270 MICRO-ORGAOTSMS IN WATER 



temperature (18-20° C), the tubes containing the B. 

 coli communis invariably exhibit after 24 to 48 hours 

 numerous conspicuous gas-bubbles distributed through 

 the solid medium, whilst no such bubbles make their 

 appearance in the tubes containing the typhoid bacilli. 

 The test possibly depends upon the meat-extract con- 

 taining sufficient dextrose (derived from the post-mortem 

 transformation of the glycogen in the blood) for a 

 visible fermentation by the B. coli communis to take 

 place. The bubbles of gas are certainly independent of 

 any ingredients present in either the gelatine or in the 

 peptone, for we have found them to form in agar-agar- 

 peptone (of the composition given on p. 16), and also 

 in meat-extract-gelatine to which no peptone had been 

 added. The great convenience of the test depends upon 

 its involving only the use of a medium which must in- 

 variably at all times be at hand in every bacteriological 

 laboratory, and also in its dispensing with the use of 

 an incubating temperature. 



From the above sketch it will be seen how many 

 difficulties attend the successful identification of the 

 typhoid bacillus in the presence of its constant com- 

 panion, the B. coli communis, but as many of the methods 

 which have been devised for its isolation are not only 

 extremely ingenious, but also of great service in eliminat- 

 ing other organisms, a brief account is here given of 

 the more important of these. 



TJffelmann's Method} — This method is based upon 

 the idea that few micro-organisms can flourish in as 

 acid a culture medium as the typhoid bacillus. Thus, 

 for the demonstration of the latter in the presence of 

 other microbes, Uffelmann uses gelatine to which a defi- 

 nite quantity of citric acid (eight drops of a 5 per cent, 

 citric acid solution to 10 c.c. of gelatine) and methyl 



1 Berliner Idinische WochenscTirift, 1891, No. 35, p. 857. 



