ISOLATION OF THE COLON BACILLUS. 63 



ber of colon bacilli present is small, as is clearly shown by 

 the following table from Irons. 



PROPORTION OF POSITIVE RESULTS IN TESTS OF POLLUTED AND 

 UNPOLLUTED WATERS BY DEXTROSE FERMENTATION-TUBE AND 

 CARBOL-BROTH METHODS. 



(Irons, 1001.) 



Dextrose 

 Fermentation- 

 tube. 



Carbol-broth 

 Method. 



Polluted waters 



Relatively unpolluted waters. 



+ - ? 

 33 3i 5 



56 38 25 



+ - ? 

 38 3° 1 



37 6l 2I 



Furthermore, the dextrose fermentation-tube possesses 

 a second advantage in the fact that a decisive negative 

 test may be obtained within twenty-four hours of the 

 original planting of the sample. If no gas is formed in 

 the tube, we commonly assume that B. coli is absent and 

 carry the examination no farther. 



When it is desired to examine samples larger than 1 c.c. 

 for B. coli it becomes necessary to modify the enrichment 

 process by adding the nutrient material to the water 

 instead of the reverse. For this purpose phenol dex- 

 trose broth (consisting of broth with 10 per cent dex- 

 trose, s per cent peptone, and .25 per cent phenol) may 

 be added to the sample of water to be enriched as sug- 

 gested by Gage (Gage, 1901). Generally 10 c.c. of the 

 broth is added to 100 c.c. of the water. The sample is 

 then incubated at 37 for twenty-four hours, and if at the 

 end of that time growth has taken place, a cubic centi- 



