Cultivation 489 



colonies of the typhoid bacillus and the colon bacillus are so 

 similar as to make it impossible to recognize a single colony 

 of either with certainty, and until some satisfactory method 

 for their differentiation is found, the only solution of the 

 problem will be to transfer a large number of colonies to 

 some culture medium, in which specific differences can be 

 recognized, and study the resulting growths. 



Special media devised for the purpose of developing the 

 specific differences, such as rapidity of growth, acid-pro- 

 duction, etc., are numerous. Thus, Eisner* has suggested 

 the employment of a special medium made by allowing i 



Fig. 150. Bacillus typhi abdominalis ; superficial colony two days 

 old, as seen upon the surface of a gelatin plate. X 20 (Heim). 



kilogram of grated potatoes (the small red German potatoes 

 are best) to macerate over night in i liter of water. 

 The juice is carefully pressed out, and filtered cold to get 

 rid of as much starch as possible. The filtrate is boiled and 

 again filtered. The next step is a neutralization, for which 

 Eisner used litmus as an indicator, and added 2.5-3 c - c - 

 of a y 1 -^ normal sodium hydrate solution to each 10 c.c. 

 of the juice. Abbott prefers to use phenolphthalein as an 

 indicator. The final reaction should be slightly acid. Ten 

 per cent, of gelatin (no peptone or sodium chlorid) is dis- 

 solved in the solution, which is boiled, and must then be 

 again neutralized to the same point as before. After filtra- 



* "Zeitschrift fur Hygiene," xxn, Heft 1, 1895; Dec. 6, 1896. 



