20 CULTURE MEDIA 



Makers of apparatus for the bacteriological laboratory now furnish a vessel for 

 making media which has two bottoms with an intervening air space. This hot 

 air layer prevents the scorching of the media as is so liable to occur when a plain 

 saucepan is used. Advantages over the rice cooker are that time is saved in bringing 

 the media to a boil, and also in the maintaining of a brisk boiling temperature. 



A 15% solution of salt raises the boiling-point 2^2C.; a 20%, 3HC., and a 

 25%, 4K C. The raising of the boiling-point by calcium chloride is about the 

 same for similar strength solutions. 



Although the Bacteriological Committee of the A. P. H. Association recommends 

 special steps to be taken in the preparation of gelatin and agar, yet for clinical pur- 

 poses it will be found satisfactory to keep on hand a stock of bouillon, and when it is 

 desired to make agar or gelatin to simply prepare such media from the stock bouillon 

 in the way to be subsequently given. 



NUTRIENT BOUILLON 



This may be made either from fresh meat or from meat extract. 

 Media from fresh meat are usually lighter in color and possibly clearer. 

 In the Philippines, however, certain measures employed for the preser- 

 vation of the meat made it very difficult to prepare clear bouillon 

 from it, so that meat extract was used entirely. There is very little 

 difference, if any, in the nutritive power of media made in either way. 



The chief objections to fresh meat as a base are: i. It takes more time and trouble. 

 2. The reaction, due to sarcolactic acid and acid salts, is quite acid, so that it is 

 necessary to titrate and neutralize the excess of acidity. 3. The reaction of the 

 finished media tends to change unless the boiling at the time of making was very 

 prolonged. 4. It is not infrequent to have a heavy precipitate of phosphates 

 thrown down at the time of sterilization, thus making it necessary to repeat the 

 process of nitration and sterilization. 



If fresh meat is used, take about 500 grams (i pound), remove fat and cut it 

 up with a sausage mill or purchase the meat already cut up as for a Hamburg steak. 

 It makes little difference whether the amount be 100 grams more or less. Place 

 the chopped-up meat in a receptacle and pour 1000 c.c. of water over it. Keep in 

 the ice chest over night and the next morning skim off with a piece of absorbent cot- 

 ton the scum of fat; then squeeze out the infusion with a strong muslin cloth, mak- 

 ing the amount up to 1000 c.c. This meat infusion contains all the albuminous mate- 

 rial necessary for the clarification of the bouillon. It is convenient to designate this 

 meat base as Meat Infusion to distinguish from the base containing meat extract. 



Having obtained 1000 c.c. of this 50% meat infusion, we dissolve in it i% of 

 Witte's peptone and K% of sodium chloride. While there is a sufficiency of the vari- 

 ous salts necessary for bacterial development in the meat juices, yet there is not 

 enough to give the best results when bouillon cultures of various organisms are used 

 for agglutination tests; and furthermore, when bouillon is used for blood cultures, 

 disintegration of the red cells, with clouding of the clear medium, may occur if there 

 t>e not sufficient salt present to prevent this. 



