Phylum Bacteroidetes: BAL-Fall 2026-General Microbiology

General Characteristics and Habitats

  • Overview: 2nd largest bacterial phylum (~7,000 species); primarily Gram-negative, anaerobic bacilli.

  • Habitats: Soil, ocean, freshwater, and human gastrointestinal tract (GIT).

  • Human Colonization:

    • Dominates fecal microbiota and accounts for ~50% of healthy mucosal tissue strains.

    • Found in oral cavity (plaque/saliva) and upper respiratory tract.

    • Prevotella is the 3rd most abundant genus in the stomach.

Symbiotic Role in Human Health

  • Carbohydrate Degradation: Supplies enzymes to break down host-resistant plant cell wall polymers (cellulose, pectin, xylan).

  • Energy Production: Fermenting complex carbohydrates generates short-chain fatty acids (SCFAs), providing energy to colon cells.

Pathogenic Capabilities and Specific Pathogens

  • Opportunistic Infections: Cause disease when spread outside normal mucosal sites (isolated from blood, urine, wounds, stool).

  • Virulence Factors: Produce enzymes (hyaluronidase, glucosidases, proteases, N-acetylglucosaminidase) that degrade host extracellular matrix to facilitate tissue colonization.

  • Key Pathogens:

    • Bacteremia: Presence in bloodstream.

    • Oral Pathogens: Porphyromonas, Prevotella, and Tannerella (cause periodontal disease).

    • Elizabethkingia meningoseptica: Causes infant meningitis.

    • Myroides: Causes nosocomial infections (UTIs, surgical wound infections, pneumonia, meningitis).

Taxonomy and Clinically Relevant Species

  • Genus Porphyromonas: Gram-negative bacilli; associated with periodontal disease.

  • Genus Prevotella (P. intermedia): Gram-negative bacilli; associated with periodontal disease.