68 



CULTURES IN SOLID MEDIA. 



with the sides of the tube. It is best always to hold the tube in- 

 verted during the inoculation, and not to remove the cotton air filter 

 until we are ready to make it. The cotton plug is then returned to 

 its place and the platinum needle again brought to a red heat to 

 destroy any bacteria which remain attached to it. 



Sometimes it is an advantage to have the culture medium with a 



FIG. 40. 



sloping surface, as shown in Fig. 40. We may then draw the nee- 

 dle over the surface in a longitudinal direction, and by this means 

 distribute the seed in a line along which development will take place. 

 The characters of growth in these stick cultures in gelatin are 



very various. Non-liquefying bacteria may grow only on the sur- 

 face, as at a, Fig. 40A ; or both on the surface and along the line 

 of puncture, as at b; or only at the bottom, as at c. In the first 

 case the microorganism is aerobic that is, it requires oxygen, and 

 grows only in the presence of this gas. In the second case it is 

 not strictly aerobic, but may grow either in the presence of oxygen 



