Growing your own bacteria.
Scientists are becoming increasingly aware that microorganisms can be found in nearly every ecosystem and on nearly every surface imaginable. This includes rocks, even those found underground in caves and caverns throughout the world. Although microorganisms living in caves are something most people don’t think about, our work in Kartchner Caverns, Arizona, has revealed complex bacterial and fungal communities on nearly every surface that has been sampled. It is also known that our bodies have a number of different types of bacteria and fungi naturally present on our skin. What happens if we touch surfaces that have their own unique microbe community with our fingers, which have their own unique microbe community? The result is that we often transfer what we have on our skin to the surface that we touch. This can have interesting, unusual, and often damaging consequences for the surface that was touched! To demonstrate to students the prevalence of bacteria on their skin, this experiment will allow you to make a microbial growth medium from gelatin that will allow you to grow bacteria from your skin, soil, rocks, grass, or anything else you can think of.
What you will need:
1) Knox gelatin (Roughly 7.0 grams of gelatin (1 pack) per 100 ml of nutrient base or plain water)
3) nutrient base (fruit juice or sugar water)
4) Petri plates (baby food jars or half-pint jelly jar will also work if sterilized by boiling or pressure cooking).
6) toothpicks and cotton swabs
7) an eyedropper
8 sterile spoons (you can sterilize these by boiling)
9) an assortment of small rocks and pebbles
10) dirty hands
So, first make your microbial growth media by mixing your nutrient substrate or plain water with the gelatin (150 ml of substrate to ~ 7 g of gelatin). 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 or other surface 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. After pouring, and when the medium is completely cool and has solidified, inoculate the plates or jars from the following sources:
- Microbes in the soil: collect 5-10 small pebbles with a sterile spoon and place in a sealable jar. Add 40 ml of water, and swirl vigorously (but not so vigorously that the jar breaks!!). Then use an eyedropper to add 3 drops of the dirty water to the surface of the medium and spread it over the surface with a glass plate spreader, back of a sterile spoon, or sterile cotton swab.
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 everyday. It is better if you incubate your plates or jars upside down (then condensation from the lid will not drip onto the gelatin surface). ***IMPORTANT*** As a control to show that the medium itself doesn’t have any contaminating microbes, set aside one or two plates/jars that have not been inoculated and set them beside the inoculated jars. There should be no growth on these plates/jars. If there is, then the medium itself was not sterile to begin with.
- Microbes from fingers or environmental swabs: follow the instructions as above, except allow the students to use a sterile swab to sample any surface they want (sinks, toilets, etc) and streak on the surface of the media. Alternatively, they can just get their hands dirty and touch the media on one or two places (this will show them why they need to wash their hands before eating!!).
- During the course of the experiment, have the student record all the bacteria they see and include the various characteristics of each bacterial colony. If the students would like to obtain some of their bacteria 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 sterile toothpick and then gently rub the toothpick onto the surface of clean media.
Allow to incubate as before and note any differences in growth of the bacteria when there are no other competing bacterial strains around.
This all relates to the Kartchner caverns project by demonstrating that there are living things everywhere, even in places where you wouldn’t expect it, like rock surfaces. It also emphasizes why it is so important not to touch the cave formations, because you may introduce foreign bacteria from your hands onto the formation, altering the balance of the native microbial populations, and possibly killing them.