TYPES OF COLON-AEROGENES GROUP IN WATER 319 



the change in the ratio of B. aerogenes to B. coli was corre- 

 spondingly abrupt. The initial determinations showed three or 

 four times as many B. coli as B. aerogenes but at 7 days there 

 were ten times as many B. aerogenes as B. coli. 



There is a possibihty that on account of imperfections in the 

 parchment there was a mechanical loss of bacteria. The results 

 obtained were however consistent and in accord with those 

 reported by other investigators. Moreover it is very improb- 

 able that a mechanical loss would have resulted in the relative 

 changes observed in the abundance of the two species. 



RELATIVE CHANGES IN COLON-AEROGENES BACTERIA IN POL- 

 LUTED STREAMS 



It is difficult to even approximate the total number of colon- 

 aerogenes bacteria in a polluted stream but the relative number 

 of B. coli and B. aerogenes may be obtained with a fair degree of 

 correctness by isolating a considerable number of colonies and 

 determining the group to which they belong by the methyl red 

 test. 



This was done on two representative streams. The samples, 

 which were collected by following down the stream in an auto- 

 mobile, were held in ice water and taken to the laboratory at 

 once. They were plated on asparagin agar, a medium on which 

 colon types grow well, but which is not favorable to many other 

 bacteria particularly the streptococci. 



One of these streams was Wolf Creek, a rather sluggish stream 

 originating in swamps and flowing through Grove City, a town 

 of about 4000 inhabitants. On its entire course through the 

 town it is rendered stagnant by a dam and is polluted by houses 

 and stables on its banks. At the lower limits of the town it 

 receives the untreated city sewage. Below Grove City it flows 

 through a partly wooded farming country and for 15 miles 

 receives no sewage. Samples were collected at approximately 

 2 mile intervals and a number of colon cultures obtained from 

 each by direct plating. There is of course an element of chance 

 in picking cultures in this way but the results shown in table 3 

 are probably fairly representative. 



