54 METHODS OF CULTIVATION OF BACTERIA. 



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 

 gauze and sterilised. If all the contents of a flask of 

 medium be not filled into tubes, the remainder must be 

 re -sterilised before being stored. 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 gelatine 

 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 gelatine 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 

 when held upright after solidification. Thus agar is 

 commonly used in such tubes (less frequently gelatine 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 such that only 

 one organism is present. The methods of obtaining pure 

 cultures will presently be described. They vary according 

 as we are dealing with aerobic or anaerobic organisms. 

 When a fresh tube of medium is inoculated from an already 

 existing culture, the resulting growth is said to be a " sub- 

 culture" of the first. All manipulations involving the 

 transference of small portions of growth either from one 

 medium to another, as in the inoculation of tubes, or, as 



