696 BACTERIA IN AIR, SOIL, WATER, AND MILK 



B. typhosus in the stools of typhoid-fever patients. It is not infre- 

 quent, 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, 2 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 attempted 

 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 pollu- 

 tion. This so-called colon test, however, should always be approached 

 with conservatism and should never be carried out qualitatively only. 

 Careful quantitative estimation 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 in- 

 testines of 'domestic animals. According to Poujol, B. coli does not 

 even always point to fecal contamination, since this author was able to 

 find the bacillus in the water of a number, of wells where no possible 

 contamination of any sort could be traced. Prescott 3 explains this, -as 

 well as similar cases, by the fact that organisms of the colon group may 

 occasionally be parasitic upon plants. 



The opinions of hygienists are widely at variance as to the value of 

 the colon test. While the discovery of isolated bacilli of the colon group 

 may therefore be of little value, it is nevertheless safe to follow the opin- 

 ion of Houston, 4 who states that the discovery of B. coli in considerable 

 numbers invariably points to sewage pollution, and that the absolute 

 absence of B. coli is, as a rule, reliable evidence of purity. 



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



2 Stokes and Haechtel, see Report 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 litre to which is added 10 grams of soluble starch. 



3 Prescott, Science, xv, 1903. 



4 Houston, Rep. Medical Officer, Local Gov. Board, London, 1900. 



