26: ANAEROBIC GROWTH
- Page ID
- 157094
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Correctly set up and handle both aerobic and anaerobic culture systems.
- Inoculate and incubate bacteria under conditions that test their oxygen requirements.
- Interpret experimental data to classify organisms by oxygen requirement.
BACKGROUND
Microorganisms are found in nearly every environment on Earth, and their ability to grow often depends on the presence or absence of oxygen. These differences reflect how they generate energy and what type of metabolism they use.
Some bacteria grow only when oxygen is present. These are known as strict aerobes. They rely on aerobic respiration, where oxygen serves as the final electron acceptor in the electron transport chain. These organisms cannot ferment sugar and will not grow in oxygen-free environments. Strict aerobes are commonly found on skin, exposed surfaces, dust, and in the upper layers of soil—places where oxygen is readily available.
Other bacteria grow only when oxygen is absent. These are called strict anaerobes. They generate energy exclusively through fermentation and are often harmed or even killed by exposure to oxygen. Strict anaerobes thrive in environments such as deep soil, sediments, swamps, marshes, sewage, the gastrointestinal tract of animals, and deep tissue wounds. A related group, aerotolerant anaerobes, also use fermentation but are not damaged by oxygen exposure.
Some bacteria are more flexible. Facultative anaerobes can switch between aerobic respiration and fermentation depending on whether oxygen is available. When oxygen is present, they use it to carry out respiration, which allows them to extract more energy from their nutrients. When oxygen is not available, they switch to fermentation. Because fermentation is less efficient, organisms growing anaerobically typically grow more slowly and to lower population densities than they would in oxygen-rich conditions.
In a clinical setting, several important human pathogens are anaerobes. For example, Clostridium tetani (the causative agent of tetanus), Clostridium botulinum (which causes botulism), and Clostridium perfringens (a common cause of gas gangrene and foodborne illness) are all strict anaerobes. These organisms can flourish in deep wounds or tissues where oxygen supply is limited. Other anaerobic pathogens are part of the body’s normal microbiota, particularly in the intestines or oral cavity, and may only cause disease when they enter normally sterile areas of the body or when the balance of normal microbes is disrupted.
Growing anaerobic organisms in the lab can be challenging, especially for strict anaerobes, because most lab procedures expose cultures to air. Even brief contact with oxygen may inhibit their growth or kill them. In research and diagnostic laboratories, anaerobic organisms are cultured using special oxygen-free environments, including sealed anaerobe jars, anaerobic glove boxes, or systems that use gas-generating chemical packets to remove oxygen. Some specialized liquid media, such as thioglycolate broth, contain reducing agents that bind oxygen and allow anaerobes to grow in deeper parts of the tube while limiting oxygen diffusion.
Understanding how microorganisms respond to oxygen is important not only for culturing bacteria in the lab but also for diagnosing infections, choosing the correct antibiotics, and predicting where pathogens may be found in the body or in the environment.
In this lab, you will investigate how different bacteria respond to the presence or absence of oxygen. You will work with three organisms: one that requires oxygen to grow (aerobe), one that is killed or inhibited by oxygen (anaerobe), and one that can grow with or without oxygen (facultative anaerobe). By inoculating all three onto both an aerobic and an anaerobic plate, you will observe how each organism grows under different conditions. This experiment will help you understand the connection between oxygen availability, microbial metabolism, and the classification of bacteria based on their oxygen requirements.
1.21: Bacterial Oxygen Requirements is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.
MATERIALS
1 Anaerobic culture
1 Aerobic culture
1 Facultative culture
2 Nutrient agar plates
METHODS/PROCEDURES
1. Label your agar plate with your name, date, and the condition you are assigned (aerobic or anaerobic).
2. Divide the plate into three sections.
3. Spot inoculate one culture into each of three separate sections on the plate. Only one organism should be placed in each section, but all three
organisms will be included on the same plate. Do not streak. Keep each spot of culture well within its own section to avoid mixing the organisms.
Cross-contamination will interfere with your results.
4. After inoculation, place the plate labeled “aerobic” directly into the incubator.
5. Place the “anaerobic” plate into an anaerobe jar or box with a gas-generating packet, then seal the container properly and incubate.
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Exercise #31 Bacterial Oxygen Requirements
NAME ______________________
EXPECTATIONS
Do you think the facultative organism will look the same on both plates? Why or why not?
RESULTS
Draw your observations of the cultures on both the aerobic and anaerobic plates after incubation. Note any differences in growth, such as colony size, density, and pigmentation. Be sure to record any distinctive differences between the organisms under each condition.
|
Organism |
Aerobic Plate |
Anaerobic Plate |
Oxygen Requirement |
CONCLUSIONS
1. What additional tests or media could you use to confirm the oxygen requirement of an unknown organism?
2. Why might facultative anaerobes be more commonly isolated from infections than strict aerobes or
anaerobes?
3 Which of the three organisms texted do you think would be most likely to thrive in a deep puncture wound?
Why?
4. In cases of suspected tetanus or botulism, what clues about the infection site or patient history might alert a
clinician to suspect an anaerobic pathogen?