EXAMINATION OF "WATER 401 



incubated for twenty-four hours. In use the centre of the 

 medium is touched with a platinum loop charged with the 

 material to be examined, the flask or dish is enclosed in a 

 much larger one in order to prevent condensation, and the 

 whole is placed in an incubator kept at 35° 0. for twenty- 

 four hours. Pure cultures of typhoid so treated produce 

 an opalescence occupying about two-thirds of the medium ; 

 B. coli gives a flat plate somewhat thicker and moister than 

 the usual form. If the inoculation is made with both 

 organisms, either from separate cultures or from a mixed 

 culture, we get a flat plate of B. coli in the centre, with an 

 opalescent halo of pure typhoid. Plates of this medium, 

 inoculated direct from typhoid stools, gave without 

 difficulty the same pure culture of typhoid bacilli; and 

 it is anticipated that this will become a valuable diagnostic 

 test, as easy of application, though not quite as rapid, as 

 the serum test. It is best applied by putting two or three 

 loops of stool into a little sterile broth, shaking and 

 inoculating as described. Tap-water, also inoculated with 

 a trace of a broth culture of typhoid, or typhoid and coli 

 mixed, readily yielded pure cultures of typhoid. 



It was found, however, that on applying it to polluted 

 waters, that in many cases there were present organisms, of 

 which more than a dozen were isolated, which responded 

 to all the accepted tests for the B. coli, but differed 

 from it in growing like typhoid on the agar-gelatine 

 medium. Moreover, when mixed cultures of typhoid and 

 one or more of these coli-like forms were inoculated on to 

 this medium, the typhoid was suppressed, and a pure 

 culture of the non-pathogenic organism obtained. 



The same inhibition of typhoid also resulted when a 

 mixed culture in broth was attempted, though the true 

 B. coli and typhoid grow normally together. As these 

 organisms retain their vitality in media so highly car- 



26 



