108 ELEMENTS OF WATER BACTERIOLOGY 



colon bacilli often results, while other bacteria multiply 

 more slowly. With highly polluted waters gas forma- 

 tion will probably begin within 1 2 hours, but with fewer 

 colon bacilli present the duration must be increased. 

 If the period of incubation be too long continued, 

 trouble in the subsequent steps of the isolation may be 

 encountered because of overgrowths by the sewage 

 streptococci, or other forms which check the growth 

 of the colon bacilli in the later stages of fermentation 

 and finally kill them out. Even with pure cultures of 

 colon bacilli Clemesha (1912'^) has shown that sugar- 

 broth tubes may be almost sterile after 4 days. 



When it is desired to examine samples larger than 

 10 c.c. it becomes necessary to modify the enrichment 

 process by adding the nutrient material to the water 

 instead of the reverse. For this purpose dextrose 

 broth or phenol-dextrose broth (consisting of broth 

 with 10 per cent dextrose, 5 per cent peptone, and 

 .25 per cent phenol) may be added to the sample of 

 water to be enriched as suggested 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 24 hours, 

 and if at the end of that time growth has taken place, a 

 cubic centimeter is inoculated into a dextrose tube. 



Advantages and Disadvantages of the Dextrose Broth 

 Fermentation Tube. Experience with the dextrose 

 broth fermentation tube as a first step in the isola- 

 tion of colon bacilli soon led to the conclusion that a 

 fair idea of the sanitary quality of water could be 

 obtained from the results of this test taken by them- 

 selves and without the further process of isolating 



