160 BIOLOGY AND TECHNIQUE 



tions which give the greatest delicacy of reaction as tested with 

 formaldehyd. The proportions so determined are then added to 

 the hot 3 per cent agar. 



Although Endo described his medium as dependent upon the 

 formation 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 

 aldehyds. Acids decolorize the red caused by aldehyds, 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 dis- 

 sociates in the hot solution. 



Robinson and Rettger's Modification of Endo. 24 This seems to be at 

 present the most useful modification of Endo available. 



1. To 1 liter of water add 25 grams of agar, 10 grams of pepton, and 

 5 grams of meat extract. Dissolve the agar, meat extract and pepton. Bring 

 to P H 6.8, and heat in autoclave for l /2 hour at 15 pounds. Filter through 

 cotton. 



To this add 10 c.c. of a 10 per cent sodium carbonate solution. Heat 

 for a few minutes, and add 1 per cent lactose. The fuchsin sulphite indicator 

 is then added in the form of 5 c.c. of saturated alcoholic fuchsin, and 10 c.c. 

 of a 10 per cent solution of sodium bisulphite. This is tubed and sterilized. 

 Krumwiede recommends preparing the medium by adjusting the reaction 

 to P H 8.5 in the first place, relying upon sterilization and the addition of 

 the bisulphite to bring the reaction to the desired end point. 



The best results are obtained by adding the lactose, fuchsin and bisulphite 

 just before use, and this can be done most conveniently if the agar basis 

 is bottled in 100 c.c. amounts. The final reaction of the medium should be 

 P# 8. A more acid reaction favors the diffusion of the indicator. In our 

 own laboratory we have found that the addition of these amounts of fuchsin 

 and sodium bisulphite to the medium inhibit the growth of typhoid in some 

 instances. We have obtained equally good differentiation by using 0.25 per 

 cent fuchsin instead of 0.5 per cent, and 0.5 per cent sodium bisulphite 

 instead of 1 per cent. 



Kendall's Modification of Endo's Medium. 2 ^ 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 



2 *Kobinson and Eettger, Jour, of Med. Kes., 24, 1916, 363. 

 28 Kendall, Boston Med. & Surg. Jour. 



