CULTURE-MEDIA. 



77 



,S<S#tK?^^ 2=^ 



The necessity for sterilization of the tubes before filling them 

 with the medium has been questioned, and it is probably un- 

 necessary 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 subse- 

 quent 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 recom- 

 mended. They may readily be filled with an ordinary 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. Or- 

 dinarily about 7 to lo c.c. are placed in 

 a test-tube. For Esmarch's roll-tubes a 

 somewhat 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 pro- 

 longed 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 sterilization has been effective. It is the universal 

 experience in bacteriological laboratories that occasionally 

 culture-media will become contaminated with extremely resist- 

 ant spores which fail to be sterilized by the ordinary pro- 

 cesses, 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. 



Fig. 18. — Wire basket 

 for test-tubes. 



