test, 

 however, should always be approached with conservatism and should 

 never be carried out qualitatively only. Careful quantitative es- 

 timation should be made in every case. 



B. coli in water is by no means always the result of human con- 

 tamination, since this bacillus is found in great abundance in the 



13 Willson, Jour, of Hyg., v, 1905. 



"Koch, Zeit. f. Hyg., xiv, 1893. 



15 Stokes and Haechtel, see Eeport 1915 A. P. H. A., on Water Analysis. The 

 medium is an agar with 5.5 grams agar, 5.0 meat extract, 10 Pepton and 8.5 NaCl 

 to liter to which is added 10 grams of soluble starch. 



