THE USE OF THE ORDINARY CULTURE MEDIA 53 



120 C. the temperature being very gradually raised. These tubes are- 

 used for the primary inoculations, and during incubation, which is 

 necessarily prolonged and usually carried out at 22 C., should be placed 

 in a covered glass jar the lid of which is kept slightly raised at one side 

 with a pad of wool to permit the access of a certain amount of air, 

 by this device undue drying of the medium is at the same time prevented ; 

 the inoculated tubes should not be covered with rubber caps. The 

 study of the characters of the large colonies of trichophyta, etc., is best 

 carried oat with media distributed in 250 or even 500 c.c. Erlenmyer 

 flasks in which the requisite surface of medium with a suitably moist 

 atmosphere is oba hied. 



THE USE OF THE ORDINARY CULTURE MEDIA. 



The culture of bacteria is usually carried on in test-tubes 

 conveniently 6 x f in. These ought to be very thoroughly 

 washed and dripped, and their mouths plugged with plain 

 cotton wool. They are then sterilised for one hour at 170 C. 

 If the tubes be new, the glass, being usually packed in straw, 

 may be contaminated with the extremely resisting spores of 

 the b. subtilis. Cotton-wool plugs are universally used for 

 protecting the sterile contents of flasks and tubes from con- 

 tamination with the bacteria of the air. A medium thus 

 protected will remain sterile for years. Whenever a protecting 

 plug is removed for even a short time, the sterility of the 

 contents may be endangered. It is well to place the bouillon, 

 gelatin, and agar media in the test-tubes directly after nitration. 

 The media can then be sterilised in the test-tubes. 



In filling tubes, care must be taken to run the liquid down 

 the centre, so that none of it drops on the inside of the upper 

 part of the tube with which the cotton- wool plug will be in 

 contact, otherwise the latter will subsequently stick to 

 the glass and its removal will be difficult. In the case of 

 liquid media, test-tubes are filled about one-third full. With 

 the solid media the amount varies. In the case of gelatin 

 media, tubes filled one-third full and allowed to solidify 

 while standing upright, are those commonly used. With 

 organisms needing an abundant supply of oxygen the best 

 growth takes place on the surface of the medium, and for 

 practical purposes the surface ought thus to be as large as 

 possible. To this end " sloped " agar and gelatin tubes are 

 used. To prepare these, tubes are filled only about one-sixth 

 full, and after sterilisation are allowed to solidify lying on their 

 sides with their necks supported so that the contents extend 

 3 to 4 inches up, giving an oblique surface after solidification. 

 Thus agaT is commonly used in such tubes (less frequently 



