GENERAL CHARACTERISTICS
- Oxygen tolerance: Obligate anaerobes (killed by O₂) or aerotolerant (tolerate minimal O₂)
- Spore formation: Do not form spores (distinguishes from Clostridia)
- Enzyme deficiency: Lack catalase and/or superoxide dismutase, making them vulnerable to ROS
- Metabolism: Obtain energy via fermentation
- Habitat: Normal flora of:
- Oral cavity
- Skin
- GI tract
- Female genital tract
- Ratio: Outnumber aerobes significantly (e.g., 1000:1 in colon)
PATHOGENESIS
- Entry via mucosal breaches, trauma, surgery
- Anaerobic conditions (↓ O₂ tension) + necrosis favor overgrowth
- Often mixed infections with aerobes that consume O₂
- Lead to:
- Suppuration (pus)
- Necrosis
- Abscess formation
- Malodorous discharge (due to fermentation byproducts)
LABORATORY DIAGNOSIS
Specimen Collection
- Deep tissue aspirates or biopsy preferred
- Avoid swabs (surface flora contamination)
- Use anaerobic transport media (e.g., gassed vials, rubber-bung syringes)
Direct Microscopy
- Gram stain: Look for pus cells + variable anaerobic rods/cocci
- Multiple morphologies may be seen
Culture
- Incubate in anaerobic conditions (anaerobic jar, GasPak, microanaerostat)
- Common media:
- Blood agar (with hemin, vitamin K)
- Fastidious Anaerobe Agar (FAA)
- Phenylethyl alcohol agar (PEA)
- Thioglycollate broth
Cultures require 5–10 days incubation
Identification
- Colony morphology, pigment production (black/brown in Porphyromonas)
- UV fluorescence (red glow for P. melaninogenica)
- Gas-liquid chromatography (GLC) for detecting short-chain fatty acids
- Molecular methods and MALDI-TOF in modern labs