ISOLATION OF SPECIFIC PATHOGENES 75 



containing increasing amounts of a solution of 4 per 

 cent hydrochloric acid and 5 per cent phenol. From 

 tubes in which growth occurs after 24 hours at 37 degrees 

 the organisms present may be isolated in pure cultures 

 by some plating method and identified by subcultures. 



The great difficulty with a majority of the enrichment 

 processes is that the conditions which favor the multipli- 

 cation of the typhoid bacillus are frequently suited in an 

 even higher degree to B. coli and other intestinal organ- 

 isms. Being present in almost all cases in much 

 higher numbers than B. typhi, these bacteria develop 

 more abundantly, and effectually mask any disease 

 germs originally present. In order to obviate this 

 difficulty, Hankin (Hankin, 1899), a f ter adding suc- 

 cessively increasing portions of Parietti solution to 

 tubes inoculated with the water to be tested, selected 

 the second highest tube of the series in which growth 

 occurred for the inoculation of a new set, finally plating 

 as above. He believed that the chance for overgrowth 

 by this method is somewhat decreased; but in the hands 

 of other investigators it has not met with marked suc- 

 cess. Klein (Thomson, 1894) in his investigations, 

 made use of the Berkefeld filter to concentrate the 

 organisms in the sample. Some observers abandoned 

 the enrichment process altogether and recommended 

 direct plating upon solid media such as phenolated 

 gelatin or the Eisner (Eisner, 1896) medium, made by 

 adding 10 per cent of gelatin and i per cent of potas- 

 sium iodide to an infusion of potato whose reaction 

 has been adjusted to 30 on Fuller's scale. 



In the last five years considerable progress has been 



