Anaerobes – Types of Bacteria, Classification, And Examples

Bacteria have been classified into three different types. These types include anaerobes, facultative, and aerobic bacteria.

Anaerobes - Types of Bacteria, Classification, And Examples

These classifications have been made based on the reactions the bacteria undergo when they generate energy in order to grow or for other activities. 

If you are not familiar with bacteria, they are tiny single-celled organisms.

The interesting thing about bacteria is that they can be found almost everywhere around the world. In fact, human bodies are full of bacteria. 

As mentioned before, one of the classifications of bacteria is anaerobes. In this guide, we are going to walk you through everything you need to know about anaerobic bacteria. 

What Are Bacteria? 

Bacteria are microscopic organisms that have a simple cell structure. Bacteria are often found together in communities made up of millions of bacteria. 

As mentioned before, bacteria have a very simple and specific cell structure.

They are different from the cells found in plants and animals as bacteria are cells that have no nucleus.

A nucleus is usually the part of the cell that contains the chromosomes and also controls the cell. 

So, what is the structure of bacteria? Well, some bacteria cells have a capsule which is a layer around the cell wall on the outside.

Bacteria cells have a cell wall that gives the bacteria cell its shape. Within the cell walls of bacteria cells, there is a plasma membrane.

This plasma membrane allows the bacteria to generate energy and substances can pass through the plasma membrane. 

In the plasma membrane, there is cytoplasm. The cytoplasm contains the genetic material and ribosomes.

Protein is made in the ribosomes. Bacteria cells also have DNA which is found in the cytoplasm. 

Bacteria cells also have flagellum which is what is used for movement. On the outside of bacteria cells, there are pili.

Pili are hairs that allow bacteria to stick to the exterior of things and transfer genetic material. A fun fact about pili is that they can help bacteria to spread illnesses. 

What Are Anaerobes? 

Anaerobes are microorganisms that do not need oxygen in order to survive. Organisms that require oxygen to grow are called aerobes.

Anaerobes are able to use other substances to help them produce energy to grow such as sulfate. 

When it comes to working with anaerobes, it is important to minimize or cut out any exposure to oxygen.

This is due to the fact that some anaerobic organisms only survive when there is no oxygen present.

With these anaerobic organisms, if they were to be exposed to oxygen, there is the possibility that they would die. 

There are some anaerobes that can tolerate oxygen for a short amount of time. 

Examples Of Anaerobes

As mentioned earlier, there are many different examples of anaerobic organisms. Here are some of these examples. 


Clostridium is a genus group of bacteria that is rod-shaped. There are many different species of Clostridium.

Only a small number of these species are pathogenic and some of those that are non-pathogenic, are beneficial. 

Clostridium can be found in soil, water, and the intestinal tracts of animals and humans.

Some of the species of Clostridium are very strict anaerobes, meaning they cannot survive in a normal oxygen environment. 


Bacteroides are a genus of Gram-negative bacteria. They are obligate anaerobic bacteria. There have been more than thirty species of Bacteroides recognized.

There are three main groups of Bacteroides that have been classified. 


Prevotella are species that are related to colonic Bacteroides. Prevotella are Gram-negative rods that are often associated with human oral colonization.

Prevotella are often recovered after respiratory tract infections. 

Anaerobic Bacteria

Anaerobic bacteria refers to bacteria that do not need oxygen for respiration.

Although, some anaerobic bacteria can tolerate oxygen and some are even able to oxygen for respiration.

Then there are some anaerobic bacteria where oxygen is essentially poisonous to them and can kill them. 

Classification Of Anaerobes

We have previously discussed that there are some anaerobes that cannot be exposed to oxygen and some that are able to tolerate oxygen.

This is why there are different classifications of anaerobic organisms. 

In fact, there are two main types of anaerobic organisms. These two classifications are facultative and obligate.

Facultative anaerobes are able to tolerate and use oxygen, whereas obligate anaerobes need to live without oxygen. 

There is also another classification of anaerobic organisms that we will cover, these are aerotolerant anaerobes. 


When anaerobic bacteria are classified as facultative, it means that this type of bacteria is able to grow well when in an environment with oxygen, but they are also able to grow without oxygen present. 

An example of anaerobes that are facultative would be human muscle cells.

There are times in the human body when muscle cells are provided with a lot of oxygen which they use for aerobic respiration. 

However, there are times when the muscles in the human body do not get enough oxygen such as during intense exercise.

During exercise, there can be times when the body’s need for oxygen cannot be met by the lungs. At this point, the muscle cells will turn to lactic acid fermentation.

Lactic acid fermentation is not as efficient and produces lactic acid which makes our muscles cramp. 

There are also bacteria that can be classed as facultative anaerobic bacteria. An example of this would be Escherichia coli, more commonly known as E.coli.

When E.coli is not in an environment where oxygen is present, it needs to use a different process in order to make energy.

For example, when E.coli is in the intestines they use fermentation to make energy. 

E.coli has a bad reputation for being associated with food poisoning. However, E.coli is really important for our gut health.

E.coli bacteria help with digestion, they help provide protection from infections, and they help with absorbing vitamins. 



When anaerobic bacteria are classified as obligate, it means that these types of bacteria live in environments where there is no oxygen present.

Obligate anaerobic bacteria cannot survive in environments where there is oxygen.

This is because obligate anaerobic bacteria do not have the defenses to protect themselves from oxygen.

Lacking the defenses to help them with oxygen and not being able to survive in an environment with oxygen is also known as oxygen toxicity. 

An example of obligate anaerobic bacteria would be Clostridium. As mentioned earlier there are multiple species of Clostridium.

Two obligate species of Clostridium are Clostridium tetani and Clostridium botulinum. Clostridium botulinum is a bacteria that produces dangerous toxins which can be fatal. 

Clostridium botulinum is often found in canned food. Clostridium botulinum is able to survive in bad conditions by producing spores that have the ability to survive for many years.

When these bacteria find themselves in better conditions it then produces toxins as the bacteria start to grow.  

As mentioned before, Clostridium botulinum can be fatal. If a person consumes these bacteria they can develop botulism.

Botulism is a life-threatening condition that is caused by this bacteria. Clostridium botulinum produces one of the most powerful toxins that science currently knows of.

Symptoms of botulism include vomiting, diarrhea, constipation, and stomach cramps. If botulism is left untreated it can lead to more severe symptoms such as paralysis. 

Many obligate anaerobes are found on the deep sea bed. This is because the oxygen levels at the bottom of the deep sea are very low.

Some obligate anaerobes are able to survive being exposed to the smallest amount of oxygen. 

Some notable and defining characteristics of obligate anaerobes are that they are not able to carry out oxidative phosphorylation.

They lack the enzymes that can convert molecules to produce oxygen. They get their energy from glycolysis. 

Glycolysis is the first stage of metabolism. Metabolism is the chemical reaction that takes place, leading to the production of energy.

Glycolysis is where simple sugars are broken down into pyruvate. When there is a lack of oxygen, pyruvate is broken down further by enzymes.

At the end of this process, pyruvate is converted to lactate which leads to the formation of lactic acid.

This is what facultative bacteria go through when there is no oxygen. However, compared to aerobic respiration, this method is less efficient. 


Aerotolerant anaerobic bacteria are able to tolerate oxygen for a short period of time. This time period is usually between 8 hours to 72 hours.

Although aerotolerant anaerobic bacteria can tolerate oxygen for a small amount of time, they are not able to grow in the presence of oxygen.

Aerotolerant anaerobic bacteria are not able to carry out metabolism processes when they are in contact with oxygen. 

Aerotolerant anaerobic bacteria are similar to obligate anaerobic bacteria as they are only able to use fermentation to produce energy.

An example of aerotolerant anaerobic bacteria is Streptococcus. 

Aerotolerant anaerobes are able to survive being exposed to oxygen as they have defenses they can use.

For example, aerotolerant anaerobes are able to produce enzymes that help to protect them from the destructive effects of oxygen.

They use the enzymes to convert the oxygen into other molecules which they can destroy with other enzymes. 

Culturing Anaerobes

Culturing anaerobic bacteria is expensive and can be quite difficult to do so.

This is due to the fact that in order to culture these bacteria you have to remove oxygen from the environment. 

One method of culturing anaerobes is to fill a vessel with a liquid medium. This is often described as the Hungate technique.

When you have successfully cultured the bacteria, there are a number of notable differences compared to cultures of aerobic bacteria. Here are some of the differences. 

Obligate aerobic bacteria, when cultured, will grow at the top of the vessel.

These types of bacteria grow at the top of the test tube vessels as a way to get the maximum amount of oxygen they can possibly get.

In comparison, obligate anaerobic bacteria will grow at the very bottom of the test tube vessel.

This is so the obligate anaerobic bacteria can avoid as much oxygen as they can. 

When culturing facultative bacteria, you should expect to find them at the top of the test tube vessel.

This is due to the fact that oxygen is beneficial to them for aerobic respiration, as it allows them to produce energy.

However, what makes culturing facultative bacteria different from obligate aerobic bacteria is that you can find facultative bacteria spread all over the test tube vessel.

This includes finding them at the bottom of the test tube. This result is due to the fact that a lack of oxygen does not harm the facultative bacteria. 

Aerotolerant anaerobic bacteria are often found all over the test tube vessel. This is because they are not affected by the concentration of oxygen in the test tube vessel. 

Final Thoughts

To conclude, anaerobes are microorganisms that do not require oxygen in order to survive and thrive.

However, there are different classifications of anaerobes that help us to determine how they react in the presence of oxygen.

The three classifications of anaerobes are facultative, obligate, and aerotolerant. 

Facultative anaerobic bacteria are defined as bacteria that can grow in the presence of oxygen and without it.

They use both aerobic respiration and fermentation to produce energy, depending on whether they are exposed to oxygen or not. 

Obligate anaerobic bacteria are bacteria that absolutely cannot grow in the presence of oxygen.

In fact, for some types of obligate anaerobic bacteria, being in the presence of oxygen can be fatal. Obligate anaerobic bacteria use fermentation to produce energy. 

Lastly, aerotolerant bacteria are able to tolerate oxygen for a short period of time.

However, they are not able to grow when they are exposed to oxygen. Aerotolerant bacteria also use fermentation to produce energy. 

We hope that this guide on anaerobes has helped you to understand all you need to know about anaerobes, including classifying them and some examples of each type.

Please share this guide with others who may benefit from reading this. Thank you for reading. 

Jennifer Dawkins