50 



METHODS OF CULTIVATION OF BACTERIA. 



of the contents may be endangered. It is well to place the 



bouillon, gelatin, and agar media in 

 the test-tubes directly after filtration. 

 The media can then be sterilized 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. The tubes may, when 

 filled, be placed in cages made of 

 fine wire netting and sterilised. If 

 all the contents of a flask of medium 

 be not filled into tubes, the remain- 

 FiG. 14.— Apparatus for delivering der must be re-stcrilised before 



measured quantities of media into ^^^^^ ^^^^^^ j^. ^^^ ^^^^ ^^ jj^^j^ 



media, test-tubes are filled about 

 one-third full. With solid media the amount varies. In 

 the case of gelatin media, tubes 

 filled one-third full and al- 

 lowed to solidify while standing 

 upright are those commonly used. 

 With organisms needing an abun- 

 dant 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 gela- 

 tin tubes are used. To prepare 

 these, tubes are filled only about 

 one-sixth full, and after sterilisa- 

 tion 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 when held 



Fig. 15. — Tubes of media. 

 a Ordinary upright tube. i. Sloped tube. 

 c. " Deep " tube for cultures of anaerobes. 



