312 
SMALLEST PLANTS ( BACTERIA ) 
LABORATORY STUDY OF BACTERIA 
Prepare culture plates of agar-agar from the following formula: 
Agar-agar Formula for 1000 c.c. 
Agar-agar 1 
Beef extract 
Peptone 
Salt . . . 
Water . . 
15 grams 
3 grams 
10 grams 
5 grams 
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 cheesecloth 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, in a slanting 
position, 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 unwashed 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. 
4. To show that heating milk reduces the number of active bacteria, 
sprinkle drops of milk and water mixture on agar-agar petri dish, 
(1) natural milk, (2) pasteurized, (3) boiled. (Use one tenth milk 
and nine tenths sterilized water.) 
5. To show that bacteria change the medium in which they grow, 
note, besides the number, form, size, and color of the colonies, whether 
any change takes place in the agar-agar. 
6. To show that bacteria grow best in the presence of warmth and 
moisture, compare those grown under such conditions with those grown 
1 Secured at most drug stores. 2 Flat, round dish with cover. 
