Phylum Chlamydiae: BAL-Fall 2026-General Microbiology

General Characteristics of Phylum Chlamydiae

Members of the genus Chlamydia are gram-negative, obligate intracellular pathogens. Transmission occurs via elementary bodies (EBs), which are metabolically inactive, endospore-like forms specialized for extracellular survival and host cell entry.

Life Cycle and Infection Process

  1. Entry: Elementary bodies enter host epithelial cells.

  2. Transformation & Replication: EBs transform into active reticulate bodies (RBs), which replicate inside the cell by consuming host ATP.

  3. Lysis & Spread: Host cell lysis releases newly formed EBs to infect adjacent cells or new hosts.

Pathogenic Species and Associated Diseases

  • Chlamydia trachomatis:

    • Trachoma: Eye infection that frequently leads to blindness.

    • Lymphogranuloma Venereum (LGV): Highly contagious STI common on college campuses; presents asymptomatically or with regional lymph node swelling.

  • Chlamydia psittaci:

    • Psittacosis: Systemic infection caused by Chlamydia psittaci.

Diagnostic Staining and Laboratory Characteristics

Chlamydia species stain poorly with traditional Gram staining despite their gram-negative classification. Visualization relies on identifying perinuclear inclusions using Giemsa stain on McCoy cell tissue culture (derived from human synovial fluid).