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Facultative Anaerobe Definition
Facultative organisms are defined as one of the most adaptable organisms. The organisms can survive in the present as well as the absence of molecular oxygen and are thus considered as one of the most adaptable organisms. Generally, some bacteria and archaea species can generate ATP in the presence of oxygen through aerobic respiration however they can also derive energy in absence of oxygen from anaerobic respiration or fermentation. Some eukaryotes are also able to survive in anaerobic respiration.
What is Facultative Anaerobe?
The facultative anaerobes can survive in oxygen-rich as well as oxygen-deficient conditions. They can switch their respiratory mode between aerobic and anaerobic respiration according to oxygen availability. Facultative anaerobes can survive in extreme environmental conditions by adopting alternate electron acceptors in the electron transport chain. The electron transport chain is the basic chemical reaction that generates ATP during cellular respiration and photosynthesis. The elements such as nitrate, nitrite, fumarate, elemental sulfur, metal ions are utilized as electron acceptors by facultative anaerobes in the absence of oxygen. Therefore they can easily survive in deoxygenated environments also.
Here, the question arises that in which condition they can grow better or which condition do they prefer? It was observed that based on ATP yield, aerobic conditions are much preferable by facultative anaerobes. The total ATP yield in aerobic respiration is 36/38 ATP molecules whereas anaerobic respiration generates 2 ATP molecules in the fermentation process.
Let’s understand the basics of the electron transport chain to understand the transition of facultative anaerobes to aerobic respiration. The electron transport chain is a bioenergetic cycle that generates proton motive force (pmf) across the membrane. The force generates due to the reduction of the electron acceptor substrate that results in ATP generation. The facultative anaerobes use several alternative pathways for pmf generation, that includes-
• Nitrate Respiration– When nitrate is used in place of oxygen as an electron acceptor, the process is named nitrate respiration. Here, the nitrate is converted into molecular nitrogen by the process of denitrification or by ammonification, it is converted into ammonia.
• Fumarate Respiration– It is a form of respiration where succinate is formed by quinol oxidation. This respiration mode is commonly found in E.coli.Sulfur respiration- The respiration method use polysulfide form of the sulfur that is reduced by the membrane-bound polysulfide reductase. The sulfur respiration can be seen in Desulfuromonas acetoxidans and W. succinogenes.
• Electron Transport to Oxidized Metal Ions– Some bacterial species such as Geobacter and Shewanella spp. Utilize oxidized metal ions as electron acceptors. E.g. Fe3+, Mn4+ is used in the electron transport chain that forms ferromagnetic mineral.
The facultative anaerobes have an enzyme named superoxide dismutase or catalase that eliminates the harmful superoxide anions. The enzyme converts the anions into ground-state oxygen along with hydrogen peroxide. The chemical reaction for the process is-
2O2– + 2H+ ⇒ O2 + H2O2
Hydrogen peroxide is an oxidizing agent that tends to diffuse out from the cell. The hydrogen peroxide is eliminated from the cell by catalase enzymes and convert into the water and molecular oxygen. Bacteria are the most common facultative anaerobes. Examples include E. coli, Pseudomonas aeruginosa, Staphylococcus spp. Salmonella, Shewanella oneidensis, and Yersinia pestis. Based upon the physical environmental conditions of these organisms, they have been classified into different categories.
Types of Organisms: Based on Energy Requirements
Based on the oxygen requirement the organisms can be classified into the following types:
Obligate Aerobe: The organisms that require molecular oxygen for their survival and growth are considered obligate aerobes. Obligate aerobes use oxygen as a final electron acceptor and utilize it to generate energy. Examples of these organisms include Mycobacterium tuberculosis, Nocardia asteroids, etc. The organisms are classified as strict, moderate, and Aerotolerant based on the amount of oxygen they require for survival.
Obligate Anaerobes (occasionally called aerophones): Obligate anaerobes do not require molecular oxygen for respiration. O2 can kill or cause complete inhibition in these organisms thus molecular oxygen is toxic for anaerobes. They adopt some other methods such as fermentation, anaerobic respiration, methanogenesis, or bacterial photosynthesis to derive energy for their survival. Examples include Actinomyces, Bacteroides, and Clostridium, etc.
Facultative Anaerobes: Facultative anaerobes are those organisms that can survive easily in both oxygens-rich as well as oxygen deficient environments. They can easily switch their respiratory mode from anaerobic to aerobic respiration in presence of molecular oxygen similarly adopt an anaerobic mode in absence of oxygen. They derive energy by fermentation or anaerobic respiration in the deoxygenated environment. Examples of facultative anaerobes are E.coli, Pseudomonas aeruginosa, Staphylococcus spp., Listeria spp., Salmonella, etc.
Aerotolerant Anaerobes: The organisms do not require molecular oxygen for their survival however the presence of oxygen does not show any toxic effect in these anaerobes. Thus, they are considered tolerant or insensitive to molecular oxygen. The energy is derived from fermentation in such organisms. Examples are Campylobacter jejuni, lactobacilli, and streptococci.
Microaerophile: Microaerophiles require a low amount of molecular oxygen thus they are present in such an environment where the concentration of oxygen is low (below 21%). However they require oxygen for their survival in low amounts, they are differentiated from anaerobes. Examples of microaerophiles include Actinomyces, Clostridium, Propionibacterium, Bifidobacterium, Bacteroides, and Prevotella.
Thioglycolate Tube Culture Identification Test
This test is performed to identify different kinds of bacteria, depending upon their oxygen requirement. In this test, the bacteria are grown in thioglycolate culture media in vitro test tubes and observe their behavior under an oxygenated and deoxygenated environment respectively.
The low amount of agar in the thioglycolate culture medium permits the complete motility of the test bacterium. The culture media is properly sterilized that remove oxygen from it to observe the bacteria in oxygen-deficient conditions. After inoculation, an optimum temperature is provided and maintained to culture test tubes to grow properly.
After some time, the culture media get oxygen by diffusion and the bacteria occupy their desirable positions in the culture media according to their oxygen requirement. The facultative anaerobes are distributed throughout the culture media due to their ability to survive in both oxygen-rich and oxygen deficient conditions.
The aerobes have a high density in the mouth of the culture media whereas the bottom has a high density of anaerobes due to the absence of oxygen. The aerotolerant bacteria are also present throughout the culture media as they are not affected by the presence of oxygen.
Facultative Anaerobes vs Obligate Anaerobes
| Obligate Anaerobes | Facultative Anaerobes |
| They do not require molecular oxygen for their survival. The word obligate means necessarily | They can easily survive in both oxygen rich as well as oxygen deficient conditions. They can switch their respiratory mode |
| In presence of oxygen, they cannot survive, i.e., oxygen is toxic for obligate anaerobes | They can survive in presence of molecular oxygen as well as in absence of oxygen. Oxygen is not toxic for these organisms |
| They derive energy by fermentation or anaerobic respiration | They perform aerobic, anaerobic respiration and fermentation to derive energy |
| They accumulate in bottom of the test tube in the culture media | They are present throughout the test tube |
| Examples- Actinomyces, Bacteroides, Clostridium, Fusobacterium, Peptostreptococcus, Prevotella | Examples – Streptococcus spp., Staphylococus spp., Salmonella, Listeria, Corynebacterium, Shewanella oneidensis |
Facultative Anaerobe Examples
The common examples of facultative anaerobe include-
• E. coli is a bacteria that commonly resides in the large intestine of vertebrates including humans.
• Yeast is also a facultative anaerobe that is used in the alcohol industry and many other industries to perform fermentation.
• B. anthracosis is an exotoxin that causes various diseases such as respiratory failure, anoxia, and sometimes death.
• Some multicellular organisms also act as facultative anaerobes such as Mussels.
Ecological Importance of Facultative Anaerobes
The organisms utilize gases such as nitrogen, sulfur, and iron and metabolize them. Anaerobic bacteria play an important role in biogeochemical cycles that led to the evolution of life on earth. The organisms are also involved in the preservation of the essential elements of life, i.e., carbon, nitrogen, and sulfur. The autotrophic anaerobes use carbon dioxide and reduce the amount of CO2 in the environment. It also conserves the waste gases to chemical forms such as biofuels.
Yeast and some other anaerobes are also useful in the industrial field. It is widely used in the alcohol industry and bakeries to carry out fermentation. However, some species of facultative anaerobes also cause infection and diseases in humans as well as in other organisms, they are called pathogens. E. coli and streptococcus are common examples of pathogens.
Facultative Anaerobe Citations
- Multi-omics of the gut microbial ecosystem in inflammatory bowel diseases. Nature . 2019 May;569(7758):655-662.
- Mechanisms and Targeted Therapies for Pseudomonas aeruginosa Lung Infection. Am J Respir Crit Care Med . 2018 Mar 15;197(6):708-727.
- Colonocyte metabolism shapes the gut microbiota. Science . 2018 Nov 30;362(6418):eaat9076.
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