160 TYPHOID FEVER. 



been added are allowed to melt and a sufficient quantity of 

 neutral litmus tincture is added to them to give a deep- 

 violet color. The tubes are sterilized and are inoculated, 

 one with the Bacillus typhosus, and the other with the Ba- 

 cillus coli communis. If agar tubes are used, they- are placed 

 in the incubator at 37 C. When the colonies grow, those 

 of the Bacillus typhosus retain the blue color, while the 

 colonies of the Bacillus coli communis become of a bright-red 

 color, and at the bottom of the tube can be seen bubbles of gas. 



III. Eisner's Method of Differentiation. This consists in 

 employing an acid mixture of gelatin, potato juice, and potas- 

 sium iodide, which contains neither peptone nor sodium chlo- 

 ride. It is used to separate not only the coli communis, but 

 also the ordinary saprophytes from the Bacillus typhosus. 

 The saprophytes do not develop at all in this medium, and 

 the colonies of coli communis and Bacillus typhosus show 

 marked differences in their behavior on plates made of this 

 mixture, and are easily separated. At the end of twenty- 

 four hours tubes of this mixture inoculated with the sus- 

 pected material will contain a large number of coli communis 

 colonies, which have the same appearance as cultures of this 

 bacillus on ordinary agar plates, whereas there will scarcely 

 be visible development of colonies of the Bacillus typhosus. 

 After forty-eight hours the Bacillus typhosus will appear as 

 small, pale, almost transparent colonies, easily distinguished 

 from the dark granular colonies of the coli bacillus. Accord- 

 ing to Abbott, Eisner's medium is thus prepared : 



"Grate 1 kilogram of pealed potato and allow this to stand 

 over night in a refrigerator ; then press out all juice, using 

 an ordinary meat-press for the purpose ; filter this fresh juice 

 cold to remove as much of the starch-granules as possible. 

 If this is not done, the starch when heated swells to such an 

 extent as to render filtration almost impracticable. Boil the 

 filtrate and again filter. Test the filtrate for acidity by 

 titrating 10 c.c. with a decinormal solution of sodium hy- 

 droxide, the indicator used being 6 drops of the ordinary 

 0.5 per cent, solution of phenolphtalein in 50 per cent, 

 alcohol. The acidity of the juice should be such as to re- 



