Learning Objective: By the end of this topic, the learner will be able to describe the characteristics, examples, and clinical significance of anaerobic bacteria, differentiate them from facultative anaerobes, and explain why aminoglycosides are ineffective against them.
Anaerobes are bacteria that thrive in oxygen-free environments. They lack essential enzymes—catalase and/or superoxide dismutase—that neutralize reactive oxygen species, making them highly susceptible to oxidative damage.
Common Examples
- Clostridioides
- Clostridium
- Bacteroides
- Fusobacterium
- Actinomyces israelii
Activity: Drag and drop
Key Characteristics
- Foul-smelling infections (due to short-chain fatty acids)
- Gas production in tissues (CO₂, H₂)
- Difficult to culture under aerobic conditions
Mnemonic: “Anaerobes Can’t Breathe Fresh Air.”
Clinical Relevance
- Part of the normal gastrointestinal flora, but becomes pathogenic when located elsewhere (e.g., aspiration pneumonia, intra-abdominal abscess).
- Aminoglycosides are ineffective against anaerobes because these antibiotics require oxygen for active uptake into bacterial cells.
Activity:
Facultative Anaerobes
Definition:
Bacteria that can grow in both aerobic and anaerobic environments. They use oxygen as a terminal electron acceptor when available, but can switch to fermentation or other anaerobic pathways in its absence.
Examples:
- Streptococcus spp.
- Staphylococcus spp.
- Enteric gram-negative bacteria
