224 BACTERIOLOGY. 



this would cause the fibres of the cotton plugs to adhere 

 to the sides of the tubes when the media dried and 

 make it difficult to remove the plugs wholly when we 

 wished to inoculate the contents of the tubes. 



The tubes and flasks, plugged with sterile cotton 

 and full of media, are put in the steam sterilizer for 

 one half hour on three consecutive days, or in the 

 autoclave for twenty minutes for two consecutive days. 

 A portion of the tubes containing nutrient agar are 

 laid in a slanted position before cooling, after the final 

 sterilization, so that a larger surface may be obtained. 



Technique of Making Plate Cultures. 



When we make cultures from any material, we are 

 very apt to find that instead of one variety of bacteria 

 only there are a number present. If such material 

 is placed in fluid media contained in test-tubes, we find 

 that the different varieties all grow together and be- 

 come hopelessly mixed. When, on the other hand, the 

 bacteria are placed on solid media they develop about the 

 spot where they were inoculated. If different varieties, 

 however, are placed too near together, they overgrow 

 one another ; it is thus advisable to have a greater sur- 

 face of nutrient material than is given on the slanted 

 surface of nutrient agar or blood-serum contained in 

 test-tubes. This need is met by pouring the media 

 while warm on flat, cool, glass plates or into shallow 

 dishes. In making plate cultures two methods are 

 carried out. In the first the material with its contained 

 bacteria is scattered throughout the fluid before it 

 hardens; in the second it is streaked over the surface of 

 the medium after it has solidified. Nutrient agar and 

 nutrient gelatin, the two substances used for plate 



