288 VETERINARY BACTERIOLOGY 



Usually the search for the specific organism in the water-supply 

 is not begun until there is an outbreak of the disease, and the 

 probabilities are that the organism by that time has disappeared 

 from the supply. Many methods for isolation have been devised, 

 but are not commonly used. For the most part, they are de- 

 pendent upon enrichment by placing the suspected water in broth 

 containing antiseptics which will inhibit the growth of other bac- 

 teria, but not of the intestinal forms. This material is then plated, 

 and the typhoid-like colonies are fished out and tested one at a 

 time, the crucial test usually applied being the ability to agglutin- 

 ate with high dilution of typhoid antiserum. 



The specific organism of Asiatic cholera may be more readily 

 isolated than that of typhoid. Flasks of peptone salt solution are 

 inoculated with the suspected water, and incubated at blood-heat 

 for twenty-four hours or less, and transfers made to fresh flasks 

 from the surface layer. The cholera spirillum has a considerable 

 avidity for free oxygen, and swarms just below the surface in much 

 greater numbers than elsewhere in the medium. Plates made 

 from this surface film should show the characteristic colonies. 



Isolation of B. coli. Advantage may be taken of the physiologic 

 and cultural characteristics of the B. coli to isolate it from water. 

 In examination of large numbers of .samples it is often found 

 useful to make what is termed a preliminary or presumptive test 

 for the presence of the colon bacillus. Fermentation tubes con- 

 taining 1 per cent, dextrose (glucose) broth are inoculated with 

 varying amounts of the water to be tested. If gas is not produced 

 in any of the tubes, it is evident that B. coli is not present; at least 

 in any considerable numbers. A negative result is, therefore, 

 good evidence of the purity of the water examined. A positive 

 test makes it probable that the water contains the colon bacillus, 

 although further tests are necessary to establish the fact; hence 

 the name, presumptive test. The evidence that B. coli is present 

 is much strengthened if gas to the amount of 30 per cent, or more, 



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and having a composition of ^7^~ = 7, is produced. An approxima- 

 tion of the number of colon bacilli present may sometimes be made 

 by observing the dilutions of the water in which gas is produced. 

 For example, if gas is produced in dilutions of 1 : 0, 1 : 10, and 1 : 100, 



