106 ROYAL SOCIETY OF CANADA 



cent of the real number. On the oilier hand, the positive presumptive 

 tests in bile, cansidc'rin»,^ all gus formation as positive, would have given 

 78 positive results, or 111 per cent, only slightly in excess of the true 

 value. As a presumptive test the lactose-bile medium seems to yield a 

 fair approximation to the troth, being far superior in this respect to 

 dextrose broth. 



Savage** strongly recommends the use of neutral red bile salt broth, 

 the bile salt checking the growth of organisms other than B. coli, and the 

 reduction changes in the neutral red ser^■ing as an indicating agent. 



In the examination of water for B. coli none of these " presumptive '' 

 tests are reliable^ in themselves, either " to indicate that a water is good 

 " or bad or that it contains or is free from B. coli. These preliminary 

 " tests have a great value in that they indicate the probable numerical 

 " presence of B. coli, and so save the labour of plating all the different 

 " dilutions made. That is their object, and as such they are of extreme 

 " value, but they are not sufficiently relial)le to take the place of the 

 " isolation and numerical determination of B. coli itself." The American 

 Standard Committee come to similar conclusions. 



The experiences of the investigators mentioned above have been fully 

 borne out by our own. During the last few years one of us has had 

 occasion to analyse several hundred samples of water from many sources, 

 the enrichment medium usually employed being glucose broth in fer- 

 mentation tubes. Latterly, however, the neutral red bile salt medium 

 has been used with fairly satisfactory results. 



Eecently, we have had occasion to make a large number of analyses 

 of the Ottawa Elver water, and as B. coli or its varieties was constantly 

 present in amounts of 5 c.c. or less, it was necessary to find out the 

 exact number of this organism, and for this purpose we employed the 

 neutral red bile salt lactose agar of Griinbaum & Hume. The count- 

 ing and isolation of B. coli from this medium gave us fair results, but 

 even with considerable practice, we were at times unable to determine 

 from the appearance of the colony whether it was that of B. coli or 

 some other organism which produced red colonies with the surrounding 

 medium red; and on this account wo determined to use aesculin instead 

 of neutral red. 



Aesculin is a glucoside and undergoes a hydrolitic fermentation 

 when B. coli, and some other organisms are grown in a medium contain- 

 ing this sugar. The reaction, as nearly as one can express by this means 

 a bacterial reaction is probably as follows: — 



C„ H„ O, -f H, O - CV, H,2 0« + (; H, O, 

 Aesculin -J- Water = Sugar + Aesculetin 



