PRESUMPTIVE TESTS FOR B. COLI. 



97 



Results harmonizing well with the above were obtained 

 by Nibecker and one of us (Winslow and Nibecker, 1903), 

 as quoted in Chapter IV. Of 775 dextrose tubes inocu- 

 lated from 259 samples of water from apparently unpol- 

 luted sources, only 41 showed gas and only 3 gave the gas 

 formula characteristic of B. coli. 



The litmus-lactose-agar plate furnishes another pre- 

 sumptive test of considerable value as indicated in Chap- 

 ter IV, although it is probably less delicate than the dex- 

 trose-broth method. With polluted waters in particular 

 this test will be found of value, since streptococci may 

 interfere with the dextrose-broth method if dilution is 

 insufficient or incubation too prolonged. 



Certain special media have also been suggested for 

 rapid routine water analysis of which those containing 

 " neutral red," one of the safranine dyes, has been most 

 fully studied. Rothberger (Rothberger, 1898) first pointed 

 out that B. coli reduces solutions of this substance, the 

 color changing to canary-yellow accompanied by green 

 fluorescence. Makgill (Makgill, 1901), Savage (Savage, 

 1901), and other English observers report favorable 



