132 



BACTERIOLOGY 



FIG. 18 



If glycerin is to be added to the agar-agar, it is done after 

 filtration and before sterilization. The nutritive properties 

 of the media for certain organisms, particularly the tubercle 

 bacillus, are increased by the addition of glycerin in the 

 proportion of 5 to 7 per cent. 



If after filtration a fine flocculent precipitate is seen, look 

 to the reaction of the medium. If it is quite alkaline, boilj 

 neutralize, and filter again. If the reac- 

 tion is neutral or only very slightly acid, 

 dissolve and again clarify with egg-albu- 

 men by the method given. 



The most important feature of all the 

 media, aside from the correct proportion 

 of the ingredients, is their reaction. It 

 must be neutral or very slightly alkaline 

 to litmus. (See remarks on Neutralization 

 of Media.) Only a few organisms develop 

 well on media of an acid reaction. 

 * Preparation of Potatoes. With an ordi- 

 nary cork borer punch out from sound 

 potatoes cylindrical bits that will slip 

 easily into the test-tubes to be used. Cut 

 away all particles of the skin. Then cut 

 on each cylinder a slanting surface extend- 

 Potato in test-tube, ing from about the middle diagonally to 

 the end. Leave the cylinders in running 

 water over night to prevent them from becoming dis- 

 colored when they are sterilized. 



One potato cylinder thus prepared is then to be placed 

 in each of the already cleaned, plugged and sterilized test- 

 tubes, after which they are sterilized by either the inter- 

 mittent method with streaming steam or by steam under 



