Lecture Notes: Bacteroides fraudulus

Overview

• Bacter: Means rod
• Oides: Means shape
• Fraudulus: Means fragile
• Bacteroides fraudulus: A gram-negative, rod-shaped bacterium
  • Pleomorphic: Ranges from sphere to rod shape
  • Inhabits human colon
  • Responsible for most human infections among Bacteroides species

Characteristics

• Gram-negative:
  • Thin peptidoglycan cell wall
  • Does not retain purple dye on gram-staining
• Non-spore-forming, Non-motile
• Obligate anaerobe: Can only live without oxygen
• Bile-resistant: Thrives in bile
• Growth mediums:
  • Blood agar
  • Bile esculin agar (Bacteroides bile esculin)
  • Forms dark colonies with brown-black halos after 48 hours at 35°C
• Antibiotic resistance: Resistant to kanamycin, vancomycin, and colistin

Pathogenicity

• Normal colonizer: Peacefully colonizes the human colon
• Infection pathway:
  • Trauma/surgery may allow it to enter the bloodstream or peritoneal cavity
  • Peritoneal infections are polymicrobial (often with E. coli)
• Virulence factors:
  • Lipopolysaccharide capsule (LPS): Aids attachment and immune evasion
  • Succinic acid: Inhibits neutrophils
  • Promotes abscess formation

Enterotoxigenic Bacteroides fragilis

• Secretes heat-labile zinc metalloprotease toxin
• Possible mutation: May increase virulence
• Effects:
  • Stimulates IL-8 secretion (pro-inflammatory)
  • Causes inflammatory diarrhea

Clinical Manifestations

• Intra-abdominal infections: Fever, abdominal pain, nausea, vomiting, distended abdomen
• Skin infections: Pus, necrotic tissue, bad odor
• Inflammatory diarrhea: Abdominal pain, watery diarrhea

Diagnosis

• Isolation in cultures from:
  • Peritoneal fluid
  • Pus
  • Stool
• Gas-liquid chromatography: Identifies acetic and succinic acids
• Gene sequencing
• CT scan: May show abscesses or gas presence

Treatment

• Antibiotics:
  • Metronidazole
  • Beta-lactam/beta-lactamase inhibitors (e.g., ampicillin, sulbactam)
  • Note: Cannot use beta-lactam antibiotics like penicillin
• Surgical:
  • Drainage of abscesses
  • Debridement of necrotic tissue

Summary Recap

• Bacteroides fraudulus: Gram-negative, pleomorphic, rod-shaped, inhabits colon
• Grows on blood and bile esculin agar
• Resistant to kanamycin, vancomycin, colistin
• Common in peritoneal and diabetic foot infections
• Treatment involves surgical intervention and specific antibiotics