Surface Microbiology

Introductory Lessons in Microbiology

It is important to learn a little bit about microbiology before designing a science fair project. Microbiology is all about microscopic life. A good place to start learning about microorganisms is learning about the tools used for collecting data.

Test bacteria on any surface with the Surface Microbes Science Fair Projects Kit: after hand sanitizer, hands, disinfectant.. for example. You can use antibiotics to test the bacteria too.

Anytime you want to test how many bacteria are On Top of a surface like a desk, skin, chicken, computer keyboard, bread dough, a hard piece of chocolate, cheese, inside of an animal's cheek, etc. then use the surface experimenter kit. The kit will let you calculate how many bacteria there are per unit surface area on the object. You can also test for e-coli, however, only the microbe water kit will let you distinguish e. coli from other coliforms and bacteria.

Objectives

1. For all grades: to illustrate the usefulness of models to represent things which are too small (cells or molecules) or too large in science.

2. For primary grades: to learn to measure with a ruler, to cut a cube, and determine smaller particles react faster than larger particles.

3. For middle grades: to determine surface area and volume of a cube in addition to the above.

4. For upper grades: to determine the surface area to volume ratio and relate this to cell size, to determine why cells divide and 1-3 above.

Materials

2500 ml 2% agar solution (sufficient for 15-20 set-ups or pairs)
a cake pan
phenolphthalein powder
1 250 ml beaker or cup
50 ml .4% NaOH solution
a metric ruler, stirrer or spoon, plastic knife, and paper towels

Strategy

1.Advance Preparation:
Mix enough agar powder in boiling water to make a 2% agar solution. Use enough water to fill a cake pan to a depth of 3 cm (approximately 2500 ml). Stir until all the powder is dissolved. As the agar cools, add 1 g of phenolphthalein (if solid is unavailable, add several ml of liquid phenolphthalein indicator) per liter of solution and stir thoroughly. If the color is pink, add dilute acid drop by drop until the solution turns colorless. Pour the mixture into the cake pan to solidify. This will provide the agar for the model of the cells. If agar is unavailable, substitute potatoes, but then razor blades must be used and a dye found which will penetrate the potato in a short time.

2. Discuss models and their importance with the class. In this activity we will use agar blocks to represent cells.

3. Give the students a 6x3x3 cm block of agar cut from the cake pan, a plastic knife, and metric ruler. Ask them to cut three separate cubes 1x1x1, 2x2x2, and 3x3x3 cm from the block.

4. Ask the students, "If you were a cell which cell would you rather be (small, medium, or large) and why?" Write this down.

5. Ask the students to place the cubes into the beaker. Then the teacher pours the NaOH into the beaker to just cover the cubes. (CAUTION: Sodium hydroxide is caustic and can burn the skin and eyes.)

6. The cubes should remain in the solution for 10 minutes. They should be stirred occasionally with the spoon. When the NaOH comes into contact with the agar blocks, the blocks and perhaps the solution will turn a pink color. The students enjoy this.

7. Depending on the grade level, students should be given a task to do while the cubes are "soaking". Primary grades may be asked if this were a cell, what type of things might move into it. Older students may be asked the same as well as to explain diffusion since this is what is happening. They should also be asked to set up a data table in which they determine the surface area, volume, and surface area to volume ratio for each cube.

8. After 10 minutes the cubes are taken out of the beaker with a spoon and dried off with a paper towel. The students should cut the cubes in half and measure the distance from the outer edge inward that has turned pink and record this.

9. Students will discover that the distance that the solution travelled in each cube is the same (5 mm). There is a pink border around the 2x2x2 cm and 3x3x3 cm cube, but the 1x1x1 cm cube is pink throughout. Ask if the pink represented food, water or something else needed by the cell to survive, which "cell" got the needed substance distributed to all its parts. They should see that the smallest cell is most efficient since it is pink throughout.

10. Mathematically, students should observe that the smallest cube has the largest surface area to volume ratio (SA:VOL). Therefore this illustrates that a large SA:VOL promotes better efficiency in moving things into and out of cells and thus survival. This can also be related to smaller particles reacting faster than larger particles in chemical reactions (i.e. Granular sugar dissolves easier than sugar cubes.)

Assessment

Students can be asked which type cell they think would have a better chance for survival, one which is 1x1x1 cm or one which is .1x.1x.1 cm. They need to justify their response. 5 points for a proper mathematical as well as written explanation. 4 points for an explanation which is a little unclear. 3 points for a proper explanation but improper or no math. 2 points for an unclear explanation but shows thought. 1 point for an honest attempt.