CUL TI VA TION OF BA CTERIA . 1 93 



Allegar informs me that he finds the use of powdered 

 agar-agar very satisfactory because of the readiness with 

 which it dissolves and the ease with which it niters. 



If agar-agar is to be made with beef-extract, the bouil- 

 lon should be made first and filtered when cofd } after 

 which the agar-agar is added and dissolved. If this is 

 not done, it is sure to contain a precipitate of crystalline 

 urates from the meat-extract, which makes it unsightly 

 though it does not interfere with its nutrient qual- 

 ities. 



Agar-agar is dispensed in tubes like the gelatin and 

 bouillon, sterilized by steam by the intermittent process, 

 or in the autoclave, and after the last sterilization, before 

 cooling, each tube is inclined against a slight elevation, 

 so as to offer an extensive flat surface for the culture. 



After the agar-agar jelly solidifies its contraction causes 

 some water to collect at the lower part of the tube. This 

 should not be removed, as it keeps the material moist, 

 and also because it has a distinct influence upon the cha- 

 racter of the growth of the bacteria. 



Glycerin Agar-agar. — For an unknown reason certain 

 of the bacteria which will not grow upon the agar-agar 

 as prepared above will do so if 3-7 per cent, of glycerin 

 be added. Among these is the tubercle bacillus, which, 

 not growing at all upon plain agar-agar, will grow well 

 when glycerin is added — a fact discovered by Roux and 

 Nocard. The glycerin may also be added to bouillon or 

 any other medium. 



Blood Agar-agar was recommended by R. Pfeiffer for 

 the cultivation of the influenza bacillus. It is ordinary 

 agar-agar whose surface is coated with a little blood 

 secured under antiseptic precautions from the finger-tip, 

 ear-lobule, etc., of man, or the veins of one of the tower 

 animals. Some bacteriologists prepare a hemoglobin 

 agar-agar by spreading a little powdered hemoglobin 

 upon the surface of the agar-agar. This has the disad- 

 vantage that powdered hemoglobin is not sterile, and the 

 medium must be sterilized after its addition. 

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