226 CIVIC BIOLOGY 



plug the test tubes. To make plugs, tear a strip of cotton 

 about two inches wide and as long as needed, fold length- 

 wise, and roll into a plug. Insert this not more than half 

 an inch into the test tube. Cotton plugs are quite generally 

 used in bacteriological work, since they allow a free circu- 

 lation of air and prevent the entrance of germs. If material 



FIG. 105. Preparing culture media 

 Photograph by the author 



is properly sterilized and plugged with cotton, it will keep 

 indefinitely. (3) Do not open the hot-air sterilizer until the 

 temperature is down to 40 or 45 C. It is preferable to leave 

 the dishes undisturbed in the sterilizer until used. 



Before planting (inoculating) your culture media with 

 bacteria observe the following: 



Unless otherwise directed, always inoculate media with 

 platinum loop or needle. (1) Heat the wire in the flame 

 just before and immediately after using. (2) Avoid having 



