234 Cultivation of Micro-organisms 



After the agar-agar jelly solidifies it retracts so that a little 

 water collects at the lower part of the tube. This should not 

 be removed, as it keeps the jelly moist, and also distinctly 

 influences the character of the growth of the bacteria. 



Glycerin Agar=agar. For an unknown reason certain 

 bacteria that will not grow upon agar-agar prepared as de- 

 scribed will do so if 3 to 7 per cent, of glycerin be added after 

 filtration. 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 added to bouillon or any other medium has the 

 same advantageous influence. 



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 from the vein of one of the lower animals. 

 Some bacteriologists prepare a hemoglobin agar-agar by 

 spreading a little powdered hemoglobin upon the surface of 

 the agar-agar. This has the disadvantage that powdered 

 hemoglobin is not sterile, and the medium must be again 

 sterilized after its addition. 



The blood agar-agar should be kept in the incubator a day 

 or two before use so as to insure perfect sterility. 



BLOOD-SERUM. 



The great advantage possessed by this medium is that it 

 is itself a constituent of the body, and hence offers oppor- 

 tunities for the development of the parasitic forms of bac- 

 teria. If the blood-serum is to be employed fresh, it must 

 either be heated or kept sufficiently long to lose its natural 

 germicidal properties. The statement that serum represents 

 the normal body-juice is erroneous, as it is minus the fibrin 

 factors and some of the salts, and contains new bodies liber- 

 ated from the destroyed leukocytes. Solidified blood-serum, 

 exposed to the heat of the sterilizing apparatus, in no sense 

 resembles the body- juices. 



It is one of the most difficult media to prepare. The 

 blood must be obtained either by bleeding some good-sized 

 animal or from a slaughter-house in appropriate receptacles, 

 the best things for the purpose being i -quart fruit jars with 

 tightly fitting lids. The jars are sterilized by heat, closed, 



