112 ELEMENTS OF WATER BACTERIOLOGY. 



forms indicate the presence of an excess of organic matter 

 derived in all probability either from sewage or from the 

 fresh washings of the surface of the ground. In either 

 case danger is indicated. 



A still closer measure of polluting material may be 

 obtained from the numbers of colonies which develop 

 on litmus-lactose-agar at 37 , since organisms which 

 thrive at the body temperature, and particularly those 

 which ferment lactose, are characteristic of the intestinal 

 tract and but rarely occur in normal waters. 



Finally, the search for the Bacillus coli furnishes the 

 most satisfactory of all single tests for fecal contamination. 

 This organism is pre-eminently a denizen of the alimen- 

 tary canal and may be isolated with ease from waters to 

 which even a small proportion of sewage has been added. 

 On the other hand, it is never found in abundance in 

 waters of good sanitary quality; and its numbers form 

 an excellent index of the value of waters of an interme- 

 diate grade. The streptococci appear to be forms of a 

 similar significance useful as yielding a certain amount of 

 confirmatory evidence. The full bacteriological analysis 

 should then consist of three parts — the gelatin-plate count, 

 as an estimate of the amount of organic decomposition 

 in process; the total count, and the count of red colonies, 

 on litmus-lactose-agar, as a measure of the organisms 

 which form acids and thrive at the body temperature; 

 and the study of a series of dextrose-broth tubes for the 

 isolation of colon bacilli and streptococci. The simple 

 examination of the dextrose-broth tube and the count on 



