TYPES OF COLON-AEROGENES GROUP IN WATER 323 



grains which fermented adonite had all the characters of the 

 B. aerogenes cultures from feces. If these tabulations should be 

 made on this basis, it would be found that the cultures of the 

 fecal B. aerogenes type from water would agree very closely 

 with those from feces while they would be quite distinct from the 

 grain cultures. There is a decided difference in the character- 

 istics of the liquefying cultures from grains and from feces but 

 in the light of our present knowledge of this sub-group, it would 

 be unsafe to make any definite deductions from these data. 

 Greenfield, (1916) found that of 405 cultures from ground and 

 surface waters 70 per cent were of the B. coll type as indicated 

 by the methyl red and Voges-Proskauer tests. 



More light can be thrown on the value of a quaUtative exami- 

 nation on the colon bacteria by a study of the results from indi- 

 vidual samples. Space will not permit a consideration of all 

 the samples but a few representative ones are given. 



No. 31. Rock Creek. This is a polluted stream previously described. 

 Two samples were taken from which 13 cultures were isolated. Eleven 

 of these were high ratio, gelatin -, indol -, adonite +, dulcite -, 

 sucrose and salicin +. One differed from these in bemg mdol + and 

 dulcite +. The characters of these cultures agreed very closely with 

 those of the high ratio cultures isolated from human feces. 



In view of the results obtained in the survey of Rock Creek it is 

 rather surprising that no B. coli cultures were obtained from these 

 samples. The samples from which B. coli were isolated were taken 

 about a year later than those giving all B. aerogenes. The disappear- 

 ance of the B. coli type may be looked upon as evidence of self purifi- 

 cation and in this the time element is an important factor. The rate 

 of flow which varies greatly in a small stream has a direct influence on 

 the time for which sewage is exposed to purifying influences before it 

 reaches a given point. There is also a possibility that the pollution 

 in the lower part of Rock Creek may have become materially increased 

 after the first samples were taken. 



No 10 The Potomac River. The pollution of this river has been 

 very thoroughly studied (Cumming, 1915). The principal source of 

 pollution is the city of Cumberland about 180 miles above Washington. 

 Sewage is emptied into the river or its tributaries at other points nearer 

 Washington but, considering the volum.e of water flowing m the river 

 they are relatively unimportant. 



