354 BACTERIOLOGY. 



low power of the microscope as small, pale, finely gran- 

 ular, almost transparent bodies that are easily distin- 

 guished from the coarser, brownish colonies of the colon 

 bacillus. 



While the method is useful, it has its limitations, and 

 is not always reliable. At times colon colonies will 

 develop in a way that would readily cause one to mis- 

 take them for typhoid colonies, while again typhoid col- 

 onies will take on the characteristics of those due to the 

 growth of the colon bacillus. This is especially the 

 case in plates over forty-eight hours old that have been 

 kept at ordinary room temperature. 



In our experience the most serviceable feature of this 

 method is the elimination of many of the common sap- 

 rophytes usually present in mixtures containing the 

 typhoid and colon bacilli. The majority of them do 

 not grow upon gelatin made by this method, which will 

 now be described. 



The description given by Eisner 1 for the mode of 

 preparation of the medium is so incomplete and unsat- 

 isfactory in most of the important details that practi- 

 cally all those who have used the method have been 

 obliged to develop their own technique from the general 

 suggestions made in his original communication. The 

 " Eisner medium" that has given satisfaction in our 

 hands is prepared as follows: grate 1 kilogram of 

 peeled potatoes and allow to stand in the refrige- 

 rator over night. Then press out all the juice, using 

 an ordinary meat-press for the purpose. Filter this 

 fresh juice cold, to remove as much of the starch gran- 

 ules as possible; if this is not done the starch when 



i Eisner : Zeit. fur Hygiene und Infectionskrankheiten, 1896, Bd. 21, p. 25. 



