20 LABORATORY GUIDE IN BACTERIOLOGY 



Preparation 



1. Note the weight of an Erlenmeyer flask (500 c c.). 



2. Weigh 3.75 (ij per cent.) g. of glucose into the 

 flask. 



3. Filter 250 g. of plain agar into the flask, heat, and 

 agitate until the glucose is completely dissolved. 



4. Fill twenty tubes in the same manner as above. 



NOTE. By tying a string across a wire basket near the top, 

 the plain agar-tubes may be kept separate from the glucose-agar. 

 A slip of paper indicating the medium and the date of its prepara- 

 tion must always be inserted. These two media are very diffi- 

 cult to distinguish by the eye, and failure to label them properly 

 will lead to entirely unreliable results. 



The media should now be sterilized in the autoclav. 

 The principle of this mode of sterilization is the applica- 

 tion of steam under pressure. Certain bacteria, and 

 some of these are very widely distributed in nature, have 

 the faculty of forming spores. These spores are very 

 highly resistant to heat and do not lose their vitality either 

 by boiling or by application of heat under ordinary atmos- 

 pheric pressure, although all vegetative forms are killed. 

 By adding the pressure of one atmosphere to ordinary 

 pressure, the boiling-point is raised to 121.40, which is 

 sufficient to kill all spores during an exposure of 5 

 minutes; media in flasks should be given 10 minutes. 



The autoclav consists of a strong cylinder, made of 

 iron, with a bottom and removable lid. Inside is a 

 basket, or rack with shelves, resting on three short sup- 

 ports with a centrally located hole in the bottom. The 

 lid fits closely (in some autoclavs with a large washer), 

 and is provided with a steam-valve (Fig. 12, a), a safety- 

 valve (Fig. 12, b), and a gauge (Fig. 12, c). The latter 

 indicates the pressure and temperature. The top is 



