THE PREPARATION OF CULTURE MEDIA 



119 



The adjustment of the reaction of media is largely determined by the 

 particular uses for which the media are designed. For examinations in 

 the practice of sanitation, such as analyses of water, ice, and milk, etc., 

 the American Public Health Association recommends a standard reac- 

 tion of + 1 per cent (the plus sign is used to indicate acidity, the minus 

 alkalinity; + 1 per cent is the expression used to indicate that one per 

 per cent of ^ sodium hydrate solution would be required to neutralize 

 the medium or 10 c.c. to the liter). For general work with pathogenic 

 bacteria, the most favorable reaction for routine media is slight alka- 

 linity, neutrality, or an acidity not exceeding + 1 per cent. 



Methods of Clearing Media. Clearing with. Eggs. When culture 

 media are prepared from substances containing no coagulable proteid, 

 it is often necessary, for purposes of clearing, to add the whites of eggs, 

 and then to heat for forty-five min- 

 utes in the Arnold sterilizer. In 

 the following detailed descriptions, 

 the direction "clear with egg" has 

 been given whenever such a step is 

 deemed necessary. The exact tech- 

 nique of such a procedure is as 

 follows : 



In a small pot or pan, the 

 whites of several eggs (one or two 

 eggs to each liter of medium) are 

 beaten up thoroughly with a little 

 water (20 c.c.). This egg white is 

 then poured into the medium, 

 which, if hot, as in the case of 

 melted agar or gelatin, must first 

 be cooled to about 50 to 55 C. 

 The mixture is then thoroughly 

 shaken and steamed in the Arnold 

 sterilizer for thirty minutes. At 

 the end of this time the flask con- 

 taining the medium is removed from 

 the sterilizer and thoroughly shaken 



so as completely to break up the coagulum which has formed. It is 

 then replaced and allowed to steam for another fifteen minutes. At 

 the end of this time the medium between the coagula should be clear. 

 It is now. ready for filtration through cotton. 



FIG. 21. MEDIA IN TUBES: a, broth; 

 b, agar slant; c, potato. 



