CHAPTER VII. 



Technique of Isolating Bacteria in Pure Culture by the Plate and the 

 Tube Method. 



PLATES. 



THE plate method can be employed with both agar-agar 

 and gelatin. It cannot be practised with blood-serum, 

 because the serum when once solidified cannot again be 

 liquefied. 



Plates are usually referred to as "a set." This term 

 implies three individual plates, each representing a mixture 

 of organisms in a higher state of dilution. The first plate 

 is known usually as "the original," or "plate No. 1," the 

 first dilution from this as "plate No. 2," and the second as 

 "plate No. 3." 



In the preparation of a set of plates the following are the 

 steps to be observed: 



Three tubes, each containing from 7 to 9 c.c. of gelatin 

 or agar-agar, are placed in a warm water-bath until the 

 medium has become liquid. If agar-agar is employed, this 

 is accomplished at the boiling-point of water; if gelatin is 

 used, a much lower temperature suffices (35-40 C.). When 

 liquefaction is complete the temperature of the water, in 

 the case of agar-agar, must be reduced to 41-42 C., at 

 which temperature the agar-agar remains liquid, and the 

 organisms may be introduced into it without fear of de- 

 stroying their vitality. The medium being now liquid and 



(137) 



