n., plural: obligate aerobes
Definition: an organism that is absolutely aerobic.
Credit: Utah State University (L. bulgaricus photo)
Table of Contents
Before we define obligate aerobes, let us first understand and define aerobic organisms. Aerobic organisms are those that can only live in the presence of oxygen. They are also called aerobes. Not only animals and plants are aerobic; several microorganisms are also aerobic, meaning they need oxygen for their survival. Aerobes can be classified as follows:
- Obligate aerobes or strict aerobes – These organisms mandatorily need oxygen for their growth and survival. These organisms utilize oxygen in aerobic respiration as a terminal electron receptor.
- Facultative aerobes – These organisms are not solely dependent upon the availability of oxygen for their survival. In an aerobic environment, these organisms carry out aerobic respiration while they switch to anaerobic respiration in anaerobic conditions.
- Microaerophile – These organisms need oxygen for their survival; however, a high amount of oxygen is toxic for such organisms.
- Aerotolerant – These organisms do not require oxygen; however, they remain unaffected by the presence of oxygen in their environment.
Obligate Aerobes Definition
To understand obligate aerobes, let us know the literal meaning of the word:
- Aerobes = survive in oxygen
- Obligate = necessarily
Hence, their name literally means organisms that categorically require oxygen to grow and survive.
So, obligate aerobe definition, in biology, refers to any organisms that can grow and survive only in the presence of oxygen as they derive energy from aerobic respiration utilizing oxygen. These organisms produce energy by the process of oxidative phosphorylation in oxidative (aerobic) respiration utilizing oxygen as the terminal electron acceptor.
Obligate anaerobes require ~21% of oxygen in their environment to grow and survive. Accordingly, obligate aerobes are found in an environment having molecular oxygen.
Oxidative respiration yields a higher amount of energy as compared to anaerobic respiration. Aerobic respiration generates 38 ATP molecules from the oxidation of each glucose molecule:
- C6H12O6+ 6 O2 + 38 ADP + 38 Phosphate 6 CO2 + 6 H2O + 38 ATP
These organisms use oxygen as the terminal electron acceptor in the Krebs TCA cycle, glycolysis, and electron transport chain and generate carbon dioxide and water along with energy molecules (e.g., ATP).
Aerobic respiration that utilizes molecular oxygen, thus, seems to be an energy-efficient process. However, the organisms undergoing aerobic respiration are under a lot of oxidative stress.
The oxidation process that involves the utilization of molecular oxygen results in the generation of toxic free radicals or reactive oxidative species (ROS) (hydrogen peroxide (H2O2) and singlet oxygen (O2-). These free radicals or ROS are potent oxidizing agents that are highly lethal to a cell. However, obligate aerobes and aerotolerant anaerobes have enzymes that neutralize these ROS or free radicals. All aerobes possess superoxide dismutase (SOD) that neutralize these reactive potent singlet oxygen species (figure).
Certain aerobes possess a catalase enzyme that decomposes H2O2, while some microbes may have a peroxidase enzyme to neutralize ROS. In addition, photosynthetic organisms contain carotenoids that quench the reactive singlet oxygen species to the non-toxic ground state. Thus, carotenoids are known as quenchers for ROS.
In contrast to aerobic organisms, anaerobic organisms do not possess these enzymes (SOD, catalase, peroxidase, carotenoid), and hence they are not able to survive from the toxic effect of oxygen.
Thus, it can be summed up that the presence of enzymes, namely, SOD, catalase, peroxidase is the obligate aerobes’ defense mechanism that protects them from the highly oxidative conditions created by the oxygen (figure).
In general, most of the life we see around us can be classified as ‘aerobes’ like humans, animals, plants, as these organisms need oxygen for their growth and survival. However, in the microbial world, especially bacteria, different types of microbes may or may not need oxygen for their growth and survival. Thus, depending upon the need for oxygen, the organisms can be broadly classified as aerobes and anaerobes. As the name suggests, aerobes depend on oxygen for their survival; anaerobes do not need oxygen.
Characteristics of Obligate Aerobes
Obligate aerobes have characteristic features that distinguish them from other types of organisms; these features are:
- Obligate aerobes utilize oxygen during metabolism for the generation of energy.
- Oxygen is utilized as the terminal electron acceptor during the energy generation step, wherein the breakdown of carbon-containing complex compounds occurs in obligate aerobes.
- The presence of oxygen is critical for the growth of the obligate aerobes in the laboratory.
- In liquid growth media in a laboratory, obligate aerobes generally grow at the surface of the liquid media. The surface of such media thus appears to be thick, while the rest of the media seems transparent and practically sterile.
- All obligate aerobes possess enzymes (like catalase, superoxide dismutase, etc.) or chemical substances (like carotenoid) that detoxify them from the toxic oxygen radicals
Obligate Aerobe Examples
The majority of the animals and fungi are aerobes. As for bacteria, here are some of them:
- Obligate Aerobic Gram-negative (-ve) bacteria include Pseudomonas aeruginosa, E. coli, Klebsiella pneumonia. Obligate Aerobic Gram (+ve) bacteria are Bacillus, Nocardia, Micrococcus spp.
- Interestingly, Micrococcus luteus is an obligate aerobic gram (+)ve bacteria whose primary habitat is the human skin. Micrococcus luteus that was dormant for 34,000 to 170,000 years was found in amber. Micrococcus luteus can remain dormant for centuries together in an oligotrophic environment, and that raised a doubt whether Micrococcus luteus is an aerobic or anaerobic bacteria. However, based upon the Micrococcus luteus oxygen requirement, it has now been classified as an obligate aerobe that resides on human skin.
- Obligate Aerobic Acid-fast bacteria, Mycobacterium tuberculosis (an obligate pathogen that causes tuberculosis)
Note: Based on Pseudomonas characteristics, previously, it was considered to be an obligate aerobic bacteria; however, it has now been classified as a facultative anaerobe.
Obligate Aerobes vs. Obligate Anaerobes
What’s the difference between obligate aerobes and obligate anaerobes? Let us find out below.
Question: What is an obligate aerobe?
Answer: Obligate Aerobes are the organisms that compulsorily require oxygen in their environment for growth and survival.
Question: What is an obligate anaerobe?
Answer: Obligate anaerobes are organisms that do not require oxygen. In fact, oxygen is toxic for them, and these organisms can not tolerate oxygen in their environment. Instead, these organisms depend on fermentation or anaerobic respiration for generating energy for their growth and survival.
Table 1: Difference between Obligate Aerobes and Obligate Anaerobes
|Obligate Aerobes||Obligate Anaerobes|
|Aerobic organisms need oxygen for their growth and survival||Organisms that do not require oxygen for their survival and growth and survive in an anaerobic environment|
|Need sufficient amount of oxygen (~21%) for survival||Oxygen is toxic for them and does not survive in the presence of oxygen|
|Undertake aerobic respiration||Undertake anaerobic respiration|
|Produce more energy due to aerobic respiration||Produces less amount of energy as it shows anaerobic respiration|
|In culture tube test, grow near the surface of the growth media in the tube||In culture tube test, grow at the bottom of the tube|
|Mycobacterium tuberculosis, Lactobacillus, Nocardia are some of the obligate aerobic bacteria||Bacterium genus Clostridium, Fusobacterium, Bacteroides, Prevotella, and the majority of the marine bacterium are obligate anaerobes|
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