196 



Cultivation of Micro-organisms 



sterile cylinders and allowed to sediment for twelve hours, then 

 repipetted into tubes. 



As the demand for serum has been considerable during the last 

 few years, commercial houses dealing in biologic products now 

 market fresh horse serum, preserved with chloroform, in liter botdes. 

 This can be employed with great satisfaction, the chloroform being 

 driven off during coagulation and sterilization. 



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

 to a temperature of 6o°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 



Fig. 45. — Koch's apparatus for coagulating and sterilizing blood-serum. 



just short of the boiling-point. During the process coagulation 

 occurs, and 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 for coagulating blood-serum. 

 The bottom should be covered with wet 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 completely closed, for if the temperature of the serum be 

 raised too rapidly it is certain to bubble, so that the desirable smooth 

 surface, upon which the culture is to be made, is ruined. 



Like other culture-media, blood-serum and its combinations may 

 be sterilized in the autoclave and much time thus saved. The serum 

 should, however, first be coagulated, else bubbling is apt to occur 



