Kartchner Caverns Microbial Observatory
National Science Foundation
photo of caverns


Grades 7-9:

Growing microbes under different conditions.

For older students, you can use a similar experimental set up as above to look at basic microbial diversity in soils (or other samples substrates).  In these experiments, several types of growth media are made and are incubated in different ways.  Much of the research in Kartchner Caverns incorporates these types of experiments to characterize the microbial diversity from different cave surface and substrates.  Students can also do this with soil, and can keep track of the different kinds of microbes (both bacteria and fungi) based on colony size, color, shape, texture, and other physical properties.  Keeping track of microbial diversity on plates made from different media or that have been incubated differently will allow the students get a basic idea of what kind of microbes live in the soil, what sort of nutrients they need to live, and the different environmental conditions necessary for their optimal growth.

What you will need:

1) Knox gelatin
2) water
3) nutritional substrates such as vegetable juice, lemon juice, oatmeal, chicken broth, and/or syrup
4) Petri plates (baby food jars or half-pint jelly jar will also work if sterilized by boiling or pressure cooking).
5) one or two different samples of different soil types
6) sterile spoons or sterile cotton swabs

The Experiment:
So, first make your microbial growth media by mixing your nutritional substrates with the gelatin, and let them set up in the Petri plates overnight.  It is important that the medium you are making be sterile so that you can be sure the microbes you are growing are from the soil sample under investigation.  You can sterilize your medium by boiling the gelatin and nutrient substrate together for 5 minutes (make sure it is covered).  Let it cool (no peeking under the cover, this will let contaminants in) to about 60°C and then pour the medium into the sterile Petri plates or jars and let it set up.  Do not stir or touch the medium with utensils that are not sterile while pouring.  It is suggested that teachers or parents perform this step as handling boiling gelatin can be dangerous.  Make all media to be used in this experiment at one time while you have your sterilizing equipment out.  Designate one medium as the “standard” that you will compare other media to.

Experiment 3, Figure 1

After the media is cool, inoculate the media using the following sources.

  1. Microorganisms in the soil:  collect 1-2 tablespoons of soil with a sterile spoon and place them in a sealable jar.  Add 100 ml of water, and swirl vigorously (but not so vigorously that the jar breaks!!).  Use an eyedropper to add 3 drops of the dirty water to the surface of the medium and spread it over the surface with the back of a sterile spoon or sterile cotton swab.

    Experiment 2, Figure 2Experiment 3, Figure 3

    Quickly replace the cover on the plates/jars and let them sit in the dark for up to 2 weeks, checking for growth of bacteria every day.  As a control to show that the medium itself doesn’t have any contaminating microorganisms, set aside one or two plates/jars that have not been inoculated and set then beside the inoculated jars.
  2. During the course of the experiment, have the student record all the bacteria they see and include the various characteristics if each colony.  If the students would like to obtain some of their bacterial isolates in pure culture (without any other bacteria present that might obscure or restrict their growth), have them touch the surface of any bacterial colony with a tooth pick ad then gently rub the toothpick onto the surface of clean media.  Allow to incubate as before and note any differences in the growth of the bacteria when there are no other competing strains of bacteria around. 
  3. Allowing the student to isolate a particular colony will enable them to observer the characteristics of the bacteria by itself growing on several different substrates.  Have the students track the growth and progress f the bacteria at different time points, using colony diameter as one metric for growth.  Also have them observe colony morphology ad look for the production of things like colored exudates.  These data can then be used to plot graphs, make tables, and write a science report.

These experiments provide a wide array of possibilities that will familiarize students with the microorganisms in their world.  As such, experiment with the experiment!!  Change in whatever way imaginable the things you can add to the growth medium that will provide different nutrients for the bacterial or fungi.  Now that students know that microorganisms are there, how do they respond to different acidities or nutrients?  Do some grow better in the dark or in the light?  For more complex experiments, change the nutritional substrates in the media such as using sugar with and with out lemon juice (to acidify the medium), adding chicken broth (for diverse amino acids), or oatmeal (for high carbohydrate content).  Grow the colonies at different environmental conditions such as with light or no light, or at different temperatures like those in Kartchner Caverns.  Depending on the lesson being taught, these experiments can aid in understanding metabolism, ecology, scientific design, or basic laboratory practice.  Have students ask their own questions about microbial communities and come up with a simple experiment to answer that question.





updated 7/2009

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