70 



CULTURES IN SOLID MEDIA. 



always to hold the tube inverted during the inoculation, and not 

 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 



die 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 stab cultures in gelatin are 



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

 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 



