Bacteriological Examination. 175 



good quality is more sharply marked by counts at 30° C. 

 than is the case with counts at 20° C. Determinations 

 at this temperature appear to be especially applicable to 

 the control of water filters, since the relative quality of 

 the raw and filtered waters and the expression of the 

 removal of bacteria by those filters are practically identical 

 with determinations made at 20° C, and, in addition, 

 they become available within a few hours after the sample 

 is collected." (Gage, 1907.) 



Finally, the search for the Bacillus coli furnishes the 

 most satisfactory of all single tests for fecal contamination. 

 This organism is preeminently 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 lactose-bile tube or the count on 



