202 Culture Media and Bacteria Cultivation 



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 contamination is obviated. 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 sterile cylinders and allowed to sediment 

 for twelve hours, then repipetted into tubes. It is evident 

 that such frequent manipulations afford numerous chances 

 of infection ; hence the sterilization of the serum becomes 

 of the greatest importance. 



As the demand for serum has been considerable during the 

 last few years, commercial houses dealing in biologic pro- 

 ducts now market fresh horse serum, preserved with chloro- 

 form, in liter bottles. This can be employed with great 

 satisfaction, the chloroform being driven off during coagu- 

 lation and sterilization. 



If it be desirable to use the serum as a liquid medium, it is 

 exposed to a temperature of 60 C. for one hour upon each 

 of five consecutive days. To coagulate the serum and make 

 a solid culture medium, it may be exposed twice, for an 

 hour each time or three times if there be reason to think it 

 badly contaminated to a temperature just short of the 

 boiling-point. During the process of coagulation the tubes 

 should be inclined, so as to offer an oblique surface for the 

 growth of the organisms. The serum thus prepared should 

 be white, but may have a reddish-gray color if many red 

 corpuscles be present. It is always opaque and cannot be 

 melted ; once solid, it remains so. 



Koch devised a special apparatus (Fig. 35) for coagulating 

 blood-serum. The bottom should be covered with cotton, a 

 single layer of tubes placed upon it, the glass lid closed and 

 covered with a layer of felt, and the temperature elevated 

 until coagulation occurs. The repeated sterilizations may 

 be conducted in this same apparatus, or may be done equally 

 well in a steam apparatus, the cover of which is not com- 

 pletely closed, for if the temperature of the serum be raised 



