CULTURE MEDIA 



8 9 



bath, although these are not essential. If it gets cold it may be 

 poured out of the funnel and warmed again in the pan. A portion 

 of the nitrate should be boiled in a test tube over the flame for a 

 minute or two. It should then remain (i) perfectly clear, (2) 

 alkaline to litmus paper, and (3) should solidify on cooling in tap 

 water. After nitration the medium is filled into tubes and steril- 

 ized in streaming steam by the fractional method, 20 minutes at 

 1 00 C. for 3 consecutive days. Gelatin 

 may be sterilized in the autoclave at 110 C. 

 for 10 minutes, but it should be chilled in 

 cold water at once after removal, and even 

 then its gelatinizing property may be seri- 

 ously impaired. 



In filling gelatin into tubes it is important 

 that the medium should not be spilled on 

 the mouth of the tube or on the cotton plug, 

 as this accident causes the latter to be glued 

 in position. The filling apparatus indicated 

 in Fig. 33 will be found convenient for filling 

 any sort of liquid medium into tubes, and 

 with proper care one may fill tubes rapidly 

 without soiling the mouths of tubes and their 



cotton plugs. Fio.33.-Apparatusfor 



Gelatin may be made from beef extract, filling media into tubes. 



mi I.L j i j.' It is held in a ring-stand 



The extract, peptone, salt and gelatin are sup port. 

 dissolved at a temperature below 60 C. or 

 the medium is cooled to this temperature after solution has been 

 accomplished. It is titrated and the proper amount of alkali 

 added. An egg is beaten up in water and then stirred into the 

 medium. It is then boiled on the water bath for an hour, 

 filtered, tested, tubed and sterilized. 



Nutrient Agar. To a liter of nutrient broth, prepared as 

 above described (page 84) add 15 grams of finely cut agar 

 shreds. Weigh the pan with its contents. Boil the material 

 over the direct flame for one to two hours, with constant stirring 



