298 BACTERIA, YEASTS, AND MOLDS 



Preparation of Agar 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 1 5 grams of agar add 985 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 absorbent cotton 

 placed in it. The liquid agar is poured into the cotton, and it will run 

 through readily, coming out as a tolerably clear solution. Some 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 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 

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

 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 agar in three or four of the test 

 tubes prepared as above described, and pour it from each into a steril- 

 ized petri dish. Replace the cover upon the dish and allow the agar 

 to harden. Sweep a littie dust from the floor and scatter over the sur- 

 face of the agar 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 agar. 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 

 agar, as in Experiment 6, and, after removing the cover, shake the dust 



