204 Cultures, and their Study 



between the thumb and upward-directed palm of the left hand, the 

 stoppers toward the operator. The position of the tubes should be 

 such as to permit one to see the contained media without the fingers 

 being in the way. The stopper of the tube toward the left is re- 

 moved by a gentle twist and placed between the index and middle 

 fingers of the left hand; the stopper of the next tube similarly re- 

 moved and placed between the middle and ring fingers of the same 

 hand. 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 incandescence 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 touch- 

 ing 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 differ- 

 ent 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, form- 

 ing 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 observation of 

 Koch, that when bacteria are equally distributed throughout some 

 liquefied nutrient medium that is subsequently solidified in a thin 

 layer, they grow in scattered groups or families, called colonies, dis- 

 tinctly isolated from one another and susceptible of transplantation. 



The plate cultures, as originally made by Koch, require con- 

 siderable apparatus, and of late years have given place to the more 

 simple and ready methods. So great is their historic interest, how- 



