THE PREPARATION OF CULTURE MEDIA 135 



The medium should be kept in dark. Plates are poured and surface smears 

 made. The typhoid colonies remain colorless, while those of coli become red. 



The preparation of Endo's medium presents difficulties due to the 

 varying purity of sodium sulphite. Kastle and Elvove * recommend the 

 use of anhydrous sodium sulphite instead of the crystallized variety. 

 Harding and Ostenberg 2 add sodium sulphite solution to a measured 

 amount of .5 per cent fuchsin to determine the proportions which give the 

 greatest delicacy of reaction as tested with formaldehyde. The propor- 

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



Although Endo described his medium 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 decol- 

 orize 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 com- 

 pound between sulphite and fuchsin dissociates in the hot solution. 



Kendall's Modification of Endo's Medium. 3 1.5 per cent meat extract agar 

 is prepared, and the reaction adjusted faintly alkaline to litmus by the addition 

 of NaOH. This agar is stored in small flasks and it is usually convenient to 

 keep flasks containing 100 c.c. each. Just before use, 1 per cent of lactose is 

 added, and then decolorized fuchsin solution, as in Endo's medium. Add 

 about 1 c.c. of decolorized fuchsin solution, made up as above by mixing roughly 

 prepared 10 per cent sodium sulphite with saturated alcoholic fuchsin. (The 

 proportions of fuchsin and sulphite are sometimes difficult to adjust, possibly 

 by reason of impurities in the sulphite due to formation of sulphate. The in- 

 structions given by most workers at present are to use 10 c.c. of a 10 per cent 

 aqueous solution of sodium sulphite, aftd to add to this 1 c.c. of a 10 per cent 

 solution of fuchsin in 96 per cenf alcohol.) When these flasks containing the 

 various ingredients are hot they are red or pink, but when plates are poured 

 and allowed to harden, the medium should be either colorless or very faintly 

 pinkish. It is best to pour a number of plates rather thickly and then allow 

 them to dry with the covers off. Inoculations from the feces solution are then 

 made by surface smear, with a bent glass rod. Colon colonies are pinkish and 

 red; typhoid colonies, smaller and grayish. 



In concluding the description of some of the most important typhoid isola- 

 tion media, we would like to add that a great deal seems to depend upon the 



1 Kastle and Elvove, Jour. Inf. Dis., xvi, 1909. 



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



3 Kendall, Boston Med. & Surg. Jour. 



