METHODS USED IN CULTIVATION OF BACTERIA 



145 



The plates when fitted together are sterilized and thus form a closed 

 cell which, if properly handled, may remain sterile indefinitely. 



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 



FIG. 30. POURING INOCULATED MEDIUM INTO PETRI PLATE. 



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 trans- 

 ferred to the second tube and a similar mixing process is repeated. 

 This second tube contains the bacteria in much greater dilution than 

 the first and the colonies which will form in the plate poured from this 

 tube will be farther apart. A third dilution is then made by transferring 

 five loopfuls of the mixture in the second tube to the third. This again 

 is mixed as before. The contents of the tubes are then poured into three 



