72 METHODS USED IN THE STUDY OF BACTERIA 



of the potato should not be changed, and to secure this 

 it is best to either boil the potato or, perhaps what is better, 

 to keep the cut potatoes in running water for twelve hours or 

 more before the cylinders are cut out. 



Other Vegetables. Carrots, turnips, and other vegetables 

 are sometimes used for special bacteria in the same way that 

 potatoes are used. 



Blood Serum. This material furnishes a specially valuable 

 medium for certain disease-producing bacteria. Beef, horse, 

 and human serum are the ones commonly used. In any case, 

 the blood is collected, allowed to clot, and then the clear serum 

 is drawn off. This is put into the appropriate vessel and 

 sterilized. The temperature of sterilization is usually low 

 in order to preserve, as much as possible, the transparency 

 of the medium. By sterilization below the coagulation 

 point, or by means of the addition of water, which raises 

 the coagulation point, it is possible to sterilize this medium in 

 a liquid condition, but it is usually solidified or inspissated. A 

 special mixture known as LoefHer's blood serum is made by 

 adding one part of dextrose bouillon to three parts of beef 

 blood serum. It is almost universally used for growing the 

 Bact. diphtherias in diagnostic laboratories. 



Eggs. Eggs are used at the present time largely for the 

 growth of Bact. tuberculosis. Fresh hens' eggs are broken 

 with greatest care, the contents put in sterile vessels, shaken 

 up and run into sterile test tubes where it is heated and coagu- 

 lated. Eggs may also be used for other work. 



