CULTIVATION OF BACTERIA 53 



slanted medium the growth is deposited by lightly smearing the 

 surface from the lower to the upper portion of the slant. A 

 " stab " culture is made by plunging the needle down the center 

 of medium that has not been slanted or the two methods may be 

 combined on an agar slant; growth may be smeared on the 

 surface and the needle plunged into the medium before it is 

 withdrawn. Surface growth and deep growth may thus be ob- 

 served in the same tube. If the organisms are transplanted 

 from one fluid medium to another a loopful is removed and gently 

 rubbed off against the glass in the upper portion of the fresh 

 medium. When the growth forms a pellicle it is sometimes 

 necessary to transfer a portion of the pellicle and so place it 

 that it rests on the surface of the fresh medium, or growth will 

 not take place. After the medium is inoculated the platinum 

 needle is removed, the plugs replaced, and the wire immediately 

 heated red hot in the flame ; at the same time several inches of the 

 glass rod should be passed through the flame also. 



Plating. If pathological material or such substances as milk 

 or water be placed in culture medium many different kinds of 

 organisms develop at the same time. Since it is impossible to 

 study the characteristics of each species unless they can be sepa- 

 rated, a method devised by Koch of plating in solid media is em- 

 ployed whereby individual bacteria are held apart and the descend- 

 ants of each are soon sufficiently numerous to appear as a colony 

 visible to the naked eye. By a procedure known as colony fishing 

 the members of a single species can be transferred to fresh sterile 

 media and a pure culture thus obtained. 



A thin layer of medium presenting a moderately large surface 

 is obtained by the use of the so-called Petri dish. Each Petri 

 dish consists of two circular glass plates, the larger forming a loosely 

 fitting lid for the smaller. 



The method of making a pour plate for the isolation of bacteria 

 is as follows : The solid medium is liquefied and then cooled to a 

 temperature of 42 C. A loopful of the material to be examined 

 is placed in a tube and thoroughly mixed with the medium by 

 rolling the tube between the hands, care being taken not to shake 



