Although breathing is essential to life, the specific role that oxygen plays in maintaining life is not easily understood. Basically, in organisms that are able to use it, oxygen allows food molecules to be completely broken down, so that every possible bit of energy is extracted for use in running the cell; cells ranging from those that make up to human body to tiny single-celled bacteria.
Aerobes and Oxygen Detoxification
Organisms that are able to use O2 for metabolism are called aerobes. Oxygen can actually be rather toxic, and for a cell to be able to use molecular oxygen, it must be able to manufacture specific enzymes that detoxify oxygen waste products. Aerobes produce catalase and superoxide dismutase (SOD) for this purpose.
Metabolism of Anaerobic Microbes
There are many types of microbes (bacteria and fungi) that are able live in the absence of oxygen. These organisms either do not have the enzymes required to detoxify oxygen waste, or they are not able to make enough of these enzymes to be able to live at normal levels of atmospheric oxygen. These microbes are called anaerobes. They are still able to break down food molecules in the absence of O2, but cannot do so as efficiently as aerobes.
Although they are not able to get as much energy from their food, being an anaerobe has its advantages. Microbes that do not require oxygen are able to live in places where aerobes cannot survive, such as the human gut, and many other places where oxygen is in low supply. For pathogenic microbes (those that cause disease), this ability is a huge advantage, allowing anaerobic pathogens to cause disease in areas of the body that are not exposed to oxygen.
Microbes and Their Relationship with Oxygen
In microbiology, there are special terms used to describe a microbe’s relationship with oxygen. Here are the main categories:
Obligate Aerobes: These organisms must have oxygen in order to survive. People, and Bacillus bacteria are just two examples of obligate anaerobes.
Obligate Anaerobes: These microbes die in the presence of oxygen. An example would be members of the bacterial genus Clostridium. Although C. tetani (the agent of tetanus), C. botulinum (the agent of botulism), C. perfringens (the agent of gas gangrene) and C. difficile (cause of pseudomembranous colitis) are killed in the presence of oxygen, members of this genus are able to produce endospores; structures that allow endospore-producers to go dormant under adverse conditions.
Facultative Anaerobes: These microbes are able to exist with or without oxygen, but, if given the choice, prefer to have access to oxygen, in order to get the maximal amount of energy from metabolizing their food. Many yeasts, enteric bacteria, such as the Gram-negative Escherichia coli, and skin-dwelling Gram-positive halophiles, like Staphylococcus, are facultative anaerobes.
Microaerophiles: These bacteria require oxygen, but at levels lower than found in the atmosphere. Helicobacter pilori, the cause of stomach ulcers, is a microaerophile.
Aerotolerant Anaerobes: These microbes are not affected by oxygen. They cannot use it, but are not harmed by it. Lactobacillus bacteria that are part of the normal gut flora are aerotolerant.
To learn more about microbiology, see Todar’s Online Book of Bacteriology or the Virtual Microbiology Classroom.
Below is a key to the information relating to the second photo (Bacteria in Liquid Culture and Oxygen) associated with this article. The following can help clarify the different types of relationships that microbes have with oxygen.
Aerobically different bacteria behave differently when grown in liquid culture...
- Obligate aerobic bacteria gather at the top of the test tube to absorb maximum amount of oxygen.
- Obligate anaerobic bacteria gather at the bottom to avoid oxygen.
- Facultative bacteria gather mostly at the top, since aerobic respiration is the most energy efficient; but as lack of oxygen does not hurt them, they can be found all along the test tube.
- Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen but at a lower concentration than found in atmosphere.
- Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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