Which Types of Bacteria Thrive in Environments That Lack Free Oxygen?
Though animals require oxygen to survive, there are bacteria that can live without it. In fact, many bacteria are harmed by the presence of oxygen. Bacteria that do not need oxygen are called anaerobic bacteria. Anaerobic bacteria are ancient organisms, older than organisms requiring oxygen. Some anaerobic bacteria live in the guts of animals, which is an arrangement beneficial to both parties. If they escape the gut, however, and enter other tissues it is very dangerous for the host.
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Facts
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One way of categorizing bacteria is by what they use as an electron acceptor when they break down carbon compounds. This breakdown of carbon compounds produces energy in a form usable for the cell. Organisms that breathe oxygen use it as the electron acceptor in aerobic respiration. Anaerobic bacteria use a variety of compounds including nitrate, fumarate and sulphur. The two methods for breaking down carbon compounds, and sometimes inorganic compounds, without oxygen are called fermentation and anaerobic respiration.
Types
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Several types of bacteria can grow without oxygen. First are the obligate anaerobes, which cannot use oxygen as an electron acceptor. Oxygen is toxic to obligate anaerobes. Second are the aerotolerant anaerobes, which cannot use oxygen as an electron acceptor, but are unharmed by it. Finally are facultative anaerobes, which prefer to use oxygen as an electron acceptor, but can live without it. In the absence of oxygen, they use fermentation or anaerobic respiration for energy.
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History
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Oxygen makes up 20 percent of the earth's atmosphere today. It is hard to imagine how anything could evolve that finds oxygen toxic in these conditions. But anaerobic bacteria evolved before the atmosphere contained very much oxygen. Oxygen in the atmosphere started to rise as bacteria that could photosynthesize sugar compounds using the sun's energy released the oxygen as a waste compound. Organisms that need oxygen, from aerobic bacteria to animals, didn't evolve until the oxygen in the atmosphere increased.
Benefits
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Anaerobic bacteria have a beneficial relationship with animals. In most animals, the gut is home to many anaerobic bacteria. These bacteria break down nutrients it is difficult for us to digest, making it easier for the host to absorb the nutrients. The bacteria get a constant source of food and an environment in which they can thrive. Antibiotics can kill these beneficial bacteria, which is why doctors recommend food like yogurt to help replace them once treatment is complete.
Warning
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While the presence of anaerobic bacteria in the gut is generally very useful, it is very dangerous if a tear forms in the wall of the gut. The bacteria that are beneficial in the gut can cause dangerous infections elsewhere in the body, like in the abdominal cavity. They can cause an infection of the lining of the abdominal cavity called infective peritonitis. Peritonitis can be life-threatening and needs to be treated immediately.
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References
Resources
- Photo Credit Heading image: E. coli Dwaipayanc: wikimedia commons (Public Domain), Section 2 image: Bacterial Growth Patterns, Pixie: wikimedia commons (Public Domain).
