CULTURAL. METHODS 85 



be free from contamination, that is, they will be pure. It 

 is much more difficult to secure pure cultures of bacteria 

 in this mechanical fashion. It may be accomplished, how- 

 ever, when the necessity arises, by the use of Barber's 

 capillary pipette. This is an instrument consisting of 

 an extremely slender capillary glass tube so arranged with 

 a thumb screw that water can be forced in and out. The 

 bacteria to be separated are placed in a drop of water 

 and examined under the microscope. By careful manipula- 

 tion the germ desired may be drawn into the capillary 

 pipette and blown out again into a drop of water free 

 from bacteria. Then it may be examined to be sure that 

 there are no other bacteria present, when it may be trans- 

 ferred to the medium upon which it is to be cultivated. 

 This method is cumbersome and the technic is difficult. 

 It has proved valuable for certain special studies but is 

 not in common use. 



Perhaps the most common method of securing pure 

 cultures of bacteria is by plating. The mixture of bac- 

 teria to be separated is placed in liquid nutrient agar or 

 nutrient gelatin, that is, in one of the liquefiable culture 

 media. When shaken thoroughly so that the bacteria are 

 well distributed, this is poured into a sterile glass Petri 

 dish and covered with a sterile glass cover. The separated 

 bacteria are soon fixed in position by the hardening of 

 the culture medium. They can no longer move about, but 

 their growth is not interfered with. In consequence they 

 develop rapidly and in the course of a few hours to a few 

 days produce a mass of bacteria large enough so that 

 it can be easily seen with the naked eye. The mass of 

 organisms is termed a colony. If the number of bacteria in- 

 troduced into the original medium is not too large and 

 they are properly mixed through the medium the colonies 

 will be well separated from each other. Theoretically, at 

 least, each organism gives rise to a separate colony, prac- 



