Isolation of Bacteria 



213 



middle and ring fingers of the same hand (Fig. 40) . If three 

 or four tubes are to be held, the third stopper can be placed 

 between the ring and little fingers of the left hand and the 

 fourth retained in the right hand. The part of each stopper 

 that enters the tube must not be touched. 



The necessary manipulation is usually made with the 

 platinum wire, which is sterilized by heating to incandes- 

 cence before using. The wire must not be used while hot, 

 but cools in a moment or two. The culture is touched, the 

 wire entering and exiting without touching the tube, and 

 the bacteria adhering to the wire are applied to the medium 

 in the other tube, the same care being exerted not to have 

 the platinum wire touch the glass. After the transfer is 

 made, the wire is made incandescent in the flame before 

 being returned to the table or stand made to hold it, and 

 the stoppers returned one after the other, each to its own 

 tube, that part entering the tube not being touched. Each 

 stopper is given a twist as it enters the mouth of the tube. 



Modifications of these directions can be made to suit the 

 different forms of containers used, but the essential features 

 must be maintained. 



When any manipulation requires that a tube or flask be 

 permitted to remain open an unusual length of time, its 

 contamination from the air can be prevented for some 

 minutes by heating its neck quite hot. The air about it, 

 being heated by the hot glass, ascends, forming a current 

 that carries the bacteria away from, rather than into, the 

 receptacle. 



Isolation of Bacteria. Three principal methods are, at 

 present, employed for securing pure cultures of bacteria. 

 Before beginning a description of them it is well to observe 

 that the peculiarities of certain pathogenic micro-organisms 

 enable us to use special means for their isolation, and that 

 these general methods are chiefly useful for the isolation of 

 non-pathogenic organisms. 



Plate Cultures. All the methods depend upon the obser- 

 vation of Koch, that when bacteria are equally distributed 

 throughout some liquefied nutrient medium that is subse- 

 quently solidified in a thin layer, they grow in scattered 

 groups or families, called colonies, distinctly isolated from 

 one another and susceptible of transplantation. 



The plate cultures, as originally made by Koch, require 

 considerable apparatus, and of late years have given place 

 to the more ready methods of Petri and von Ksmarch, 



