222 BACTERIOLOGY. 



The Methods of Obtaining and Studying Pure Cultures of 

 Single Species of Bacteria. 



In order to study bacteria, both in culture media 

 and in the living body, we must separate those devel- 

 oped from one organism from all others and study 

 them by themselves in pure cultures. In order to do 

 this we have to take the greatest precautions to insure 

 that the materials that we make use of for the growth of 

 bacteria, the flasks and tubes that hold these materials, 

 and the instruments with which we transfer the bac- 

 teria are sterile. We also carefully try to prevent any 

 bacteria entering from the air or elsewhere. 



The Cleansing and Sterilization of Apparatus. 



In bacteriological work sterilization is practically 

 always done by means of dry and moist heat, for no 

 antiseptic substances can be allowed to remain in any 

 of the media used for the growth of bacteria or on any 

 of the apparatus which would come in contact with 

 them, as such substances would inhibit the growth of 

 the bacteria which we desired to study. 



The platinum wires and loops used in transferring 

 bacteria are sterilized by holding them for a moment 

 until red-hot in a gas or alcohol flame. They should 

 not be used until time enough has elapsed for them to 

 cool sufficienly not to injure the bacteria touched by 

 them. Knives, instruments, etc., are, after thorough 

 cleansing, placed in boiling 1 per cent, soda solution 

 for three to five minutes. Hypodermic needles are 

 sterilized by boiling in soda solution, or, when this is 

 impossible, they are first frequently rinsed with boil- 

 ing or with very hot water and then filled with a 5 per 



