136 BACTERIA IN AIR, SOIL, AND WATER. 



difficulty, for each belongs to a group of organisms morpho- 

 logically similar, very widespread in nature, and many of which 

 have little or no pathogenic action. The biological characters 

 of these organisms will be given in the chapters devoted to the 

 diseases in question, but here it may be said that from the 

 public health standpoint the making of their being found a 

 criterion for the condemning of a water is impracticable. 

 There is no doubt that the typhoid and cholera bacteria can 

 exist for some time in water — at least this has been found 

 to be the case when sterile water has been inoculated with 

 these bacteria. But to what extent the same is true when they 

 are placed in natural conditions, which involve their living in 

 the presence of other organisms, is unknown, for it may be 

 safely said that by no known method can the presence of either 

 be demonstrated in the complex mixtures which occur in nature. 

 With regard to the typhoid bacillus, of late the tendency has 

 been to seek for the presence of indirect bacteriological evi- 

 dence which might point in the direction of the possibility of 

 the presence of this organism. 



Methods of detecting B. coli in Water. — The isolation of B. coli is brought 

 about in several ways. One method aims at giving the total number of colo- 

 nies per cubic centimetreby plating various fractions of i c.c. in a 2 per cent 

 lactose agar, to which neutral litmus tincture has been added, and incubating 

 at 37° C. It is found that a large number of water bacteria will not grow at this 

 temperature, whereas B. coli grows well, its colonies becoming coloured red by 

 the action of the bacilli on the lactose and thus are readily identified. 



Theobald Smith recommends the use of the ferrrientation tube. A series 

 of such tubes, containing i to 2 per cent of glucose broth, are inoculated with 

 variable quantities of water and incubated for 48 hours at 37° C, and those 

 shawing 25-40 per cent, of gas are removed and plates made from their con- 

 tents and search made for the presence of B. coli by the usual cultural tests. 



Stone has broadened the application of the fermentation method of Smith 

 by removing .5 c.c. of the contents of those tubes showing the proper quantity 

 of gas, and adding it to a tube containing 10 c.c. of neutral broth and .3 c.c. of 

 Parietti's solution,^ and placing in the thermostat for 24 hours at 38° C. If 

 growth results, .5 c.c. is removed to a fermentation tube and incubated as 

 before ; then if no gas is formed it is presumed that the fermenting organism 

 met with was not B. coli, but on the other hand, if gas is formed B. coli is 

 doubtless present and can readily be identified by plating out and cultivating 

 on the various media. 



' Parietti's solution consists of Carbolic acid . . 5 grammes. 



Hydrochloric acid (pure) . 4 „ 

 Water (distilled) . . .100 cc. 



