136 BIOLOGY AND TECHNIQUE 



2. Neutralize and clear by filtration. 



3. Add 10 c.c. of ten per cent sodium carbonate solution in order to 

 render it alkaline. 



4. Add 10 grams of chemically pure lactose. 



5. Add 5 c.c. of alcoholic fuchsin solution, filtered before using. 

 (Endo in his original contribution does not mention the strength of 

 this fuchsin solution, which, however, should be saturated.) 



This colors the medium red. 



6. Add 25 c.c. of a ten per cent sodimn sulphite solution. This 

 again decolorizes the medium, the color not entirely disappearing, how- 

 ever, until the agar is cooled. 



7. Put into test tubes, 15 c.c. each, and sterilize. 



The medium should be kept in the dark. For use, plates are poured 

 and sin-face smears of stools made. Endo claims that upon this medium 

 the typhoid bacillus outgrows the colon bacillus and its colonies remain 

 colorless, while those of bacilli coli become red. 



The preparation of Endo's medium presents certain difficulties 

 which arise largely from the varying purity of the sodium sulphite. 

 Kastle and Elvove' accordingly recommend the use of anhydrous sodium 

 sulphite instead of the crystallized variety which is hydrated. Harding 

 and Ostenberg^ recommended the following method of preparing Endo's 

 medimn which we beheve to be excellent. They adopted the method 

 largely because Na^SOa is easily oxidized and therefore varies in SO2 

 content. They add soditmi sulphite solution to a measured amount of 

 .5 per cent fuchsin solution until they determine the proportions which 

 give the greatest delicacy of reaction as tested with formaldehyde. The 

 proportions so determined are then added to the hot 3 per cent agar. 

 This insures a delicate medium. 



Although Endo described his medixun as dependent upon the forma- 

 tion of acid by the bacteria, this is not so. Acids give no coloration of 

 the sulphite-fuchsin mixture. Indeed this mixture is used by chemists 

 under the name of Schiff's reagent as a test for aldehydes. Acids decolor- 

 ize the red caused by aldehydes, and this accounts for the frequent late 

 discoloration of red colon colonies on prolonged cultivation. The 

 medium is red when hot, and colorless when cold, because the compound 

 between sulphite and fuchsin dissociates in the hot solution. 



Malachite-Green Media. ^ — The principle of these media is that mal- 



1 Kastk and Elvove, Jour, of Inf. Dis., xvi, 1909. 



^ Harding and Ostenberg, Jour, of Inf. Dis., xi, 1, 1909. 



' Loeffler, Deut. med. Woch., 32, 1906. 



