CULTURE-MEDIA. 83 



them with the medium has been questioned, and it is prob- 

 ably unnecessary as far as the preservation of the culture- 

 medium is concerned, but it will be found that the cotton 

 plugs fit much better after sterilization with dry heat. 

 During this and subsequent sterilizations the tubes are held 

 in a wire basket. 



Filling of the Tubes. A special funnel closed with a 

 stop-cock for filling tubes with liquefied media is often 

 recommended. They may readily be filled with an ordi- 

 nary funnel of small size. During the filling, the neck of 

 the test-tube where it comes in contact with the cotton must 

 not be wet with the medium. Ordinarily about 7 to 10 c.c. 

 are placed in a test-tube. For Esmarch's roll-tubes a some- 

 what smaller quantity is desirable. 



The sterilization of tubes containing culture-media is 

 always done by steam, and has been sufficiently described. 

 It is to be remembered that the solidifying power of gelatin 

 is impaired by too prolonged heating, while heating is less 

 likely to damage other culture-media. The media which 

 are sterilized at a low temperature (70 C.) should be tested 

 for two days in the incubator to determine whether steriliza- 

 tion has been effective. It is the universal experience in 

 bacteriological laboratories that occasionally culture-media 

 will become contaminated with extremely resistant spores 

 which fail to be sterilized by the ordinary processes, an 

 occurrence which causes great annoyance and calls for the 

 exercise of much patience. Sometimes, also, moulds attach 

 themselves to the plugs, especially if they are moist, and send 

 their filaments down through the cotton ; finally, having 

 reached the lower edge of the cotton, their spores may fall 

 upon the medium, grow there and ruin it. 



