206 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



hand, the other between the second and third fingers, the plugs pro- 

 jecting outward. Flame the mouths of the tubes and test coolness 

 of the platinum wire by plunging it for a distance of about a centi- 

 meter into the sterile medium. 



(e) Remove some material from the infected tube by dipping the 

 tip of the wire in it, and transfer to the sterile tube. 



(/) Replace the plugs in their respective 

 tubes and sterilize the wire before laying it 

 down. 



II. The Isolation of Pure Cultures of Bacteria. 

 A. Aerobic Organisms. It is the exception 

 rather than the rule that bacteria exist in nature 

 or in many pathological processes in pure cul- 

 ture, that is, that a single kind of organism 

 alone is present. From such mixtures of bac- 

 teria it is frequently necessary to isolate one or 

 more organisms in a pure state, uncontami- 

 nated by other microorganisms. A common 

 and efficient method of separating bacteria from 

 mixtures is to distribute them in melted gelatin 

 or agar, 1 in such a manner that individual cells 

 are somewhat widely separated. The medium 

 is then allowed to harden. The organisms are 

 immobilized in or upon the medium and sur- 

 rounded by nutrients; the descendants of each 

 individual organism thus develop locally and 



apart from the descendants of other organisms. Under favorable 

 conditions the descendants of individual cells become so numerous 

 they may be seen with the unarmed eye as spots or colonies, each 

 of which is made up of the progeny of a single organism. It is a 

 simple matter to touch such a colony with a sterile, cool platinum 

 needle, and infect sterile media with the adherent bacteria. In this 

 manner pure cultures are obtained. The technic of the isolation of 

 aerobic and facultatively anaerobic bacteria is technically termed 

 plating, or streaking, depending upon the apparatus used. 



1. Plate Method. Three tubes of nutrient agar or gelatin are melted 

 and cooled to 42 to 45 C. A platinum wire, previously sterilized 



1 Agar melts at about 95 C. and solidifies at about 40 C. It is necessary to work 

 rapidly with melted and cooled agar, to carry out the technic of inoculation before 

 solidification takes place. 



FIG. 24. Platinum needle 

 and platinum loop. 



