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Aerotolerant – Definition and Medical Relevance

Aerotolerant microorganisms can survive in the presence of oxygen without requiring it for growth. They are a key concept in microbiology and infectious disease medicine.

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Things worth knowing about "Aerotolerant"

Aerotolerant microorganisms can survive in the presence of oxygen without requiring it for growth. They are a key concept in microbiology and infectious disease medicine.

What Does Aerotolerant Mean?

Aerotolerant describes microorganisms – typically bacteria – that do not require oxygen for growth but can withstand its presence without being harmed. This distinguishes them from aerobic organisms (which depend on oxygen) and strictly anaerobic organisms (for which oxygen is toxic).

Aerotolerance Compared to Other Oxygen Classes

Microorganisms are classified according to their relationship with molecular oxygen (O₂):

Obligate aerobe: Can only grow in the presence of oxygen (e.g., Mycobacterium tuberculosis).
Obligate anaerobe: Can only grow without oxygen; oxygen is toxic to these organisms (e.g., Clostridium tetani).
Facultative anaerobe: Can grow with or without oxygen, but prefers aerobic conditions (e.g., Escherichia coli).
Microaerophile: Requires low concentrations of oxygen (e.g., Helicobacter pylori).
Aerotolerant anaerobe: Grows exclusively through fermentation but tolerates oxygen without harm (e.g., Lactobacillus spp.).

Mechanism of Aerotolerance

Aerotolerant organisms possess specialized protective mechanisms that shield them from the toxic effects of reactive oxygen species (ROS). These include:

Superoxide dismutase (SOD): An enzyme that converts superoxide radicals into hydrogen peroxide and molecular oxygen.
Catalase or peroxidase: Enzymes that break down hydrogen peroxide into water and oxygen, thereby preventing oxidative damage.
NADH oxidases: These enzymes reduce oxygen directly to water without generating harmful intermediates.

Because aerotolerant organisms lack a respiratory chain, they obtain energy exclusively through fermentation – regardless of whether oxygen is present or not.

Medical and Microbiological Significance

Aerotolerant bacteria play an important role in various medical and biological fields:

Probiotics and Gut Health

Many aerotolerant lactic acid bacteria, particularly from the genera Lactobacillus and Streptococcus, are of great importance in food production (e.g., yogurt, cheese) and as probiotics. They are able to colonize the human gastrointestinal tract even when oxygen is intermittently present.

Infectious Diseases

Some aerotolerant bacteria are human pathogens capable of causing disease. A well-known example is Erysipelothrix rhusiopathiae, the causative agent of erysipeloid. Understanding oxygen tolerance aids in planning appropriate laboratory culture conditions and in selecting suitable treatment strategies.

Laboratory Diagnostics

In microbiological diagnostics, classifying a pathogen as aerotolerant is clinically relevant. It influences how samples must be transported and cultured. Unlike strictly anaerobic organisms, aerotolerant bacteria can still be successfully cultured after brief exposure to air.

Distinction: Aerotolerant vs. Facultatively Anaerobic

A common misconception is the confusion between aerotolerant and facultatively anaerobic organisms. The key difference lies in energy metabolism: facultative anaerobes can utilize both aerobic respiration and fermentation, whereas aerotolerant anaerobes rely exclusively on fermentation – even when oxygen is available. They do not metabolically benefit from oxygen, but are not harmed by it either.

References

Madigan, M. T. et al. – Brock Biology of Microorganisms, 15th edition, Pearson Education (2018).
Tortora, G. J., Funke, B. R., Case, C. L. – Microbiology: An Introduction, 13th edition, Pearson Education (2018).
World Health Organization (WHO) – Laboratory Biosafety Manual, 4th edition, WHO Press (2020). Available at: https://www.who.int

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