Anaerobes

Anaerobes are bacteria that thrive in environments with little to no oxygen. These organisms play significant roles in both normal human physiology and in causing infections under specific conditions.

Examples of Anaerobes

Clostridium
Bacteroides
Fusobacterium
Actinomyces israelii

Key Characteristics of Anaerobes

Absence of Catalase/Superoxide Dismutase: Anaerobes typically lack enzymes like catalase and superoxide dismutase, making them susceptible to oxidative damage.
Foul Smelling: They produce short-chain fatty acids, which result in a characteristic foul odor.
Difficult to Culture: Anaerobes are often challenging to grow in standard laboratory settings due to their oxygen sensitivity.
Gas Production: These bacteria produce gases such as CO2 and H2 in tissues, a sign often observed in anaerobic infections.

Mnemonic

Anaerobes: “Can’t Breathe Fresh Air”

C: Clostridium
B: Bacteroides
F: Fusobacterium
A: Actinomyces

Pathogenicity

Anaerobes are part of the normal flora in the gastrointestinal (GI) tract, where they typically do not cause harm. However, when found outside their usual environments, they can become pathogenic, leading to infections.

Resistance to Aminoglycosides

Anaerobes are resistant to AminO2glycosides (aminoglycoside antibiotics) because these antibiotics require oxygen to enter bacterial cells, which anaerobes lack in their environments.

Table of Anaerobes and Key Features

Bacterium	Environment	Key Characteristics
Clostridium	GI tract	Spore-forming, causes gas gangrene
Bacteroides	GI tract	Common in abdominal infections
Fusobacterium	Oral/GI tract	Causes oral and GI abscesses
Actinomyces israelii	Oral cavity	Associated with oral and cervicofacial infections

Table of Anaerobes and Key Features

Points to Remember

Anaerobes lack catalase and superoxide dismutase, making them vulnerable to oxidative stress.
They produce foul-smelling short-chain fatty acids.
Anaerobes generate CO2 and H2 in tissues, which can be a diagnostic sign in anaerobic infections.
They are resistant to aminoglycosides, as these antibiotics require oxygen for bacterial entry.
Anaerobes are part of the normal GI flora but can be pathogenic in other body regions.

Bibliography

Baron, E. J., & Finegold, S. M. (1990). Bailey and Scott’s Diagnostic Microbiology. St. Louis: Mosby.
Brook, I. (2016). Anaerobic infections in children. Journal of Medical Microbiology, 65(5), 389–399.

USMLE Step 1 Review Series

Curriculum

Anaerobes

Examples of Anaerobes

Key Characteristics of Anaerobes

Mnemonic

Pathogenicity

Resistance to Aminoglycosides

Table of Anaerobes and Key Features

Points to Remember

Bibliography

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