58 THE CULTIVATION OF BACTERIA. 



for fifteen or twenty minutes, is filtered through absorbent 

 cotton several times into test-tubes and sterilized by steam 

 for twenty minutes on three successive days. 



The addition of 5 per cent, neutral glycerin to this bouillon 

 makes an excellent liquid culture-medium for tubercle bacilli, 

 and is highly recommended by Rotix. 



Any standard beef-extract, such as Liebig's, Armour's, 

 Wyeth's, etc., may be used in making this bouillon, instead 

 of the meat itself, 5 grams of the extract being added to 

 1 liter of water, the rest of the process being the same. 



II. The Most Commonly Used Solid Culture-Media. 



1. Gelatin. 



Nutrient gelatin is prepared as follows : A meat-infusion, 

 as described for bouillon, is prepared and put into a large 

 Bohemian flask of 2 liters capacity ; to this meat-infusion are 

 added : 



Chloride of sodium, 5 grams ; 



Peptone, 10 " ; 



Best quality gelatin 100 " . 



The flask, loosely closed with a plug of absorbent cotton, is 

 cooked over a water-bath until all the gelatin is dissolved. 

 This will take from an hour and a half to two hours. After 

 this the mixture, which will be found to be decidedly acid, 

 is neutralized by means of a solution of sodium carbonate 

 added drop by drop until the mixture is faintly alkaline, as 

 in the bouillon. The flask is again put over the water-bath 

 for an hour, after which the mixture is filtered hot through 

 absorbent cotton and sterilized in the steam sterilizer for 

 twenty minutes each day on three consecutive days. 



Great care must be exercised to introduce the medium into the 

 sterilizer only when steam is actively being generated, and not to 

 allow it to cool in the sterilizer. 



Advantages. Gelatin is a most excellent medium, and 

 remains solid at room temperature (22 or 24 C.), but is 

 readily liquefied when exposed to a higher temperature. 



