METHODS USED IN CULTIVATION OF BACTERIA 175 



cooled and hardened by the cold water contained in the crystallizing 

 dish. 



The original method of Koch has been modified considerably and 

 the method universally employed at present depends upon the use 

 of circular covered dishes, the so-called Petri dishes. These obviate 

 the necessity of a leveling stand and prevent contamination of the 

 plate when once poured. Each Petri dish plate consists of two 

 circular glass dishes; the smaller and bottom dish has an area of 

 63.6 square centimeters ; the larger is used as a cover for the smaller, 

 and forms a loosely fitting lid. The plates when fitted together are 

 sterilized and thus form a closed cell which, if properly handled, 

 may remain sterile indefinitely. 



FIG. 13. INOCULATING. 



The technique for making a pour plate for the purpose of isolating 

 bacteria from mixed culture is as follows : 



The actual " pouring" of plates is preceded by the preparation 

 of usually three graded dilutions of the material to be examined. 

 For this purpose three agar or gelatin tubes are melted and, in the 

 case of the agar, are cooled to a temperature of about 42 C. in a 

 water bath. A platinum loopful of the material to be examined is 

 transferred to one of these tubes. The bacteria are then thoroughly 

 distributed throughout the melted gelatin or agar by alternately 

 depressing and raising the plugged end of the tube, giving it a 

 rotary motion at the same time. This thoroughly distributes the 

 bacteria throughout the medium without allowing the formation of 

 air-bubbles, Two loopfuls of this mixture are then transferred to 

 the second tube and. a similar mixing process is repeated. This 



