TYPHOID FEVER 



99 



NaOH until the red colour is replaced by a slight brown tinge. 

 When the end point is reached the fluid should stand about fifteen 

 minutes between each addition of alkali, as the process goes on slowly. 

 This solution becomes red on boiling, but the colour disappears on 

 cooling again. Use i per cent, in preparing the sugar media enumer- 

 ated above : the sugars are to be dissolved in the peptone-salt-water, 

 the indicator added, the mixture tubed and sterilized in the steam for 

 three successive days ; not autoclaved, as this will alter the sugars. 

 The air is expelled from the inner tube in this process, being replaced 

 by steam, so that when the whole has cooled it is 

 full of medium. 



This medium turns a brilliant red in presence of 

 acid. 



Neutral red agar is ordinary agar containing 2 per 

 cent, of glucose and 0-25 per cent, of a i per cent, 

 solution (freshly prepared) of neutral red. It turns 

 crimson with acids, yellowish-brown with alkalies. 



Litmus milk is prepared by adding a sufficiency 

 of litmus solution to fresh milk, tubing off, and 

 sterilizing for thirty minutes in the steam on three 

 successive days. 



Litmus whey is prepared by heating milk to 55 C. 

 and adding i to 2 per cent, of extract of rennet. 

 The mixture is allowed to stand in the cold until it 

 has clotted firmly. The clot is then cut into small 

 pieces, and the whey filtered from it through filter- 

 paper ; 2 per cent, of agar are then added, and the 

 whole autoclaved. It is made definitely acid ( + 10), 

 cooled to 60 C., and the beaten white and shell of 

 an egg added for each 200 c.c., autoclaved at 

 1 10 C. for ten minutes, and filtered. Litmus solu- 

 tion is added to the filtrate (about 10 c.c. of litmus 

 to 100 c.c. of medium). It is then tubed off, auto- 

 claved, and allowed to cool in the sloping position. 



( + 10 indicates that 10 c.c. of the medium will 

 be exactly neutralized to phenolphthalein when 

 i c.c. of decinormal alkali are added, the titration being done whilst 

 the medium is hot.) 



Indol. This is tested for in a seven-day culture in peptone-salt-water. 

 The nitroso-indol test is the simplest, but it is not very delicate. Add 

 i c.c. of a 0-2 per cent, solution of potassium nitrite, add one drop of 

 pure nitric or sulphuric acid, and allow to stand for an hour or more. 

 The formation of a red colour indicates the presence of indol. Efirlich's 

 test is more delicate and more convenient if the reagents are at hand. 

 Prepare (i) a solution of 4 grammes of paradimethylamidobenzaldehyde 

 in 380 c.c. of absolute alcohol and 80 c.c. of concentrated hydrochloric 

 acid, and (2) a saturated watery solution of potassium persulphate. 



FIG. 23. FERMEN- 

 TATION TUBE. 



