Presumptive Tresis for B. Colt. 



147 



SELECTIVE ACTION OF BILE SALTS. 

 (JACKSON, 1906.) 



Bacteria per c.c. 



Uncon- 



taminated 



Well. 



Contami- 

 nated 

 Pond. 



Suspen- 

 sion of 

 Feces. 



Suspen- 

 sion of 



Gelatin, 20° 



Agar, 37° 



Bile agar,* 37° 



Lactose bile agar,* 37° . 

 Lactose bile agar,* 37° . 

 Bile agar, 37° 



920 

 o 



2700 

 170 

 43 

 25 

 17 

 16 



350,000 

 450,000 

 300,000 

 250,000 

 250,000 

 60,000 



900,000 

 900,000 

 900,000 

 675,000 

 600,000 

 900,000 



* Bile diluted, 1:1. 

 Jackson therefore suggested the use of fresh ox bile 

 containing i per cent of lactose as a presumptive test 

 instead of dextrose broth. In particular he hoped that 

 this medium would be free to a great degree from the 

 negative results due to overgrowths in polluted waters. 

 He reported 275 examinations of badly contaminated 

 waters, in which 65 per cent of the samples failed to 

 give the dextrose-presumptive test, and only 10 per cent 

 failed to show gas in lactose bile. In a more recent 

 communication, Jackson (1907) reports that in the ex- 

 amination of 5000 samples of water at the Mt. Prospect 

 Laboratory, the bile medium has proved uniformly satis- 

 factory. He recommends incubation for 72 hours, results 

 bemg commonly obtained, however, after 48 hours; and he 

 considers any tube showing 25 per cent gas as positive. 

 In a series of examinations recently carried out at the In- 

 stitute of Technology, 16 per cent of the positive tubes 



