108 VETERINARY BACTERIOLOGY 



over. This method has been found useful in the study of develop- 

 mental and evolutionary problems, but is not practicable for 

 routine laboratory isolations. 



Isolation by Plating. The development by Koch of the lique- 

 fiable media furnished a ready means for the isolation in pure 



Fig. 55. Isolation by successive streak cultures on an agar or gelatin 

 plate: A, First streak solidly grown; B, second streak, discontinuous; C, 

 third streak, having many isolated colonies. 



culture of most species of bacteria. Nutrient agar or gelatin 

 or one of their modifications may be used. The medium is lique- 

 fied by heat, then cooled in a water-bath to about 43. The 

 mixed culture of organisms from which it is desired to isolate 

 pure cultures is inoculated into one of the tubes. From this 



Fig. 56. Petri dish (McFarland). 



transfers are made by means of a sterile platinum loop to a second 

 tube; this is thoroughly mixed and transfers made to a third tube, 

 and from this even to a fourth. Each of these tubes of media 

 is then poured into a sterile, flat, glass, covered dish called a Petri 

 dish. These Petri dishes or " plates " are allowed to stand until 

 the medium has solidified; they are then incubated and examined 



