THE DISTRIBUTION OF MICRO-ORGANISMS 187 



produced is regarded as presumptive evidence of the approxi- 

 mate number of organisms of the B. coll type in the respective 

 volume of water. 



The recognition of pathogenic bacteria, such as the germs of 

 typhoid fever and Asiatic cholera, in water supplies has been 

 accomplished very infrequently. Search for the cholera germ 

 is best undertaken by adding a large volume, i to 10 liters, of the 

 suspected water to one-tenth its volume of a sterile solution con- 

 taining 10 per cent of peptone and 5 per cent of sodium chloride. 

 After incubation for twelve to twenty-four hours at 37 C., trans- 

 fers are made from the surface of this culture to tubes or flasks 

 containing Dunham's solution (peptone i per cent, salt 0.5 per 

 cent). At the same time gelatin plates are inoculated from 

 this surface material. The cholera organism, if present, tends 

 to outgrow all other bacteria in the surface film of such cultures, 

 and after one or two transfers in series it will so predominate 

 that it may be recognized by specific agglutination with a cholera- 

 immune serum in high dilution (i-iooo). The appearance of 

 the colonies on the gelatin plates is valuable as confirmatory 

 evidence, and from them perfectly pure cultures may be obtained 

 for further study. The search for the typhoid bacillus in water 

 is usually rather hopeless. It has occasionally been detected 

 by plating the water on special media such as litmus lactose 

 agar or gelatin, or by culture in broth at 39 C. and inoculation 

 of guinea-pigs or white rats with the culture, and subsequent 

 plating of the heart's blood from the dead animal. Supposed 

 typhoid bacilli isolated in this way must satisfy the biochemical 

 tests for B. typhosus and furthermore must show specific agglu- 

 tination with high dilutions (i -100) of typhoid-immune serum. 



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 



