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Obligate Aerobes Definition

Organisms that can thrive in the presence of oxygen, are aerobic organism or aerobes like the plants, microorganism and animals. Aerobic organism can be classified as:

  • Aerotolerant: These organism don’t require oxygen and show no impact in the presence of oxygen in the environment.
  • Microaerophile: These organism require oxygen to thrive, but higher concentration of oxygen is fatal to such organisms.
  • Facultative aerobes: These organisms are not completely reliable on oxygen for survival. Depending on the conditions, they carry out processes, like in aerobic condition they perform aerobic respiration and in anaerobic condition, they undergo anaerobic respiration.
  • Strict/ obligate aerobes: these organisms strictly require oxygen for survival and they use the oxygen during aerobic respiration as an electron receptor.

What is Obligate Aerobes?

Obligates means mandatory and aerobes means require oxygen to survive. Thus, obligate aerobes are those which require oxygen strictly to thrive. From aerobic respiration, they obtain energy by consuming oxygen.

Through the oxidative phosphorylation process, energy is generated where in aerobic respiration, oxygen is used as terminal electron acceptor. Obligate aerobes are found in surroundings with molecular oxygen, whereas obligate anaerobes for their survival require 21% of oxygen.

Aerobic respiration generates more amount of ATP than anaerobic respiration. From one glucose molecule, 38ATP molecules are generated in the aerobic respiration.

C6H1206 + 38ADP + 38Phosphate ⇒ 6CO2 + 6H20 + 38ATP

In electron transport chain, glycolysis, TCA and Kreb cycle, oxygen is used as electron acceptor which produces ATP and along with-it water and carbon dioxide. Although organism which carry out aerobic respiration have shown extreme oxidative stress, but takes up molecular oxygen, making it an energy saving process.

Usage of molecular oxygen results in the production of single O2, H2O2 and ROS, which are free radicals which can oxidize and are fatal to cells. The fatality can be balanced by aerotolerant anaerobes and obligate aerobes. The effect of single oxygen species can be balanced out by superoxide dismutase, which is found in all aerobes.

Hydrogen peroxide can be hydrolyzed by catalase enzyme which is found in aerobic organism. The ROS can also be balanced out by peroxidase found in microorganism. Carotenoids, is an accessory photosynthetic pigment which can neutralize the single oxygen to its ground state. Thus, carotenoids are ROS quenchers.

However anaerobic organism do not comprise of enzymes like carotenoid, catalase, peroxidase and others, thus its negative impact on the cell, will not let them thrive. Enzymes like catalase, SOD, peroxidase, in obligate aerobes protect them for oxidative damage, thus acting as defense barriers.

Within the category of aerobes, there are plants, animals, humans and bacteria, which need oxygen to thrive, however on the basis of their need, microorganism can be grouped into aerobes and anaerobes.

Characteristics of Obligate Aerobes

The characteristics of obligate aerobes are:

a) Obligate aerobes consist of enzymes, catalase, superoxide mutase and peroxidase, and carotenoids which protect them for oxidative damage, thus acting as defense barriers.

b) Oxygen is very vital if obligate aerobes are to be cultivated in laboratory.

c) When cultivating obligate aerobes in lab in a liquid media, they tend to grow on the surface and look quite thick in consistency, whereas the remaining is sterile and opaque.

d) Oxygen is used by obligate aerobes, while metabolism to produce energy.

e) While producing energy, oxygen is taken up as the electron acceptor, where the carbon compounds are disintegrated by obligate aerobes.

Obligate Aerobes Examples

Almost all the fungus and animals are aerobes. In bacteria, the examples are:

a) Obligate aerobic acid fast-bacteria: an example is Mycobacterium tuberculosis, which results in tuberculosis.

b) Micrococcus luteus: It resides on skin and is a gram-positive bacteria. This species can remain inactive for years, for instance, for 34,000-170,000 years this species was inactive and was first seen in amber. As it can remain inactive for thousand of years, it wasn’t sure whether it is aerobe or anaerobe, but now it is evidenced that its an obligate aerobe.

c) Obligate aerobic gram-negative bacteria: It comprises of Klebsiella pneumonia, Pseudomonas aeruginosa and E. coli.

d) Obligate aerobic gram-positive bacteria: It consists of Micrococcus sp, Nocardia and Bacillus.

Obligate Aerobes vs Obligate Anaerobes

Organism which does not require oxygen to thrive and for their growth are called as obligate anaerobes, whereas obligates aerobes require oxygen to continue their survival. Obligate anaerobes does not require oxygen as it is harmful to them. Around 21% of oxygen is required by obligate aerobes to thrive.

Obligate anaerobes carry out anaerobic respiration, whereas aerobes undergo obligate aerobes undergo aerobic respiration. As obligate aerobes carry out aerobic respiration, they generate more amount of energy, whereas obligate anaerobes generates less amount of energy.

Obligate anaerobes grow at the bottom in the test tube. Obligate aerobes grow at the surface in the test tube containing liquid. Examples of obligate anaerobes are Bacteroides, Clostridium, Fusobacterium, Prevotella and others. Lactobacillus, Mycobacterium and Nocardia are the examples of obligate aerobic bacteria.


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