SIMPLIFIED CONFIRMATORY TEST FOR B. COLI 557 



Occasionally B. coli are found in water which form non-typical 

 colonies on Endo medium, i.e., colonies without a metallic 

 sheen. Under such conditions, at least two colonies considered 

 to be most likely B. coli, are transferred into lactose broth fer- 

 mentation tubes (Standard Method) and gas formation observed 

 after twenty-four and forty-eight hours. The presence of gas 

 in these tubes constitutes a positive test, while the absence of 

 gas at the end of forty-eight hours constitutes a negative test 

 for B. coli. In view of the difficulty in differentiating non-typi- 

 cal colonies of B. coli from non-colon colonies on the Endo plate, 

 it is thus possible, by inoculating non-colon colonies into fer- 

 mentation tubes, to miss the detection of B. coli. On the other 

 hand, when such colonies are grown in Endo tubes, we have, 

 aside from their growth on the slant, also gas formation in the 

 butt. We thus have two factors which help to establish the 

 presence of the organisms. 



There is, of course, a remote possibility that we might have 

 colon-like colonies on an Endo agar slant and gas formation 

 in the butt, due to an anaerobe instead of B. coli. When such a 

 condition is suspected, inoculations of several colon-like col- 

 onies in lactose broth fermentation tubes, should be resorted to. 

 It might be mentioned however, that in our work with anaerobic 

 lactose fermenting organisms, obtained from waters of the 

 southeastern area of the United States, in not a single instance 

 did such organisms produce gas in the butt of Endo tubes. 



Another important advantage of the substitution of the Endo 

 tube for the Endo plate in confirmatory tests for B. coli, lies in 

 the fact that tubes can be kept in the ice-box from three to 

 four weeks without deterioration. Russell (1912) and Robinson 

 and Rettger (1916) have suggested that Endo medium for ty- 

 phoid work be first stored in tubes, and plates poured from these 

 when ready for use. These investigators found that this me- 

 dium keeps well in tubes from two to three weeks in the ice-box. 



It is well known that Endo plates, even if kept in the dark, 

 will begin to redden after forty-eight hours. This appears to be 

 due to the large surface exposed to air, which brings about the 

 oxidation of the unstable sodium sulfite to sulfate. Tubes con- 



