THE PREPARATION OF CULTURE MEDIA 159 



of agar. Boil for three hours (or one hour in an autoclave) until agar is 

 dissolved. Render weakly alkaline to litmus paper, filter, and boil for half 

 an hour more. 



(b) Litmus solution: Two hundred and sixty e.c. of litmus solution are 

 boiled for ten minutes. (The litmus solution used by Conradi and Drigalski 

 is the very sensitive aqueous litmus recommended by Kubel and Tiemann, 

 and purchasable under the name.) After boiling, 30 grams of chemically 

 pure lactose are added to the litmus solution. The mixture is then boiled 

 for fifteen minutes, and if, a sediment has formed, is carefully decanted. 



(c) Add to the hot lactose mixture to the hot agar solution; mix well 

 and, if necessary, again adjust to weak alkaline reaction, litmus paper being 

 used as an indicator. To this mixture add 4 c.c. of a hot, sterile ten per cent 

 solution of sodium carbonate, and 20 c.c. of a freshly made solution of 

 crystal violet (c. p. Hochst), 0.1 gram in 100 c.c, of sterile distilled water. 



Surface smears are made upon large plates. These are incubated 

 twenty-four hours. Typhoid colonies are small, blue, and trans- 

 parent. Colon colonies are large, red, and opaque. 



Endo's Medium.- 1 1. Prepare one liter of meat infusion three per cent 

 agar, containing 10 grams of pepton and 5 grams of NaCl. 



2. Neutralize and clear by filtration. 



3. Add 10 c.c. of 10% sodium carbonate to render 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 10% sodium sulphite solution. This again decolorizes 

 the medium, the color not entirely disappearing, however, until the agar is 

 cooled. 



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



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 22 recom- 

 mend the use of anhydrous sodium sulphite instead of the crystallized 

 variety. Harding and Ostenberg 23 add sodium sulphite solution to 

 a measured amount of .5 per cent fuchsin to determine the propor- 



l Endo, Cent. f. Bakt., xxxv, 1904. 



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



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



