22 A MANUAL OF BACTERIOLOGY 



using. When the cotton plugs are removed from sterile tubes, 

 the tubes should be held horizontal or sloping slightly down- 

 ward. The plugs should be held between the fingers in such a 

 way that the portions entering the tubes will not come in contact 

 with each other nor with other objects (Fig. 14). 



Stab cultures are made by thrusting the straight wire down 

 the center of the cylinder of the culture medium. Streak cul- 

 tures are made by drawing the wire once over the flat surface 

 of the medium (agar or potato). 



Fig. 14. Method of inoculating tubes of solid media 



After inoculation, tube cultures should be placed in some re- 

 ceptacle which will keep them upright, and set in a place where 

 they will not be exposed to strong light. Experimental cultures 

 can be stored in incubators at required temperatures. Stock cul- 

 tures should be kept in a room of even temperature, preferably 

 not too high, and should be transferred to new media once in 

 six or eight weeks. Before removing the cotton plugs from old 

 cultures it is well to burn over the surface of the cotton, to kill 

 germs which may have settled in it. The plugs may be protected 

 from dust by caps of paper, tin foil, or rubber. 



