CULTIVATION OF BACTERIA 57 



at all. In the latter case the method does not give absolutely 

 accurate information, nevertheless it is the best known for the 

 routine examination of water and milk. 



Fishing. When the object of plating is to obtain a pure cul- 

 ture rather than the enumeration of the organisms present, a colony 

 is fished from the Petri dish to a tube of fresh culture medium. 

 Fishing is best accomplished by using the microscope ; otherwise a 

 minute colony of another species unseen by the naked eye may be 

 touched and the resulting subculture contaminated. The lower 

 part of the Petri dish is placed on the stage of the microscope 

 and the low-power lens focused over the colony chosen. The 

 sterilized platinum needle is held in the right hand and introduced 

 between the objective and the colony. When the point of the 

 needle is visible through the microscope it is gently lowered until 

 it is seen to touch the colony and to carry away a small portion 

 of it. The needle is then withdrawn without being permitted to 

 touch anything in passing and the organisms clinging to it are 

 transferred to the medium desired. Fishing requires practice ; 

 only by careful manipulation can the subculture be obtained 

 pure. 



Incubation. The temperature at which the inoculated media 

 should be kept depends largely upon the organisms to be grown. 

 Many saprophytes are accustomed to the temperature of an ordi- 

 nary room and consequently no special apparatus is necessary for 

 their cultivation. Pathogenic organisms, on the other hand, 

 require the body temperature 37.5 C. for their best growth. 



In order to maintain a constant temperature at any required 

 degree an incubator is employed. Different makes vary somewhat 

 in detail but all are constructed on much the same principle. Gen- 

 erally speaking an incubator is a double-walled copper chamber 

 with double doors ; the space within the two walls is filled with 

 water, which being a poor conductor of heat prevents rapid changes 

 of temperature taking place within the chamber as a result of 

 changes on the exterior. It may be heated by electricity, gas, or 

 oil ; a thermo-regulator is usually attached to automatically con- 

 trol the flame. 



