(Written in collaboration with Jenny Cuccinello. Adapted from an activity developed by Fellows in the Howard Hughes Biology Institute.)
TYPE OF ENTRY:
TYPE OF ACTIVITY:
ABSTRACT OF THE ACTIVITY:In this series of exercises, the students will predict the conditions necessary for bacterial growth, test their predictions and at the same time practice the aseptic techniques and safety procedures needed when working with bacteria. In the first exercise, the students' prior knowledge of bacteria is determined including the conditions they believe necessary for bacterial growth. Armed with petri dish and cotton swabs they are dismissed to the campus to obtain samples from the places with these conditions. Students learn how to prepare their lab areas for microbiology work, pour and swab plates, and seal, label and store plates.
BACKGROUND INFORMATION:This activity is a series of exercises that introduces students to bacteria and aseptic microbiology lab techniques. The content exercises and the lab skill exercises run side by side. The students will determine the conditions necessary for bacterial growth as well as the techniques used to culture and isolate bacteria. To complete the series of exercises takes one full 55 minute class period on the initial day and two half periods on the second and third days. However, there are many logical stopping points and you can tailor it to fit your time frame.
There is no prep time needed for the first exercise. The prior knowledge activity works well as a journal entry and, if done at the end of the period prior to beginning the activity allows you to read through their responses and get an idea of what the students know and don't know. I prefer to use the four square method but the questions could be answered in narrative form . I also allow the students to draw pictures if necessary.
In exercise two the students learn how to prepare their work area. We used spray bottles filled with a solution of 50% Lysol. A 10% bleach solution is very effective also. The simulated "bacto-broth" solution is made from fluorescent Elmer's glue (any color) diluted with water. One small bottle of glue should suffice 5 classes. Spread on the students' lab tops and water faucets before each class. The "bacto-light' is a hand held black light. Any missed spots will fluoresce nicely and show the students if they were thorough.
The students practice swabbing plates in exercise three. In order to assess student competency we devised a method so we could see their swabbing pattern. Prepare a 3% agar solution. You want it thick to prevent the phenothalein solution used to simulate bacteria from absorbing in too quickly. Add 25 mL of NaOH to the melted agar. Pour the agar into petri dishes and let cool. Prepare a phenothalein solution. Adding a small amount of glycerol to the phenothalein solution helps to prevent quick absorption of the solution into the agar. Separate into test tubes (enough for each working group). These plates can be reused. Simply melt down the microwave, swirl until clear, and let cool.
In the concluding exercise the students are sent on a microbe scavenger hunt. We used this activity to show the students how to pour their own plates as well as how to label them, seal them and store in the incubator. For our incubator we used two plastic drip trays from the garden nursery and a heating pad. Works great!
exercise two: disinfectant in spray bottles (50% Lysol solution), old rags, "bacto-broth" and "bacto-light" (see background notes)
exercise three: prepared NaOH agar plates and phenolthalein solution, inoculating loops, flame source if using metal inoculating loops
exercise four: nutrient agar, sterile petri dishes, cotton swabs, permanent markers
1. Where I think bacteria growUsing the conditions you described in question one, make a list of places you have access to on campus that you would expect to find bacteria.
Obtain a spray bottle and cloth from the equipment table. Wipe down your lab station and surrounding water fixtures and handles thoroughly. When you are finished, request an inspection with the "bacto-light" and your teacher. Any missed areas will need in a recleaning! Make sure to get a stamp on your lab write up to indicate successful disinfecting.
To practice the technique of spreading bacteria on a plate we will use an indicator solution. Because bacteria are so small, you usually cannot see them when you spread bacterial solution on the agar. By using the indicator solution we may observe the spreading pattern.
Obtain a pre-poured petri dish, an inoculating loop and a vial of the simulated bacteria culture from the equipment area. The inoculating loop is sterile in its wrap and is disposable. Take the loop out of the wrap and dip it in the bacteria just to the top of the loop. Carefully lift the lid of your petri dish (at approximately a 30 degree angle) and touch the loop to the agar. Drag the bacteria solution across the surface of the agar in a zig zag pattern from the top to the bottom.
Bring your plate to your teacher for inspection.
We want to spread the bacteria on the surface of the agar to try to isolate a group of bacteria cells called a colony. Please be very careful when swabbing the surface so as not to tear the "hardened" agar.
In the data section of your lab write up, use your circle template to draw a representation of your petri dish. Label it to correspond to the labels on your petri dish. You are now ready to "hunt" bacteria. Use the cotton swabs to transfer bacterial cells by rubbing the fixture or object and then gently swabbing the surface of one quadrant of your petri dish. For each specimen collected, record the following information:
a. Location: _________________________ (be as specific as possible) b. Conditions such as moisture content. lighting, temperature c. Degree of human contact d. Any other details or information
Things to remember:
1. Keep the petri dish covered as much as possible.