58 METHODS OF CULTIVATION OF BACTERIA. 



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 



a 



FIG. 14. Platinum wires in glass handles. 



a. Straight needle for ordinary puncture inoculations, b. " Platinum loop." 

 c. Long needle for inoculating "deep" tubes. 



contents extend 3 to 4 inches up, giving an oblique sur- 

 face 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 



