178 CULTURE MEDIA 



ized in the autoclave at 15 pounds' pressure for not less 

 than twenty-five minutes, since potatoes usually harbor 

 very resistant spores, and it is not unusual for a few tubes 

 to spoil even after this thorough heating. 



Potatoes are sometimes used in "potato plate cultures." 

 The term "plate culture" is a relic of the time when flat 

 glass plates were used for this and other "plate cultures." 

 Now glass dishes of the general form shown in Fig. 112, 

 called "Petri dishes," or plates, are used for practically all 

 plate culture work. For "potato plates" slices from pota- 

 toes are cut as large and as thick as the relative sizes of 

 potato and dish permit (Fig. 113). The slices should be 

 thin enough not to touch the lid and thick enough to be 

 firm. 



It is a good plan to wrap each dish separately in paper to 

 retain the lid securely, then sterilize as for potato tubes, 

 and leave plates T\Tapped until wanted. 



It sometimes happens that the natural acidity of potatoes 

 is too great for the growth of many organisms. The acidity 

 is sufficiently corrected by soaking the pieces of potato in a 

 1 per cent solution of sodium carbonate for an hour before 

 they are put into the tubes or plates. 



Glycerinized potato tubes are convenienth" prepared by 

 covering the potato in the tube with glycerin broth, steril- 

 izing and pouring off the excess broth immediately after 

 sterilizing, taking care that the tubes do not become con- 

 taminated, which is not very probable if the work is quickly 

 done while the tubes are still hot. 



Blood Serum Media.— Blood serum, usually from the 

 larger, domestic animals, on account of convenience in 

 securing it in quantity, is used in the study of the bacteria 

 causing disease in man and animals. Most commonly the 

 serum is collected from the clotted blood after it has well 

 separated (usually about forty-eight hours is required for 

 this). It is then run into tubes which are plugged with 

 cotton and placed in an apparatus for coagulating the serum 

 by heat. A copper water-bath with a tightly closed air 

 compartment or the horizontal autoclave (Fig. 75) is suffi- 

 cient for this purpose, though special forms of apparatus are 



