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 following different categories:
- Aerobes: Aerobes grow in ambient air, which contains 21% oxygen and small amount of (0.03%) of carbon dioxide. Aerobes require molecular oxygen as a terminal electron acceptor so cannot grow in its absence. e.g., Bacillus cereus
- Obligate aerobes: They have absolute requirement for oxygen in order to grow. Pseudomonas aeruginosa, Mycobacterium tuberculosis.
- Anaerobes: Usually bacteria of this group can not grow in the presence of oxygen, oxygen is toxic for them. They use other substances as terminal electron acceptor. 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: These bacteria grow only under the condition of high reducing intensity and for which oxygen is toxic. Clostridium perfringens, Clostridium botulinum etc.
- Facultative anaerobes: They are versatile organisms, capable of growth under both aerobic and anaerobic conditions. They preferentially use oxygen as terminal electron acceptor. e.g., Enterobacteriaceae group, Staphylococcus aureus etc.
- Aerotolerant anaerobes: Are anaerobic bacteria that are not killed by exposure to oxygen.
- Capnophiles: Capnophilic 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. The examples of capnophilic bacteria includes Haemophilus influenzae, Neisseria gonorrhoeae etc.
- Microaerophiles: Microaerophiles are those groups of bacteria that can grow under reduced oxygen (5% to 10%) and increased carbon dioxide (8% to 10%). Higher oxygen tensions may be inhibitory to them. This environment can be obtained in specially designed jars or bags. Examples of microaerophiles are: Campylobacter jejuni, Helicobacter pylori etc.
|Obligate aerobes||Require oxygen, Have no fermentative pathways. Generally produce superoxide dismutase||Mycobacterium
|Microaerophilic||Requires low but not full oxygen tension||Campylobacter
|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
Can not use oxygen as terminal electron acceptor
*Mneomonics: ABCs of anaerobiosis
Why oxygen is toxic to some bacteria and how 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 condition 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.