APPENDIX C 

 MAKING PERMANENT PREPARATIONS 



A supply of demonstration material is very useful for class in- 

 struction and for general publicity work. If colonies of organisms 

 in pure culture are desired, there is nothing better than the Soyka 

 flask. The medium, when hard, can be inoculated with a needle at the 

 center and will produce a good colony. After growth begins and 

 the agar has dried somewhat, the cotton plug can be burned off and 

 the mouth of the flask sealed with red sealing wax. The flask may 

 then be mounted in one of the special holders in which there is dis- 

 played a microphotograph or authentic drawing of individual bac- 

 teria of the species exhibited. Specimens prepared in this way will 

 stand much rough handling and will endure for several years. 



The Soyka-flask method is chiefly useful for pure-culture colonies 

 or when work can be planned in advance. Teachers are well aware 

 of the fact that many instructive examples arise in the progress of 

 the daily work, and that such specimens are extremely valuable if 

 properly preserved. A method is therefore desirable which will allow 

 the preservation of cultures in test tubes and Petri dishes. Hastings l 

 has described a very satisfactory method in which glycerin agar is 

 used. The method consists in pouring over the surface of the plates 

 to be preserved some glycerin agar, which is prepared by mixing 

 equal quantities of 2 per cent agar and glycerin. In preparing the 

 glycerin agar the ordinary thread agar should be washed in tap water 

 for two or three days. A 2 per cent solution of the washed agar 

 in distilled water is made and carefully filtered through paper. An 

 equal volume of glycerin is added to this solution. No sterilization 

 is necessary, because the medium is dense enough to prevent the 

 growth of bacteria and molds. 



When one wishes to preserve an ordinary plate culture containing 

 an agar medium, a small quantity of glycerin agar is melted, cooled 

 to about 45 C, and carefully poured over the surface of the plate 

 culture. The glycerin agar solidifies and forms a firm protective 

 layer over the surface of the plate. The larger part of the water will 

 evaporate, but the glycerin, being hygroscopic, holds enough to 



1 Centralbl. f. Bakt., 2te Abt., 34 : 432. 1912. 

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