BY R. GREIG SMITH. 745 



Potassium iodide appears to be most serviceable in aiding the 

 growth of Bact. typhi, or what is practically the same thing, 

 checking concomitant bacteria to such an extent as to render the 

 isolation of colonies of typhi an easier matter than would obtain 

 in media devoid of this salt. It is used in conjunction with a 

 potato medium which was introduced into bacteriology by Holz. 

 The preparation of the potato medium, however, as recommended 

 by Eisner, to whom we are indebted for the employment of 

 potassium iodide, differs slightly from that recommended by Holz. 



Eisner's method* — Potatoes are washed, pared, and finally 

 grated ; a litre of water is added for every 500 grams of the 

 pared potatoes and the suspension is pressed through a cloth. 

 The strained fluid is allowed to stand overnight and then it is 

 filtered. The nitrate is boiled and again filtered. Ordinary 

 gelatine is dissolved in the extract, when it will be found that 

 every 10 c.c. of the potato-gelatine requires from 2*5 to 3 c.c. of 

 tenth normal alkali to neutralise (using phenolphthalein as the 

 indicator); 10 c.c. portions of the gelatine are put into tubes 

 and sterilised. When required, 1 c.c. of sterile, 10% potassium 

 iodide is added to the molten gelatine and plates are made after 

 infection. On the plates, Bact. coli commune grows to the 

 exclusion of other bacteria, as large colonies in 24 hours, while 

 Bact. typhi produces small, clear colonies like drops of water in 48 

 hours. The latter are finely granular, while the former are 

 brownish and coarsely granular. 



Eisner used potassium iodide as a result of experiments con- 

 ducted with various chemicals, salts, resins, alkaloids and amido- 

 bodies. Of those which he tried — in number they came to many 

 hundreds — potassium iodide together with an acid medium was 

 found to be the best for differentiating between the typhoid and 

 colon varieties of bacteria and for excluding others. This method 

 is used chiefly for detecting typhi in faeces, but Remlinger and 

 Schneiderf have used it in testing waters and soils. Kiibler and 



* Eisner, Centralblatt fitr Bakt. 1 Abt. xvii., 591, Ref. 

 t Remlinger unci Schneider, Centralblatt f iir Bakt. 1 Abt. xix. , 244, Ref. 

 48 



