Bacteroides Fragilis

Bacteroides fragilis is a clinically important member of the Bacteroidaceae family. Other notable microbes within this group include Bacteroides ovatus, Bacteroides thetaiotaomicron, Bacteroides distasonis, and Bacteroides vulgatus [1].

Staining and microbiologic features:

• B. fragilis, a bile resistant pathogen, is a component of normal anaerobic flora in the human intestine [1,2]. 
• It appears gram-negative on gram staining and does not form spores [1,2].
• Bile promotes the growth of B. fragilis [3].

Virulence:

• It uses fimbriae to attach to host cells [4].
• Cytolysins produced by B. fragilis also cause RBC hemolysis [5].
• B. fragilis expresses superoxide dismutase (SOD) and catalase, enzymes that help the organism to survive the oxidative stress [4]. 
• Anaerobic metabolism in P. aeruginosa produces short-chain fatty acids like succinic acid. These metabolites result in the disruption of phagocytic and intracellular killing mechanisms of immune cells [4]. 
• Polysaccharide capsule shields the organism from phagocytosis [4].
• Unlike aerobic Gram-negative LPS, Bacteroides LPS exhibits minimal endotoxin activity. It is due to phosphate-less glucosamines in lipid A and a reduced number of fatty acids linked to amino sugars [4]. 
• The heat-labile zinc metalloprotease toxin secreted by enterotoxigenic B. fragilis rearranges the F-actin cytoskeleton within intestinal epithelial cells, triggering increased chloride secretion and fluid loss. In addition, this toxin promotes the release of chemokines (like IL8) from intestinal epithelium. The subsequent influx of immune cells, while aiming to control the bacterial infection, can also contribute to epithelial damage [4].
• It also produces proteases and neuraminidases [5].

Transmission:

• Disruption of the intestinal barrier, for example, during surgery [6]

Diseases and complications:  

•  Intra-abdominal infections, such as peritonitis and abscesses [6]
• Gynecologic Infections, such as pelvic inflammatory disease and ovarian abscesses [2]
• Bacteremia and Gastroenteritis [7]

Diagnostic testing: 

• Microscopy may reveal gram-negative rods that are pleomorphic and faintly staining [8]
• Anaerobic culture [8]

File:Bacteroides fragilis on Fastidious Anaerobe Agar – Sensitive to Metronidazole – Detail.jpg by Nathan Reading from Halesowen, UK is licensed under CC BY 2.0.

• Biochemical testing and gas chromatography can be helpful in diagnosis [6]

References:  

1. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 294)
2. CMMRS edition 6, 2016-17 (page no: 86,88)
3. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 317) 
4. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 318)
5. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 297) 
6. Usmle Step 1 Lecture Notes: Microbiology / Immunology, 2020 edition, (page no: 254)
7. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 319)
8. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 320)