Difference Between Aerobic and Anaerobic Bacteria

Main Difference – Aerobic vs Anaerobic Bacteria

Bacteria represent a large domain of prokaryotic organisms. They are few micrometers in length and possess several shapes such as spheres, rods, and spirals. Bacteria live in a vast variety of habitats such as soil, water, acidic hot springs, radioactive waste, deep portions of the earth’s crust, and as parasites in or on animals and plants. They are vital in recycling nutrients by decomposing organic materials and fixation of nitrogen from the atmosphere in nutrient cycles. Bacteria exhibit a variety of metabolic types as well. Aerobic and anaerobic bacteria are the two groups of bacteria classified based on the type of respiration. The main difference between aerobic and anaerobic bacteria is that anaerobic bacteria use molecular oxygen as the final electron acceptor in the electron transport chain whereas anaerobic bacteria use other molecules or compounds as the final electron acceptor.

Key Areas Covered

1. What are Aerobic Bacteria
     – Definition, Facts, Mechanism of Respiration
2. What are Anaerobic Bacteria
     – Definition, Facts, Mechanism of Respiration
3. What are the Similarities Between Aerobic and Anaerobic Bacteria
     – Outline of Common Features
4. What is the Difference Between Aerobic and Anaerobic Bacteria
     – Comparison of Key Differences

Key Terms: Aerobic Bacteria, Anaerobic Bacteria, Electro Transport Chain, Final Electron Acceptor, Molecular Oxygen

What are Aerobic Bacteria

Aerobic bacteria refer to the microorganisms that grow in the presence of oxygen. The four types of bacteria that can utilize oxygen are obligate aerobes, facultative anaerobes, microaerophils, and aerotolerant anaerobes. Obligate aerobes use oxygen to oxidize sugars and fats to generate energy in a process called cellular respiration. If oxygen is available, facultative anaerobes use oxygen for their respiration. Microaerophils require oxygen to survive, but require environments containing lower levels of oxygen than are present in the atmosphere. Aerotolerant anaerobes do not require oxygen but, they are not harmed by oxygen as anaerobic bacteria. The behavior of different types of bacteria in a liquid culture is shown in figure 1.

Figure 1: Behavior of Different Bacterial Types in a Liquid Culture
1 – Obligate Aerobes, 2 – Obligate Anaerobes,  3 – Facultative Bacteria, 4 – Microaerophiles, 5 – Aerotolerant Bacteria

Aerobic bacteria oxidize monosaccharides such as glucose in the presence of oxygen through cellular respiration. The three steps of aerobic respiration are Krebs cycle, glycolysis, and oxidative phosphorylation. During glycolysis, glucose (C6) is broken down into two pyruvates (C3) molecules in the cytoplasm. In the presence of oxygen, pyruvate combines with oxaloacetate (C4) in order to form citrate (C6), eliminating acetyl-CoA during the citric acid cycle. The citric acid cycle is the second stage of cellular respiration, which is also called the Krebs cycle. During Krebs cycle, carbon dioxide is eliminated as a waste, while reducing NAD into NADH.  Six NADH, two FADH2 and two ATPs per one glucose molecule are produced by the Krebs cycle. Oxidative phosphorylation, which is the third stage of cellular respiration in which the electron transport chain is employed to produce 30 ATPs by the enzyme ATP synthase, use the above-mentioned NADH and FADH₂ molecules. The balanced chemical reaction of the oxidation of glucose is shown below.

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

Lactobacillus, Mycobacterium tuberculosis, and Nocardia are some of the examples of aerobic bacteria.

What are Anaerobic Bacteria

Anaerobic bacteria are microorganisms that grow in the absence of oxygen. The bacteria that are incapable of tolerating oxygen are called obligate 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. Though aerotolerant bacteria do not use oxygen, they can survive in the presence of oxygen. Anaerobic bacteria play a major role in nutrient cycles such as nitrogen cycle. The anaerobic bacteria in the nitrogen cycle and their role are shown in figure 2.

Figure 2: Nitrogen Cycle

Some of the obligate anaerobes use fermentation while the others use anaerobic respiration. Aerotolerant bacteria are strictly fermentative while facultative anaerobes use either fermentation, anaerobic respiration or aerobic respiration.

Fermentation

The two types of fermentation are lactic acid fermentation and ethanol fermentation. Both methods stats with the glycolysis. The second step is the fermentation. Electron transport chain is not employed in the fermentation. The chemical reactions for each type of fermentation are shown below.

Lactic Acid Fermentation

C₆H₁₂O₆ + 2 ADP + 2 phosphate → 2 lactic acid + 2 ATP   

Ethanol Fermentation

C₆H₁₂O₆ + 2 ADP + 2 phosphate → 2 C₂H₅OH + 2 CO₂↑ + 2 ATP

Anaerobic Respiration

The final electron acceptor of the anaerobic respiration is not the molecular oxygen as in aerobic respiration. Different types of organisms use different types of final electron acceptors. These can be ions such as sulfur, ferric, manganese (IV), cobalt (III), and Uranium (VI) and compounds such as fumarate, sulfate, nitrate or carbon dioxide. Methanogenic bacteria are one such type of organisms that use carbon dioxide as the final electron acceptor in the absence of oxygen. They produce methane gas as a byproduct. Bacteroides, Clostridium, and E. coli are some of the examples of anaerobic bacteria. 

Similarities Between Aerobic and Anaerobic Bacteria

• Both aerobic and anaerobic bacteria are prokaryotes.
• Both aerobic and anaerobic bacteria undergo the three steps of the cellular respiration; glycolysis, Krebs cycle, and electron transport chain.
• All the steps of the cellular respiration in both aerobic and anaerobic bacteria occur in the cytosol.
• Facultative bacteria can live as both aerobic or anaerobic bacteria.

Difference Between Aerobic and Anaerobic Bacteria 

Definition

Aerobic Bacteria: Aerobic bacteria refer to the microorganisms that grow in the presence of oxygen.

Anaerobic Bacteria: Anaerobic bacteria refer to the microorganisms that grow in the absence of oxygen.

Significance

Aerobic Bacteria: The final electron acceptor of the aerobic bacteria is molecular oxygen.

Anaerobic Bacteria: The final electron acceptor of the anaerobic bacteria can be ferric, sulfur, nitrate, fumarate or carbon dioxide.

Ability to Detoxify Oxygen

Aerobic Bacteria: Aerobic bacteria possess enzymes to detoxify oxygen by catalase or superoxide.

Anaerobic Bacteria: Anaerobic bacteria do not possess enzymes to detoxify oxygen.

Presence of Oxygen

Aerobic Bacteria: Aerobic bacteria can only survive in the presence of oxygen.

Anaerobic Bacteria: Anaerobic bacteria cannot survive in the presence of oxygen.

Final Electron Acceptor

Aerobic Bacteria: Water is produced from the molecular oxygen by aerobic bacteria.

Anaerobic Bacteria: Nitrate, methane, sulfide, and acetate-like substances are produced by anaerobic bacteria.

Habitat

Aerobic Bacteria: Aerobic bacteria live in the soil, water, and on different surfaces.

Anaerobic Bacteria: Anaerobic bacteria live in oxygen-depleted areas such as digestive system (stomach to the rectum) of animals.

Efficiency of the Energy Production

Aerobic Bacteria: Aerobic bacteria produce more energy.

Anaerobic Bacteria: Anaerobic bacteria produce less energy.

In a Liquid Medium

Aerobic Bacteria: Aerobic bacteria come to the surface of the medium in a liquid medium.

Anaerobic Bacteria: Anaerobic bacteria settle at the bottom of the medium.

Examples

Aerobic Bacteria: Lactobacillus, Mycobacterium tuberculosis, and Nocardia are some of the examples of aerobic bacteria.

Anaerobic Bacteria: Bacteroides, Clostridium, and E. coli are some of the examples of anaerobic bacteria. 

Conclusion

Aerobic and anaerobic bacteria are two types of bacteria that differ in the final electron acceptor of the electron transport chain. Aerobic bacteria use molecular oxygen as the final electron acceptor while anaerobic bacteria use other substances as the final electron acceptor. The main difference between aerobic and anaerobic bacteria is the type of final electron acceptor during cellular respiration.

Reference:

1. Haddock, B A, and C W Jones. “Bacterial respiration.” Bacteriological Reviews, U.S. National Library of Medicine, Mar. 1977, Available here.

Image Courtesy:

1. “Anaerobic” By Pixie – Own work (Public Domain) via Commons Wikimedia
2. “Nitrogen Cycle” By Cicle_del_nitrogen_de.svg: *Cicle_del_nitrogen_ca.svg: Johann Dréo (User:Nojhan), traduction de Joanjoc d’après Image:Cycle azote fr.svg.derivative work: Burkhard (talk)Nitrogen_Cycle.jpg: Environmental Protection Agencyderivative work: Raeky (talk) – Cicle_del_nitrogen_de.svgNitrogen_Cycle.jpg (CC BY-SA 3.0) via Commons Wikimedia