Microbiology · MU Sofia

71. Anaerobic non-spore forming bacteria. Pathogenesis, clinics, microbiological diagnosis

Covers the general features of non-spore-forming anaerobic bacteria, including oxygen tolerance, enzyme deficiencies and their normal flora habitats, then how they cause suppuration, necrosis and abscesses. Lists virulence factors and a table of key organisms with their associated infections, plus laboratory diagnosis from specimen collection through Gram staining, anaerobic culture and identification methods. Closes with surgical and antibiotic treatment, including a resistance caution for Bacteroides fragilis, and prevention measures.

Inside: 9 sections · 4 labelled pictures · 4 tables · 15 flashcards · 3 min

Topic 71 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 9 sections, about 229 words (3 min of reading), with 15 flashcards.

In this note

  1. Anaerobic Non-Spore Forming Bacteria
  2. GENERAL CHARACTERISTICS
  3. CLASSIFICATION
  4. PATHOGENESIS
  5. VIRULENCE FACTORS
  6. CLINICALLY SIGNIFICANT ORGANISMS & DISEASES
  7. LABORATORY DIAGNOSIS
  8. TREATMENT
  9. PREVENTION

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

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