Aerobe

Aerobic organisms or aerobes meaning can grow in an oxygenated environment. The organism which can survive in the absence of oxygen is known as anaerobes. Anaerobes can grow only in the absence of oxygen are either obligate anaerobes or strict anaerobes. As per the research of marine biologists in July 2020, aerobic microorganisms in quasi-suspended animation were widely found in organically poor sediments. Those organisms are living at 250 feet below the seafloor of ages are up to 101.5 million years.

Types of Aerobes

Microorganisms like bacteria are usually classified into different groups depending on their characteristics such as physical appearance, nutrient and energy, culture properties, optimal growth temperature, oxygen requirement, biosynthetic capacity..etc. They are broadly classified into two types based on their oxygen requirements. They are aerobes and anaerobes. Based on the method of consuming oxygen, the aerobes are classified into three types. They are listed below. 

1. Obligate aerobes 

2. Facultative anaerobes 

3. Microaerophiles

4. Aerotolerant anaerobes 

Obligate Aerobes: The obligate aerobic bacteria definition requires free oxygen to survive. While undergoing cellular respiration, these organisms use oxygen for the metabolism process like sugar or fats to generate energy. During this type of respiration, oxygen will serve as the terminal electron acceptor for the electron transport chain. The advantage of aerobic respiration is it can yield more energy than anaerobic respiration or fermentation process. But the obligate aerobes undergo a high level of oxidative stress. The obligate aerobes get harmed due to reactive oxygen molecules. Examples of obligate aerobes are fungi and bacteria. The list of bacterias includes Mycobacterium tuberculosis, Pseudomonas aeruginosa, Bacillus, and Nocardia asteroides. 

Streptomyces coelicolor is a gram-positive bacterium, belongs to the phylum Actinobacteria. This is known as a unique obligate aerobe because the genome of this aerobe bacterium encodes by numerous enzymes with its functions. It usually attributes to anaerobic metabolism in facultative and strictly anaerobic bacteria. 

Facultative Anaerobes: A facultative anaerobes can synthesize energy through aerobic respiration in the presence of oxygen. Also, it has the capability to switch to the fermentation process for synthesizing energy in the absence of oxygen. Example of facultative anaerobes are bacterias namely Staphylococcus spp, Salmonella, Escherichia coli, Listeria spp, Shewanella oneidensis, and Yersinia pestis, and certain eukaryotic organisms like Saccharomyces cerevisiae

Microaerophiles: The microorganism under the microaerophiles, requires oxygen for survival, but it became toxic while exploring the atmospheric concentration of oxygen. That is the atmospheric air contains 21% of the oxygen. Many microaerophiles are also capnophiles, which require an elevated concentration of carbon dioxide. Examples of microaerophiles are Campylobacter, Helicobacter pylori...etc. 

Function of Aerobes

Respiration is the important source of cells to convert fuel into energy. The end product of the respiration process is adenosine triphosphate (ATP), which uses the energy stored in phosphate bands to power chemical reactions. ATP is known as the currency of the cells. As oxygen is an excellent electron acceptor for generating ATP chemical reaction. Aerobic respiration can procedure more ATP while comparing to anaerobic respiration. The oxidation of glucose is a good example of aerobic respiration.  

C₆H₁₂O₆ + 6O₂ + 38 ADP + 38 phosphate → 6 CO₂ + 44 H₂O + 38 ATP

The energy released during the above reaction is about 2880kJ per mol, During this process, every 38 ADP get conserved and regenerated into 38 ATP molecules per glucose. This is 19 times more energy per sugar molecule than the ATP generated in an anaerobic reaction. During the oxidation of glucose and water, oxygen is used during oxidation. The above reaction is the combined form of three series of biochemical reactions glycolysis, oxidative phosphorylation, and Krebs cycle. 

Difference Between Aerobes and Anaerobes 

Oxygen is an important molecule for the metabolism, survival, and growth of many microorganisms. Some organisms cannot survive in the presence of oxygen, and some organisms can survive without oxygen. The highly reactive molecules are such as hydrogen peroxide and superoxide free radicals are formed during the biochemical reactions involving oxygen.  This causes harm to the organisms. In order to fight against the harmful effect of molecules, the enzymes in bacteria can convert free radicals into oxygen compounds such as water, microbes, possess enzymes, aerobes, peroxidase, and superoxide dismutase for oxygen metabolism.  The anaerobes do not produce any enzymes to convert free radicals. So, it cannot survive in the presence of oxygen. 

The aerobic bacteria meaning, it requires oxygen for the growth and survival of organisms. 

The anaerobic bacteria definition does not require oxygen for its survival. The anaerobes use acetone, sulfur carbon dioxide as a final electron acceptor during energy metabolism. 

The anaerobes are classified into two classes. They are listed below,

1.  Aerotolerant anaerobes

2.  Obligate anaerobes

Aerotolerant Anaerobe: The microorganisms under the classification of aerotolerant anaerobes will produce ATP using fermentation reaction without using oxygen. But they will use reactive oxygen molecules to protect themselves. 

Obligate Anaerobes: These microorganisms cannot survive once it is explored to the environment oxygen, which contains 21% of oxygen. Oxygen tolerance varies from species to species. Some species can survive up to 8% of oxygen, while other organisms lose their vulnerability even at the 0.5% concentration of oxygen. 

Using certain biochemical reactions, the bacteria are classified based on their oxygen requirements. The redox reaction indicator dyes adding to the other media can determine unknown bacterium can utilize oxygen. To demonstrate the anaerobic conditions and oxygen utilization, resazurin dye is used. This indicates the oxygen reduction reaction by changing the colour from blue to pink. Here, colour change intensity is associated with the oxygen requirement of the bacteria present in the medium. These methods are usually followed to demonstrate milk’s anaerobic condition. 

Some researchers found some problems in conducting conventional techniques to determine the oxygen requirement of bacteria. These anaerobic bacteria are highly sensitive to the oxygen environment. But new technologies with sequencing technologies can be used to determine the oxygen requirement and other biochemical characteristics to support the growth of bacteria. 

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