Obligate Anaerobes: Definition & Examples

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  • 0:01 What Are Obligate Anaerobes?
  • 1:44 Obligate Anaerobe Metabolism
  • 3:21 Examples of Obligate…
  • 5:16 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Oxygen is everywhere...or is it? This lesson will explore some organisms that can die if they are exposed to oxygen - the obligate anaerobes. In addition, it will discuss the different types of metabolisms obligate anaerobes.

What Are Obligate Anaerobes?

You're cleaning your garage when 'ouch!' you step on something. You check your foot and there is a deep puncture wound, so you get a band-aid and continue on with your cleaning. What you don't know is that bacteria, called Clostridium tetani, entered your foot and caused tetanus. These bacteria can be found in animal poop, dust, soils and whatever object you just stepped on. As this bacteria grows inside of you, it produces a powerful toxin that will eventually cause spasms, difficulty swallowing, and if you're really unlucky, respiratory failure and death.

Organisms, like Clostridium tetani, are tiny, fascinating critters that can die if they are exposed to oxygen. You might be thinking, oxygen is everywhere, how in the world can anyone live without oxygen?!.

In addition to the bacteria that causes tetanus, there are many species of obligate anaerobes, or organisms that survive in habitats without oxygen. These organisms are harmed or even killed when exposed to oxygen. Many obligate anaerobes avoid oxygen altogether by living in oxygen-free habitats, such as in the guts of animals, under deep-sea volcanoes, or deep in the mud. When oxygen exposure is unavoidable, some obligate anaerobes can produce a dormant structure called an endospore.

For example, the bacterium that causes tetanus produces an endospore when exposed to oxygen. This tough structure contains the bacteria's DNA. When the endospore ends up in an oxygen-free habitat (like the deep puncture wound on your foot), it germinates and the bacteria's DNA becomes active again. The word 'obligate' means 'restricted to,' and anaerobe is Greek and translates into 'living without air.' Not all obligate anaerobes cause disease; in fact, some are quite helpful!

Obligate Anaerobe Metabolism

Before we delve into examples, let's quickly go over how these obligate anaerobes eat. Most organisms you think of require oxygen for some part of their metabolism, or the chemical reactions that occur inside of living organisms. For example, when you breathe oxygen or eat food (some sort of carbon-containing substance), chemical reactions take place in your body, transforming the oxygen and food into energy. This energy powers everything you do, from watching this lesson to walking to the store.

Since oxygen is toxic to obligate anaerobes, they have to find another way to fuel their metabolism. Let's take a look at some different methods.

  • Anaerobic respiration is when an organism obtains energy from food without using oxygen. Using oxygen is far more efficient, but since it would kill an obligate anaerobe, it is out of the question.
  • Fermentation also does not use oxygen and is another way obligate anaerobes can break down carbon-containing compounds and create energy.
  • Methanogensis occurs when the organism consumes a carbon-containing molecule and produces methane gas as a byproduct. This is seen in methanogens (but we'll get to that later).
  • Anaerobic photosynthesis is when photosynthesis occurs without oxygen. Normally photosynthesis combines water, carbon dioxide and light, transforming them into oxygen and sugars. In anaerobic photosynthesis, oxygen is not produced.

These methods of metabolism are very complicated. For this lesson, you just need to understand that there are several ways obligate anaerobes can get energy without using oxygen. Let's take a closer look at some of specific examples of obligate anaerobes that each use some of the metabolisms listed above.

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