Blood-serum 195 



After the agar-agar jelly solidifies it retracts so that a Uttle 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. — -Certain bacteria among which is the 

 tubercle bacillus, will not grow upon agar-agar prepared as described 

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

 filtration. This fact was discovered by Roux and Nocard. 



Blood Agar-agar was recommended by R. PfeifEer for the culti- 

 vation of the influenza bacillus. It is ordinary agar-agar whose 

 surface is coated with a little blood secured under aseptic 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. As powdered hemoglobin is not sterile, the medium 

 must be 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 advantage possessed by this medium is that it is primarily a 

 constituent of the animal body, and hence offers conditions favor- 

 able for the development of the parasitic forms of bacteria. 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 liberated from the destroyed leukocytes. Solidified 

 blood-serum, exposed to the heat of the steriUzing 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, and carried to the slaughter-house, 

 where the blood is permitted to flow into them from the severed 

 vessels of the animal. It seems advisable to allow the first blood to 

 escape, as it is likely to become contaminated from the hair. By 

 waiting until a coagulum forms upon the hair the danger of con- 

 tamination is diminished. The jars, when full, are allowed to stand 

 undisturbed until firm coagula form within them, after which they 

 are carried to the laboratory and stood upon ice for forty-eight 

 hours, by which time the clots will have retracted considerably, and 

 a moderate amount of clear serum can be removed by sterile pipets 

 and placed in sterile tubes. If the serum obtained be red and 

 clouded from the presence of corpuscles, it may be pipetted into 



