Oxygen Requirements for Pathogenic Bacteria

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:

  1. 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 cereusoxygen requirements of bacteria and media
  2. Obligate aerobes: They have absolute requirement for oxygen in order to grow.  Pseudomonas aeruginosa, Mycobacterium tuberculosis.

    Basic classification of Medically Important Bacteria
    Basic classification of Medically Important Bacteria
  3. 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.
  4. Obligate anaerobes:  These bacteria grow only under the condition of high reducing intensity and for which oxygen is toxic. Clostridium perfringens, Clostridium botulinum etc.
  5. 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.
  6. Aerotolerant anaerobes: Are anaerobic bacteria that are not killed by exposure to oxygen.
  7. 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.
  8. 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.


Classification Characteristics Important Genera
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
Are fermenters
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.effects of oxygen on aerobic, anaerobic and facultative anaerobic bacteria
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.

About Acharya Tankeshwar 460 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. A question. Microaerophilic why they need low oxygen concentration ?. The high concentration of oxygen how it affects the microaerophils?

  2. thanks Tankeshwar Acharya i want clear classification of earobic bacteria and gram positive bacteria plz thank

    • P. aeruginosa is aerobic only. I cannot ferment, but it can grow in lack of O2 if NO3 is available. A facultative anaerobe by definition is an organism that grows best in oxygen present environments, but can still grow in a lack of oxygen by utilizing fermentation. So, since P. aeruginosa cannot ferment, it is not considered a facultative anaerobe.

  3. Why is Sexually Transmitted Bacteria for Mycology test be put inside the oxygen incubator but not in room
    temperature? unlike other bacteria.

  4. Hi sir , nice details, my q is if u know a person who can write chemical reactions means phd chemistry . plz let me know. Thanks.

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