Main Difference – Facultative vs Obligate
In ecology, facultative and obligate are two terms used to describe organisms based on the mechanism of obtaining energy by each organism. Generally, organisms produce energy through cellular respiration. The three main types of cellular respiration are aerobic respiration, fermentation, and anaerobic respiration. The organisms that use either facultative or obligate methods during respiration can be bacteria, fungi, or endoparasites such as protozoans and worms. The main difference between facultative and obligate is that facultative organisms obtain energy from aerobic respiration, anaerobic respiration, and fermentation whereas obligate organisms obtain energy from aerobic respiration, anaerobic respiration or fermentation.
Key Areas Covered
1. What is Facultative
– Definition, Types of Respiration, Types of Organisms
2. What is Obligate
– Definition, Types of Respiration, Types of Organisms
3. What are the Similarities Between Facultative and Obligate
– Outline of Common Features
4. What is the Difference Between Facultative and Obligate
– Comparison of Key Differences
Key Terms: Aerobic Respiration, Anaerobic Respiration, Endoparasites, Enterobacteriaceae, Facultative, Facultative Anaerobes, Facultative Fungi, Facultative Parasites, Fermentation, Obligate, Obligate Aerobes, Obligate Anaerobes, Obligate Fungi, Obligatory Parasites, Pasteurellaceae, Vibrionaceae
What is Facultative
Facultative refers to the ability to live under more than one specific environmental conditions. The three types of facultative organisms are bacteria, fungi, and endoparasites such as protozoans and nematodes.
Facultative Bacteria (Facultative Anaerobes)
The facultative bacteria are known as facultative anaerobes. Facultative anaerobes can grow without oxygen. But, they are capable of using oxygen, if it is available in the medium to generate more energy than in usual anaerobic respiration. On that account, faculatative anaerobes can use all three cellular respiration methods: aerobic respiration, anaerobic respiration, and fermentation. The three important families of facultative anaerobic bacteria are Enterobacteriaceae, Vibrionaceae, and Pasteurellaceae. Enterobacteriaceae are ubiquitous bacteria that live in soil, water, and vegetation as opportunistic normal flora (E.coli, Klebsiella pneumoniae, Proteus mirabilis), and as pathogens (Shigella, Salmonella, and Yersinia pestis). Vibrionaceae consists of catalase and oxidase enzymes to detoxify oxygen. It can be found in the external environment as well as the intestinal tract of animals. Vibrio, Aeromonas, Photobacterium are examples of Vibrionaceae. Pasteurella and Haemophilus are the two types of Pasteurellaceae. Pasteurella is pathogenic the domestic animals while Haemophilus lives in the mucous membranes of animals. Klebsiella pneumonia interacting with a human neutrophil is shown in figure 1.
Generally, fungi are saprophytes that live on dead or decaying organic matter. But some fungi are capable of entering living organisms, causing diseases to the host. Apple blotch, peach, black spot, and Panax leaf spot are some of the diseases caused by facultative fungi in plants. Facultative fungi such as candida cause vaginal cadidosis and athlete foot in humans.
Generally, facultative parasites live independent of a host. But occasionally, they become parasites. Amoeba-like protozoans and some nematodes such as Strongyloides spp are facultative parasites.
What is Obligate
Obligate means be restricted to a particular characteristic. Obligate organisms such as bacteria, fungi, algae and endoparasites can be identified in nature. Most algae are obligate aerobes.
Obligate Bacteria (Obligate Aerobes and Obligate Anaerobes)
Obligate bacteria can be categorized into two groups based on the type of cellular respiration they use to obtain energy. They are obligate aerobes and obligate anaerobes. The obligate aerobes use oxygen to oxidize sugars and fats to generate energy during cellular respiration. Thus, they use aerobic respiration. Aerobic bacteria live in the external environment where they can obtain oxygen. On the contrary, obligate anaerobes are incapable of detoxifying oxygen. So, they live in oxygen-free environments, using fermentation or anaerobic respiration to produce energy. Mycobacterium tuberculosis and Nocardia asteroides are examples of obligate aerobes while Actinomyces and Clostridium are examples of obligate bacteria. The growth of obligate aerobes and obligate anaerobes in a liquid medium is shown in figure 3.
Obligate fungi can also be categorized as obligate aerobic fungi and obligate anaerobic fungi. Most of the fungi are obligate aerobic fungi such as yeast. The fungi that live inside ruminant digestive systems such as Neocallimastix, Piromonas, and Sphaeromonas are obligate anaerobes. An obligate aerobic fungi is shown in figure 4.
Obligatory parasites can only survive inside a host. Thus, their life cycle is involved in moving from one host to another. Most obligate parasites use two hosts called the definitive host and intermediate host. Flatworms, roundworms, and pinworms are obligatory parasites that live inside the gastrointestinal tract, blood, and lymphatic system. Protozoans such as Plasmodium are also anaerobic obligatory parasites.
Similarities Between Facultative and Obligate
- Facultative and obligate are two types of organisms categorized based on the types of cellular respiration the used to obtain energy.
- The three types of cellular respiration methods used by both facultative and obligate are aerobic respiration, anaerobic respiration, and fermentation.
- Both facultative and obligate can be bacteria, fungi or endoparasites.
Difference Between Facultative and Obligate
Facultative: Facultative refers to the ability to live under more than one specific environmental condition.
Obligate: Obligate means to be restricted to a particular characteristic.
Types of Cellular Respiration
Facultative: Facultative organisms obtain energy from aerobic respiration, anaerobic respiration, and fermentation.
Obligate: Obligate organisms obtain energy from aerobic respiration, anaerobic respiration or fermentation.
Facultative: A single type of facultative organisms can be identified as facultative anaerobes.
Obligate: Two types of obligate organisms can be identified as obligate aerobes and obligate anaerobes.
Presence/Absence of Oxygen
Facultative: Facultative organisms can survive with the presence or absence of oxygen.
Obligate: Obligate aerobes can survive in the presence of oxygen while obligate anaerobes can survive in the absence of oxygen.
Facultative: Facultative organisms live in the external environment as well as inside the host.
Obligate: Obligate aerobes only live in the external environment while obligate anaerobes only live inside the host.
Facultative: Facultative parasites can survive without the host.
Obligate: Obligate parasites only survive in the presence of the host.
In a Liquid Medium
Facultative: Facultative organisms can be identified throughout the medium but, mostly near the surface.
Obligate: Obligate aerobes can be identified near the surface of the medium. Obligate anaerobes can be identified at the bottom of the medium.
Efficiency of the Energy Production
Facultative: The efficiency of the energy production in facultative organisms is high.
Obligate: The efficiency of the energy production in obligate organisms is less.
Facultative and obligate are two types of organisms that differ on the basis of cellular respiration types. The three types of cellular respiration are aerobic respiration, anaerobic respiration, and fermentation. Facultative organisms can use any of the three types of cellular respiration methods whereas obligate organisms can use only one of the three methods of cellular respiration. Thus, obligate organisms can be categorized as aerobes and anaerobes. The main difference between facultative and obligate is the type of cellular respiration mechanism used by each type of organism.
1. Facultative Anaerobic Bacteria, The Microbial Gene Research & Resources Facility, .
2. Young, Paul A. “Facultative Parasitism and Host Ranges of Fungi.” American Journal of Botany, vol. 13, no. 8, 1926, pp. 502–520. JSTOR, .
1. “Klebsiella pneumonia Bacterium” by NIAID via
2. “Armillaria mellea, Honey Fungus, UK 1″ By Stu’s Images via
3. “Pink indian pipes” By Magellan nh – Own work ) via
4. “Anaerobic” By Pixie – Own work (Public Domain) via