Bacteroides fragilis is a bacteria that is a major component of human waste.  Bacteroides species are the most common type of bacteria found in the GI tract of healthy humans.  They play an important role in preventing overgrowth of dangerous germs.   Bacteroides fragilis is usually harmless.  However, when it escapes the GI tract, (example - by way of a ruptured appendix) it can cause dangerous infections throughout the body.

Diseases

• A heathy adult can have more than 50 million in their intestines (without symptoms)
• Sepsis, intra-abdominal infection, abscess.
  

Habitat & Transmission

• Lives in normal, healthy colons. 
• Prefers an environment without oxygen. 
• Not contagious.  
  

Weapons/Defenses 

• Able to form an abscess.  
• Resistant to some antibiotics. 
• Capsule - helps prevent phagocytosis.
• Pili and fimbriae - helps the bacteria grab and hold on to intestinal walls. 
  

Weaknesses

• Can be destroyed by the immune system.  Complement and antibodies to it's capsule help alert white blood cells which can destroy the bacteria.
  

Photo - This micrograph depicts Bacteroides fragilis ss. fragilis bacteria cultured in blood agar medium for 48 hours. CDC/Dr. V.R. Dowell, Jr. 1972. Used with permission.

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