PREPARATION AND CARE OF CULTURE MEDIA. 



99 



and other substances decompose at these high temperatures and the results 

 obtained by the growth of bacteria in such media are not comparable with those 

 obtained on media sterilized at 100 C. Kitchens has recently shown that detri- 

 mental acids are formed when bouillon containing sugar is autoclaved. Peptone 

 water, agar, and bouillon may be sterilized in the autoclave. For titrating culture 

 media the writer uses the burettes shown in fig. 59. The twentieth-normal alkali 

 is stored as shown in fig. 60. Quadruple-normal sodium hydrate solution is used 

 for neutralization. The phenolphthalein solution is made by adding i gram of 



N 

 the dry powder to 100 cc. of 50 per cent alcohol, and then enough sodium hydrate 



to carry it fully into solution, removing the yellow color without making the fluid 



a very decided pink. Fluid media may 

 be filled into tubes very rapidly by means 

 of the device shown in fig. 83. For storing 

 media sterilized in test-tubes and for hold- 

 ing cultures made on such media the writer 

 has found ordinary quinine cans very use- 

 ful (fig. 84). 



The proper care of culture media after 

 sterilization involves considerable thought 

 if they are not to be used immediately. 

 Stored media lose water and along with 

 this loss, of course, there are physical 

 changes, so that the results obtained 

 are not always comparable with those 

 obtained from similar media containing 

 the standard volume of water. Various 

 devices have been recommended for pre- 

 venting this loss of water. Rubber caps 

 keep in the moisture, but are apt to 

 favor the development of fungi. Paraf- 

 fined plugs made by removing the cotton 

 plug, dipping the lower end of it quickly into and out of hot sterile paraffin, and 

 replacing it in the mouth of the tube or flask before the melted paraffin has had 

 time to cool, answer the purpose very well, but have the objection that all of the 

 tubes must be placed in turpentine or some other solvent of paraffin before they 

 can be cleaned for a second use. On the whole, the use of moderately tight plugs 

 and the storage of the media in cool or cold air are the best methods of retaining 

 the water content of the medium. Nutrient media should be made in small quan- 

 tities and often, rather than in large quantities and at infrequent intervals. The 

 cotton should be dry-heated in bulk before plugs are made from it 



*Fic. 84. Ordinary quinine cans with a little cotton in the bottom are very convenient for 

 holding cultures and culture-media in test-tubes. One-third actual size. 



