270 BACTERIA, YEASTS, AND MOLDS 



Preparation of Gelatin Culture Medium 



For the following experiments it is necessary to prepare a jelly 

 upon which molds will grow. A satisfactory jelly for this purpose is 

 as follows. 



To loo grams of gelatin add 900 cc. of water and about 5 grams 

 of Liebig's Extract of Beef, and boil for half an hour. While still 

 hot filter the material through absorbent cotton. In using absorbent 

 cotton for this purpose a large funnel should be used and the absorb- 

 ent cotton placed in it. The liquid gelatin is poured into the cotton, 

 and it will run through readily, coming out as a tolerably clear solu- 

 tion. Spme of the filtered jelly is to be placed in sterilized flasks and 

 some in test tubes, about 10 cc. in each. Plug the flasks and test tubes 

 with cotton, and steam the jelly in a common steamer for about twenty- 

 five minutes. The jelly is to be cooled and put aside for twenty-four 

 hours. At the end of that time it should again be placed in the 

 steamer and steamed for half an hour. Once more set it aside for 

 twenty-four hours, and upon the third day steam it again for half an 

 hour and cool. Material thus prepared should give a clear, slightly 

 brownish jelly, which, if properly sterilized, will keep indefinitely. It 

 should be acid to litmus paper. 



If the teacher does not care to go to the trouble of making the 

 gelatin, she can buy it of dealers in bacteriological supplies. The 

 gelatin culture medium which is sold by such dealers is slightly 

 alkaline, and should be rendered a little acid by adding HCl until 

 the mixture will just turn blue litmus paper red. Molds require an 

 acid medium, though bacteria need one with an alkaline reaction. 



6. Mold Spores in Dust. Melt the gelatin in three or four of the 

 te.st tubes prepared as above described, and pour it from each into a 

 sterilized petri dish. Replace the cover upon the dish and allow the 

 gelatin to harden. Sweep a little dust from the floor and scatter over 

 the surface of the gelatin in one petri dish. Scrape some dust from 

 a crack in the floor and sow on another dish. In the same way sow 

 dust from other places upon the gelatin. Set aside until the molds 

 begin to grow, and examine the mold colonies. 



7. Molds in a Dust Cloth. Prepare two petri dishes of hardened 

 gelatin, as in Experiment 6, and, after removing the cover, shake 



