Anaerobic bacteria are microorganisms which survive in the absence of oxygen. They cannot tolerate oxygen at all and will die if exposed to an environment which has a high quantity of oxygen. Anaerobic metabolism involves organic or inorganic redox reactions, fermentation reactions and anaerobic reaction which produces highly volatile fatty acids and gaseous molecules such as methane and alcohol.
Anaerobic Bacteria can be Classified Into:
Facultative Anaerobes: These anaerobes are the most versatile and they preferentially utilise oxygen as a terminal electron acceptor. They can also metabolise in the absence of oxygen by reducing other compounds. Much more usable energy, in the form of high-energy phosphate, is obtained when a molecule of glucose is completely catabolised to CO₂ and H₂O in the presence of O₂ (38 molecules of ATP) than when it is only partially catabolised by a fermentative process in the absence of O₂ (2 molecules of ATP).
Microaerophilic Anaerobes: These types of anaerobes can only proliferate in low concentration of oxygen (2 to 10%) or in an atmosphere with high CO₂ concentration i.e. 10% of CO₂.
Obligate Anaerobes: Obligate anaerobes are completely incapable of aerobic metabolism but they are variably tolerant to oxygen. These organisms can be categorised into three different classes. These are:
1. Strict: Tolerance level of only ≤0.5% of Oxygen
2. Moderate: Tolerance level of 2-8 % of Oxygen
3. Aero Tolerant Anaerobes: These obligate anaerobes can tolerate atmospheric oxygen to a limited time. These anaerobic bacteria require oxygen to live.
Anaerobic Bacteria Examples: Bacteroides, Fusobacterium, Porphyromonas, Prevotella, Actinomyces, Clostridia etc.
Anaerobic bacteria are medically significant as they cause many infections in the human body. For example, various species of Clostridia can cause food poisoning, soft-tissue infection etc, whereas Actinomyces can cause head, neck, abdominal and pelvic infections.
As the name suggests, aerobic bacteria come under the class of microorganisms which grow and survive in an oxygenated environment, in short, aerobic bacteria require oxygen to live.
Aerobic Bacteria can be Classified Into:
Obligate Aerobes: Obligate aerobes need oxygen to survive. The utilise atmospheric oxygen to oxidise substrates such as sugar and fats to generate energy and carry out metabolic activities. Aerobic microorganisms use glycolysis, the Krebs TCA cycle and electron transport chain with oxygen as the final electron acceptor. The enzymes used are catalase, peroxidase and superoxide dismutase.
Facultative Aerobes: The aerobes use oxygen when it is available but are not solely reliant on it. They have anaerobic methods of energy production.
Microaerophiles: They require oxygen for energy production. However, a high concentration of atmospheric oxygen is harmful to them. They use fermentation reactions for energy and do not have an electron transport system.
Aerotolerant Aerobes: They do not use oxygen for metabolic activities and are not harmed by it. They lack all three enzymes needed to break down oxygen.
Aerobic Bacteria Examples: Some examples of aerobic bacteria are Nocardia sp. Pseudomonas aeruginosa, Mycobacterium tuberculosis etc.
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In this section, we will discuss the differences between aerobic and anaerobic bacteria
Aerobic and Anaerobic Bacteria List:
Aerobic Bacteria List:
Anaerobic Bacteria List:
Q1: Does Oxygen Kill Anaerobic Bacteria?
Obligate anaerobes are microbes which are killed by normal atmospheric concentrations of oxygen (20.95% O₂). Oxygen tolerance varies between anaerobic bacterial species. Some of them are capable of surviving in up to 8% oxygen whereas other species lose viability unless the oxygen concentration is less than 0.5%. Anaerobic microorganisms lack certain enzymes such as catalase, oxidase, superoxide dismutase that are essential for bacteria to survive in the presence of oxygen. That’s why oxygen is toxic to anaerobes because they can use oxygen metabolically.
Q2: What is the Clinical Significance of Aerobic Bacteria?
Aerobic bacteria also cause a variety of human diseases and infections. The primary aerobic microorganism of skin and tissue infections include S. aureus, P. aeruginosa are members of the enterobacteriaceae, and beta-hemolytic streptococci. Therefore, proper specimen collection and transport, media and incubation are important criteria for the recovery of aerobes. The results of aerobic cultures assist in the diagnosis and treatment of patients with bacterial infections.
Q3: What is the Difference Between Aerobic and Anaerobic Bacteria?
Please have a look at the ‘Differentiate between aerobic and anaerobic bacteria’ section.