Anaerobic Organism - Definition, Examples and Quiz | Biology Dictionary (2023)

Anaerobic Organism Definition

Anaerobic organisms are those that live an anoxic environment – one which lacks oxygen. While most living things require oxygen to survive – they’re aerobic – oxygen can actually be toxic to anaerobic organisms. The great majority of organisms produce energy molecules called ATP (adenosine triphosphate) through a process of aerobic cellular respiration. This complex set of chemical interactions takes place in the cytoplasm and cell membrane of prokaryotes, and in the mitochondria of eukaryotes. During respiration, oxygen acts as the final electron acceptor at the end of an electron transport chain, which is why aerobic organisms must breathe air containing oxygen in order to survive. However, anaerobic organisms use either fermentation or anaerobic cellular respiration to produce ATP. In this case, an atom other than oxygen is the final electron acceptor. For example, some anaerobic bacteria that live deep in mud in swampy areas use a sulfate ion instead of oxygen, and hydrogen sulfide is produced as a byproduct, rather than water. This explains the sulfurous smell in many swamps and mudflats.

Two Types of Anaerobes

There are two main types of anaerobes: facultative and obligate. Facultative anaerobes can live with or without oxygen. When oxygen is present in their environment, they use aerobic cellular respiration to produce energy in the form of ATP. If oxygen becomes depleted, they can switch to anaerobic respiration or fermentation. In contrast, obligate anaerobes must live without oxygen. They are only equipped to undergo anaerobic respiration or fermentation, and the presence of oxygen kills them.

Facultative Anaerobes

Human muscle cells are facultative anaerobes. During exercise in which a person gets plenty of oxygen to their muscles, like distance running, the cells undergo aerobic respiration. But during intense exercise such as sprinting, in which the body’s oxygen needs outstrip the lungs’ ability to provide it, muscle cells will switch to lactic acid fermentation. This process is much less efficient than aerobic respiration and produces lactic acid as a byproduct, which builds up in the muscles and causes the burning sensation commonly felt during strenuous exercise. Because this is so much less efficient, a person can only do such intense activity for a very short period of time before “hitting the wall” and having to stop.

Another familiar facultative anaerobe is the bacterium Escherichia coli. While E.coli has had a bad rap in the press due to incidents of food poisoning, E.coli are actually very important and beneficial residents of the human gastrointestinal tract. They aid in digestion of food and absorption of necessary vitamins, as well as protection from potentially harmful infections. These bacteria can easily function with or without oxygen, which makes them highly adaptable to different environments. In the anaerobic intestine, they use fermentation to produce energy. If found in the oxygen-rich environment outside the gut, they switch to aerobic respiration.

Other Examples of Facultative Anaerobes

  • Staphylococcus aureus: Causes staph infections. Methicillin-resistant S. aureus is responsible for MRSA.
  • Lactococcus lactis: Its lactic acid fermentation is used in processing many types of cheese.

Obligate Anaerobes

One infamous example of an obligate anaerobe is Clostridium botulinum. This common bacterium produces a potent neurotoxin that can be fatal in even small amounts. It is found growing in items such as home-canned products, baked potatoes wrapped in aluminum foil, and honey. Under poor survival conditions, C. botulinum produces spores with a tough coat that allows them to survive for years. When conditions improve, the bacteria begin to grow and produce potentially lethal toxins. If a person consumes food contaminated with actively growing C. botulinum they are likely to succumb to a deadly food poisoning called botulism, the early symptoms of which are nausea, vomiting, and weakness. Then come the neurological effects: blurred vision, difficulty speaking and swallowing, and impaired muscle control, followed by difficulty in breathing and possibly death by asphyxia. Infantile botulism occurs after a baby ingests C. botulinum spores, which may be found in soil, dust, or honey. This is why young babies should never be given honey; before one year of age, their immune system isn’t strong enough to handle the spores, so they begin to grow and cause severe illness.

Possibly the largest aggregation of obligate anaerobes on the planet is found on the deep-sea floor, where they populate hydrothermal vents. These underwater hot springs erupting from the Earth’s crust are laden with minerals, which the bacteria use to energize their process of chemosynthesis, thereby building organic molecules. First discovered in 1977 by researchers off the Galapagos Islands, their existence re-wrote all biology textbooks. Before then, it was thought that photosynthesis was the only means by which autotrophic organisms could convert energy into food for themselves. Bob Ballard, the deep-sea explorer who discovered the wreck of the Titanic, was on the Alvin submersible the day it went down to film the vents. He later said that the discovery of chemosynthesis in vent bacteria was one of the biggest biological discoveries of the 20th century – far more important than any historical wreck. Evolutionists speculate that life began on the deep-sea floor, energized by chemosynthesis.

Other Examples of Obligate Anaerobes

  • Clostridium tetani: Causes tetanus
  • Chlorobium, Chloroflexus and several other species contribute to the prismatic colors of Yellowstone National Park’s hot springs

Anaerobes: Friend or Foe?

It is clear that our planet is well-populated with diverse anaerobic organisms. Some are pathogenic, causing severe infections such as MRSA, botulism, and tetanus. Others are beneficial, adding beauty to hot springs, flavoring cheeses, and shaping the communities of the ocean. For others, like E. coli, their status depends on their location: while E. coli is a necessary, helpful resident of the human gut, it can become pathogenic if ingested orally or some other way. In summary, anaerobes are important residents of Earth which brilliantly fulfill their ecological niches.


1. Which of the following metabolic pathways requires oxygen?
A. Aerobic cellular respiration.
B. Anaerobic cellular respiration.
C. Lactic acid fermentation.
D. Alcoholic fermentation.

Answer to Question #1

A is correct. The term “aerobic” refers to oxygen. The other three processes all require the absence of oxygen in order to occur.

2. Which of the following is not a facultative anaerobe?
A. Escherichia coli
B. Staphylococcus aureus
C. Clostridium botulinum
D. Human muscle cell

Answer to Question #2

B is correct. C. botulinum is the obligate anaerobe responsible for botulism food poisoning. E. coli, S. aureus, and human muscle cells are all facultative anaerobes, able to switch back and forth between aerobic and anaerobic respiration, depending on their environment.

3. Where is the largest aggregation of obligate anaerobes found on the planet?
A. Swamps and marshes
B. Temperate and tropical rainforests
C. The deep-sea floor
D. Grassland and agricultural soils

Answer to Question #3

C is correct. The world’s largest population of obligate anaerobes inhabits hydrothermal vents found along undersea ridges, which stretch for 40,000 miles along the edges of Earth’s tectonic plates.


  • Arnold, P. (2009). Examples of Anaerobic Bacteria. Retrieved from
  • Reece, J. B., & Campbell, N. A. (2011). Campbell Biology. Boston: Benjamin Cummings / Pearson.
  • Clostridium botulinum. (2013). United States Department of Agriculture, Food Inspection and Safety Service. Retrieved from
  • Hoecker, J. (2015). How can I protect my baby from infant botulism? The Mayo Clinic. Retrieved from
  • Taylor, A. (2016). What Makes Yellowstone’s Hot Springs So Colorful? Retrieved from
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