139a ELEMENTS OF WATER BACTERIOLOGY 



the fact (pointed out on page 127) that lactose bile is 

 less sensitive than lactose broth as a presumptive test, 

 as shown by the number of positive isolations of B. 

 coli recorded. Thus Young, of the Kansas State Water 

 Survey, notes that out of 140 samples showing fermen- 

 tation in the presumptive test, 116 were found to give 



Station. 



Giesboro Point 



Fort Foote 



Fort Washington 



Mt. Vernon to White- 

 stone Point 



Indian Head 



Possum Point 



Maryland Point 



Pope's Creek 



Lower Cedar Point . . . 



All below Lower Cedar 

 Point 



No. of 

 Samples. 



770 



, 789 



778 



851 

 241 

 212 

 681 

 740 

 476 



2261 



Average 

 B. coli. 



29s 

 254 

 123 



102 

 123 

 66.2 

 1.44 

 .19 

 •14 



.052 



Avg. Fer- 

 menting 



Organ- 

 isms not 



B. coli 



7 

 2 



5 

 7 

 8 



75 

 33 

 16 



•057 



Percent- 

 age not 

 B. coli to 

 B. coli. 



8.1 

 3-8 

 4.8 



8.3 

 12.8 



20.8 



52.1 



174.0 



114. o 



IIO.O 



Percent- 

 age Fer- 

 menting 

 Organ- 

 isms Con- 

 firmed 

 B. coli. 



92-5 

 96.4 



95-2 



92.2 

 88.6 

 82.4 

 68.9 

 36.6 

 46.7 



47-7 



positive confirmatory results for B. coli. Of these, 

 95 were positive in the lactose bile presumptive test, 

 but 20 samples showed B. coli with lactose broth which 

 were negative with lactose bile. In order to over- 

 come these irregularities, a method of cross-inoculation 

 was followed, using three media — lactose bile, lactose 

 broth and dextrose bile. The procedure was as follows: 

 Each sample of water was inoculated into the three 

 media, using one large fermentation tube with 10 c.c. 

 of the sample and five ordinary tubes with i c.c. each. 

 Incubation for 24 hours was maintained. If the fer- 



