40 A MANUAL OP BACTERIOLOGY 



Place two sterile Petri dishes on the table and label them 

 Second Dilution and Third Dilution. Remove the plug from the 

 mouth of tube B and flame the mouth of the tube ; slightly 

 raise the cover of the Second Dilution dish and pour in the 

 melted medium (Fig. 20). Take care that none of the liquid 

 medium rises over the edge of the Petri dish. If the medium 

 fails to cover the entire surface of the plate, gently tilt the dish 

 back and forth until the fault is rectified. Keep the Petri dishes 

 level until the medium has hardened ; then they may be placed 

 in the culture chamber. 



After twenty-four to forty-eight hours, depending upon the 

 temperature, the plates will be dotted with colonies, each of 



wliich is supposed 

 to consist of the 

 descendants of a 

 single organism. 

 Study the colonies 

 by placing the in- 

 verted Petri dish 

 Fig. 20. Method of pouring melted media into xj^g sta^e of 



Petri dishes " 



the microscope 



and using the | objective with weak light from the substage. 



Sterilize a straight platinum needle in the flame and, after 

 allowing it to cool a few seconds, dip the tip of it into one of the 

 colonies. Inoculate both an agar slant and a gelatin stab with 

 bacteria obtained in this manner. Label the tube, giving it a 

 number, and make a record in a notebook stating the kind of 

 colony from which the bacteria were taken and the original 

 source of the Petri dish. Place the inoculated tubes in the 

 culture chamber. 



If the bacteria used for these inoculations were the descendants 

 of a single organism, these tubes will contain pure cultures. The 

 bacteria may be used for other subcultures, for stained prepara- 

 tions, etc. It often happens, however, that the first isolations are 

 not pure cultures. In that case another series of plates must be 

 poured, using the growth in the test-tube culture as the source. 

 In fact, it is always safer to observe this precaution. 



