2: EXPOSURE PLATES
- Page ID
- 157061
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Observe and describe microbial growth on nutrient agar plates resulting from environmental exposure.
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Identify the appearance of colonies formed by different microorganisms.
BACKGROUND
Although they are invisible to the naked eye, millions of microorganisms surround us at all times. Becoming aware of their presence is an important first step in learning to work with them safely and effectively. While a microscope can help you observe microorganisms, it is not practical to scan every inch of your lab bench or hands to locate them.
Instead, we will use a method that allows individual microbial cells to multiply until they form visible colonies which can be seen without a microscope. This approach lets us observe microorganisms present in our environment by growing them on solid nutrient media.
In this exercise, you will test various environmental surfaces by exposing sterile agar plates to them. As each cell multiplies, visible colonies will appear where microorganisms landed and grew. However, it's important to note that we can’t create conditions that support the growth of all types of microorganisms, and we intentionally avoid growing fungi (a group of eukaryotic microorganisms that includes molds and yeasts) in large quantities, as they can be difficult to control and may pose health risks in a classroom setting.
To make this experiment more engaging and informative, each student should choose a different surface to test. You may expose plates to surfaces such as doorknobs, countertops, elevator buttons, or personal items like coins or backpacks.
Important: Do not test your own skin, mouth, or any part of your body at this time. The purpose of this activity is to safely visualize environmental contamination, not to culture potentially infectious microbes from your own body. Later in the semester, once you have developed the skills necessary to safely handle human specimens, you will have opportunities to examine your own skin and throat for microbial presence.
MATERIALS
4 sterile cotton swabs
1 tube of sterile water
2 marking pens to label plates
4 nutrient agar plates
METHODS/PROCEDURES
- Carefully open the sterile cotton swab without touching the cotton tip.
- If you are sampling a dry surface, moisten the swab slightly by dipping it into the tube of sterile water. This helps lift microorganisms more effectively.
- Rub the swab firmly and thoroughly over a surface area approximately 4 inches by 4 inches.
- Gently lift the lid of your Petri dish just enough to access the agar surface. Rub the swab lightly across the agar in a zigzag or sweeping motion. As you move the swab, gently roll it between your fingers to ensure maximum transfer of any collected microorganisms.
- Label your agar plate according to the instructions provided in Exercise 1. Include your name, the date, the source of the sample, and “Ex. 2.”
- Place the plate in the incubator set to 30°C with the agar side facing up. (You should be able to see your label.)
The following is to be completed during lab then turned in on Canvas as a PDF
*If you are using an iPad or tablet you will need to take screen shots of your competed work, save the screenshots as one PDF then submit them on Canvas by the due date designated on Canvas.
*You can also print out the entire exercise to bring to lab with you. If you choose to complete the lab on paper, take pictures of the completed results and conclusions sections only, save them as one PDF, then submit to Canvas by the due date designated on Canvas.
EXPOSURE PLATES
NAME ______________________
EXPECTATIONS
Talk to your three bench partners and record the sources (yours and theirs) tested below. Which of the four surfaces tested by your group do you predict will have the most microbial growth? Which do you expect will have the least? Explain your reasoning.
RESULTS
Use color pencils to draw and fully label your own plate and those of the three other students in your group. Place all labels below each drawing. Be sure to include the incubation time, temperature for each plate and sample source.
CONCLUSIONS
1. Which plate showed the most microbial growth? Describe the characteristics of the colonies you observed
(e.g., size, shape, color, texture).
2. Were there any plates with very little or no visible growth? What are some possible reasons for this outcome?
3. After examining all four plates, yours and those of your three classmates—did the results match your expectations about where the most microbial growth would occur? If not, what might explain the differences?
4. A single swab may collect many different types of live microorganisms from a surface, but only some will grow into visible colonies on nutrient agar. This is because the medium or growth conditions may not support all types of organisms. For example, bacteria that require high temperatures won’t grow if incubated at room temperature. What is one other reason a live microorganism might not grow into a colony on the plate?
Attributions: title iage sources from: Asawari, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons