Oxygen requirements of bacteria reflect the mechanism used by them, to satisfy their energy needs. On the basis of oxygen requirements, bacteria can be divided into the following different categories:
Table of Contents
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 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 in order to grow. For example, 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 for 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 less of the metabolic pathways, they produce fewer ATP molecules than do 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
Obligates 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 both 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
Are anaerobic bacteria that are not killed by exposure to oxygen. Aerotolerant anaerobes can survive in the presence of oxygen but do 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 bags. Examples of capnophilic bacteria include Haemophilus influenzae, Neisseria gonorrhoeae 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 | Important Genera |
| Obligate aerobes | Require oxygen, Have no fermentative pathways. Generally produce superoxide dismutase | Mycobacterium Pseudomonas Bacillus |
| 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 survive in the presence of oxygen only by virtue of 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 superoxide is reduced by the enzyme superoxide dismutase 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 which are 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.
