346 THE SMALLEST PLANTS (BACTERIA) 



plants all do this, but the simpler the plant, the more 

 easily does it resist. It is this ability to withstand un- 

 favorable conditions and to resume growth when condi- 

 tions change for the better that makes bacteria such " good 

 friends and such bad foes." 



LABORATORY STUDY OF BACTERIA 



Prepare culture plates of agar-agar from the following formula : 



Agar-agar Formula for 1000 c.c. 



Agar-agar 1 15 grams 



Beef extract 3 grams 



Peptone 10 grams 



Salt 5 grams 



Water 1000 grams 



Boil material for the agar-agar formula ; add sodium hydrate till the 

 color of litmus paper is not changed ; cool to about 56 C, and beat 

 into this one whole egg, including the shell. Warm slowly to the boiling 

 point and continue till the egg is firmly coagulated; then strain the clear 

 medium through a cheese-cloth on to moist cotton in a filter funnel. 



Work rapidly. Cool, and then boil once more. Filter through cotton 

 into test tubes. Each tube should not be more than a quarter full. Plug 

 the tubes with cotton. Then sterilize this mixture in the test tubes by 

 placing them upright in water and boiling twenty minutes on each of 

 three successive days. Let part of the test tubes cool, having the 

 plugged end elevated half an inch. These are called slant agar tubes. 

 When petri 2 cultures are needed, melt up a sterile agar tube and pour 

 into a sterile petri dish. 



1. To show that bacteria are present on one's hands. Draw the fingers 

 of the u.nwashed hand across the surface of the agar-agar in petri dish. 

 Cover and set away for four days at room temperature or two days at 

 body temperature. 



2. To show that fewer bacteria are present on freshly washed hands. 

 Draw the fingers of the washed hand across the surface of the agar-agar. 

 Cover and set away. 



3. To show that bacteria lodge under the nails. Place on culture plates 

 scrapings from under finger nails, (1) before washing the hands, (2) after 

 washing the hands. 



1 Secured at most drug stores. 2 Flat, round dish with cover. 



