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CULTURE MEDIUM

A study of bacteria requires some nutrient medium for bacteria growth. Test tubes and petri dishes containing sterile nutrient can be purchased, or students can prepare
their own in several ways.

Different mediums will allow different bacterial colonies to develop. For each of the following suggestions, be certain that containers are clean and sterilized.

(Caution: Before discarding any medium on which bacteria have grown, be sure to sterilize the medium by heating or by soaking it overnight in a disinfectant such as 3 percent Lysol.)

Glassware and dishes can be sterilized in an oven at 110 degrees F (45 degrees C) for one hour or in a pressure cooker at fifteen pounds for thirty minutes.

Hands should be thoroughly washed (with soap) before and after handling materials.

(Note: Shallow glass dishes or glass coasters with glass covers can be substituted for petri dishes. Baby food jars and test tubes also work well.)

a. Agar medium.

Add 6 level teaspoonfuls of nutrient agar to 1 cup of water. Shake vigorously to get as much into solution as possible. Add another cup of water, cover, then sterilize in a pressure cooker. Do this by placing the agar container in a basket just above the water level. (A small can or rubber stopper can be used to raise the basket.) Put a lid on the cooker, and clamp it tight. Allow steam to flow freely at a moderate rate for ten minutes. Place the pressure control on the vent pipe, raise pressure to fifteen pounds, then lower the heat just enough to maintain the pressure for thirty minutes. At the end of this time, turn off the heat and allow the pressure to return to zero of its own accord. Let the cooker cool for two minutes, then pour the agar into sterilized petri dishes, test tubes, or other containers. As a variation, clear beef broth can be substituted for the water.

b. Gelatin medium.

Dissolve one beef bouillon cube in 1 cup of boiling water. Add a spoonful of sugar. Prepare unflavored gelatin according to directions on the package (use Knox gelatin or a bacteriological gelatin from a scientific supply house), but substitute the beef broth for the boiling water. Cool the solution, and pour it into sterilized petri dishes or test tubes. Cover and let containers cool.

c. Potato medium.

Cut wedges from a large, peeled, uncooked potato. Place one wedge into each test tube to be used. Add 1 teaspoon of water to each tube, plug with cotton, and sterilize
the tubes in a pressure cooker at 15 pounds for thirty minutes or in a double boiler for one hour.

Similarly, thin slices of white potato can be placed into petri dishes, sealed, and sterilized. Wedges or slices of carrot, sweet potato, apple, pear, or orange can be substituted for the white potato.

When you have sterilized your culture media, you can set about to grow the cultures you want.

 
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