PREPARATION OF NUTRIENT CULTURE-MEDIA 67 



placed obliquely to drain, and when dry, corked with cotton 

 plugs. Then put in the hot-air oven (little wire cages, Fig. 

 17, being used to contain them) for fifteen minutes, after 

 which they are ready to be filled with the nutrient medium. 

 (The cotton should fit firmly in the tube and extend a short 

 space beyond it.) 



Test-tubes without flaring edges are more desirable, since 

 the edges can easily be drawn out so as to seal the tube. 



Instead of test-tubes, ordinary 3-ounce panel medicine 

 bottles can be used for retaining the nutrient media and 

 cultures. 



According to investigations, the glass tubes become suffi- 

 ciently sterile in the steam-chest without the preliminary 

 sterilization in the dry oven. 



Sterilization by Filtration. Germ Filters. Kaolin or por- 

 celain bougies, such as are used in the Berkefeld, Chamber- 

 land, and Pasteur filters, restrain most bacteria, except those 

 now known as ultramicroscopic. In the making of toxins 

 this method is used, heat or disinfectants being undesirable. 

 With the knowledge of smaller forms of life, the filter will 

 need further improvement. 



CHAPTER X 

 PREPARATION OF NUTRIENT CULTURE-MEDIA 



OF the many different media recommended and used since 

 bacteriology became a science, we can describe only the more 

 important ones now in use. Each investigator changes them 

 according to his taste. 



Potato as Medium. The knowledge of bacteria and 

 germs or molds settling and growing upon slices of potato 



