' STREAK ' CULTURES 73 



in an oblique position, so as to expose as much surface as 

 possible. The tube is held in a horizontal position, to 

 prevent aerial organisms ^rom falling into the tube, and 

 the plug is carefully withdrawn with the third and fourth 

 fingers of the right hand, using the back of the hand. The 

 platinum wire (which has been previously sterilised by 

 heating to redness in flame), with a trace of the colony on 

 the point, is carefully passed down the tube, so as not to 

 touch the sides, and is gently drawn along the centre of the 

 nutrient medium, using a light but even pressure. The wire, 

 on removal, is at once sterilised by heating, and the cotton- 

 wool plug returned to the tube, having been previously 



Fig. 16.— Method of making a ' Streak ' Culture. 



singed in the flame. The whole operation is carried out 

 as quickly as possible, so as to reduce the chance of outside 

 contamination to a minimum. 



The ' streak ' cultures in the case of gelatine are kept at 

 a temperature of 70° F, ; when on agar, they are incubated 

 at about 100° F. The colonies may confine themselves 

 to the actual inoculation stroke, or they may spread them- 

 selves out until the whole surface of the medium is covered 

 with the growth. The growth, again, may flourish only on 

 the surface, as is generally the case, or the organisms may 

 grow downwards into the medium in the form of hair- like or 

 radiating runners. Some organisms will develop on the 



