54 METHODS OF CULTIVATION OF BACTERIA 



The Use of the Ordinary Culture Media. 



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

 conveniently 6 x § in., but for many purposes smaller tubes, 

 5x| in., are equally suitable and medium is thus saved. The 

 tubes 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 ex- 

 tremely resisting spores of the b. subtilis. Cotton-wool plugs 

 are universally used for protecting the sterile contents of flasks 

 and tubes from contamination 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 

 filtration. 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 agar is commonly used in such tubes (less frequently 

 gelatin is also " sloped "), and this is the position in which blood 

 serum is inspissated. Tubes, especially those of the less commonly 

 used media, should be placed in large jars provided with stoppers, 

 otherwise the contents are apt to evaporate. A tube of medium 

 which has been inoculated with a bacterium, and on which 

 growth has taken place, is called a " culture." A " pure culture " 

 is one in which only one species is present. The methods of 

 obtaining pure cultures will presently bo described. When a 

 fresh tube of medium is inoculated from an already existing 



