Microorganisms can be characterized by their requirements for oxygen. The oxygen requirements of bacteria reflect the mechanism used to satisfy their energy needs.
Depending on the oxygen requirements, microorganisms can be classified as aerobic (requires O2), facultative (can grow in the presence or absence of O2), anaerobic (growth in the absence of O2), capnophilic ( CO2 stimulates growth), and microaerophilic (require slightly decreased O2 tension).
Aerobes
Aerobes grow in ambient air, which contains 21% oxygen and a small amount of (0.03%) carbon dioxide. Aerobes obtain some of their energy from glycolysis but they get most of the energy through aerobic respiration (via the Krebs cycle and oxidative phosphorylation). Aerobes require molecular oxygen as a terminal electron acceptor, so they cannot grow in its absence. For example., Bacillus cereus.
Among the aerobes, cultures of rapidly dividing cells require more oxygen than do cultures of slowly dividing cells.
In a culture tube containing nutrient broth, obligate aerobes grow near the surface; where atmospheric oxygen diffuses into the medium; obligate anaerobes grow near the bottom of the tube, where little or no free oxygen reaches them.
Obligate aerobes
They have an absolute requirement of free oxygen to grow. For Examples, Pseudomonas aeruginosa, Mycobacterium tuberculosis. Most obligate aerobes obtain sufficient oxygen from nutrient broth or on the surface of solidified agar medium, but some need more; in such cases, oxygen gas is bubbled through the medium or into the incubation environment.
Anaerobes
Anaerobes can not grow in the presence of oxygen, oxygen is toxic to them. Anaerobes do not use free O2 as their final electron acceptor. Instead, they use inorganic oxygen-containing molecules such as nitrate (NO3-), nitrite (NO2−), and sulfate (SO₄²-), in a process called anaerobic respiration. As anaerobes use fewer metabolic pathways, they produce fewer ATP molecules than aerobic organisms.
Their metabolism frequently is a fermentative type in which they reduce available organic compounds to various end products such as organic acids and alcohols.
Obligate anaerobes
Obligate anaerobes are killed by free oxygen. These bacteria grow only under the condition of high reducing intensity. Clostridium perfringens, Clostridium botulinum etc.
Obligate anaerobes are killed not by gaseous oxygen but by a highly reactive and toxic form of oxygen called superoxide and hydrogen peroxide.
obligate anaerobes lack superoxide dismutase and catalase enzymes to neutralize oxygen free radicals thus, they succumb to the toxic effects of superoxide and hydrogen peroxide.
You can find detailed mechanisms about ‘how oxygen kills‘ at the end of this blog post.
Facultative anaerobes
Facultative anaerobes are versatile organisms capable of growth under aerobic and anaerobic conditions. They use oxygen if it is available but can function without it. When oxygen is present, they preferentially use oxygen as a terminal electron acceptor and carry on aerobic metabolism, but they shift to anaerobic metabolism when oxygen is absent. e.g., Enterobacteriaceae family, Staphylococcus aureus, etc.
Aerotolerant anaerobes
They are anaerobic bacteria that are not killed by exposure to oxygen. Aerotolerant anaerobes can survive in the presence of oxygen but not use it in their metabolism. Lactobacillus, for example, always captures energy by fermentation, regardless of whether the environment contains oxygen.
Capnophiles
Capnophilic (or carbon dioxide–loving) bacteria require increased concentration of carbon dioxide (5% – 10%) and approximately 15% oxygen. This condition can be achieved by a candle jar (3% carbon dioxide) or carbon dioxide incubator, jar, or bag. Examples of capnophilic bacteria include Haemophilus influenzae, Neisseria gonorrhea, etc.
Microaerophiles
Microaerophiles (mipkro-aer`o-filz) are those groups of bacteria that can grow best under reduced oxygen (5% to 10%) and increased carbon dioxide (8% to 10%) concentrations. Higher oxygen tensions may be inhibitory to them.
In a nutrient broth, they grow below the surface of the medium in a culture tube at the level where oxygen availability matches their needs. This environment can be obtained in specially designed jars or bags. Microaerophiles such as Campylobacter are also capnophiles. They thrive under conditions of low oxygen and high carbon dioxide concentration. Examples of microaerophiles are Campylobacter jejuni, Helicobacter pylori, etc.
Summary
| Classification | Characteristics | Examples |
| Obligate aerobes | Require oxygen and have no fermentative pathways. Generally, produce superoxide dismutase. | Mycobacterium Pseudomonas Bacillus |
| Capnophilic | CO2 stimulates growth | Neisseria gonorrhoeae Campylobacter |
| Microaerophilic | Requires low but not full oxygen tension | Campylobacter Helicobacter |
| Facultative anaerobes | Will respire aerobically until oxygen is depleted and then ferment or respire anaerobically | Most bacteria,i.e., Enterobacteriaceae |
| Obligate anaerobes | Lack superoxide dismutase Generally lack catalase Are fermenters Can not use oxygen as terminal electron acceptor | Actinomyces* Bacteroides Clostridium *Mnemonics: ABCs of anaerobiosis |
Why oxygen is toxic to some bacteria and how do bacteria detoxify toxic oxygen metabolites?
Several studies indicate that aerobes can only survive in the presence of oxygen through an elaborate system of defenses. Without these defenses, key enzyme systems in the organisms fail to function, and the organisms die.
Obligate anaerobes, which live only in the absence of oxygen, do not possess the defenses that make aerobic life possible and, therefore, can not survive in air.
The tolerance to oxygen is related to the ability of the bacterium to detoxify superoxide and hydrogen peroxide, produced as a byproduct of aerobic respiration.
The assimilation of glucose in aerobic conditions results in the terminal generation of free radical superoxide (O2–). The enzyme superoxide dismutase reduces the superoxide to oxygen gas and hydrogen peroxide (H2O2). Subsequently, the toxic hydrogen peroxide generated in this reaction is converted to water and oxygen by the enzyme catalase, which is found in aerobic and facultative bacteria, or by various peroxidases found in several aerotolerant anaerobes.
- Obligate aerobes and most facultative anaerobes have both superoxide dismutase and catalase.
- Some facultative and aerotolerant anaerobes have superoxide dismutase but lack catalase.
- Most obligate anaerobes lack both enzymes.
References and further readings
- Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
- Madigan, M., Martinko, J., Stahl, D., & Clark, D. (2012). Brock Biology of Microorganisms (13th ed). Pearson Education
- Review of Medical Microbiology and Immunology, Lange Medical Books, 13th edition
- Koneman’s Color Atlas and Textbook of Diagnostic Microbiology (Color Atlas & Textbook of Diagnostic Microbiology), 7th edition
