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Chlamydiae are obligate intracellular bacteria whose cells lack peptidoglycan and who share a common replicativc cycle that involves two forms: elementary and reticulate bodies (Figure 39–1). Elementary bodies (EBs) are smaller, have rigid cell walls, can survive outside cells, and are infectious. Once EBs attach to the cell membranes of susceptible cells they enter the cell by endocytosis and transform into larger, but fragile reticulate bodies (RBs) that multiply by binary fission and form more EBs that are released by exocytosis or cell rupture to infect adjacent cells and begin the cycle anew. Despite their biologic similarities, the Chlamydia are diverse in their tropisms and clinical features even within a single species. Chlamydia trachomatis primarily produces infections of the conjunctiva or genital tract depending on which biovar is involved. Trachoma is a progressive conjunctivitis with inflammation and scarring leading to blindness and is caused by C trachomatis biovars A, B, and C. Sexually transmitted biovars D-K cause urethritis, cervicitis, salpingitis, and neonatal infections of the eye and respiratory tract after vaginal delivery of infected mothers. L biovars of C trachomatis causes lymphogranuloma venereum (LGV), a sexually transmitted disease that manifests as painless genital ulcers followed by painful suppuration of regional inguinal lymph nodes. LGV biovars can also cause ulcerative proctitis, rectal fistulae, and strictures.

Chlamydophila species cause atypical pneumonia as the genus name implies. Chlamydophila psittici causes psittacosis, a zoonotic pneumonia contracted by inhalation of respiratory secretions or aerosols of cloacal droppings of infected birds of diverse species. Chlamydophila pneumoniae is a cause of community-acquired pneumonia that mirrors Mycoplama pneumoniae in its person-to-person transmission, clinical features, and treatment.

FIGURE 39–1.

Chlamydia life cycle. A. Fluorescence light micrograph of human cells (red) infected with C. trachomatis (green). B. A transmission electron micrograph of human cells that contain RBs (white arrows), EDs (black arrows), and an intermediate form called “aberrent bodies” (black arrowhead). C. A schematic representation of the infectious cycle of Chlamydiae.

Members of the genus Chlamydia are obligate intracellular bacteria which lack peptidoglycan in their cell wall. Three of the nine species cause disease in humans. Chlamydia trachomatis is the most important human pathogen as a major cause of genital infection and conjunctivitis. A chronic form of C trachomatis conjunctivitis, called trachoma, is the leading preventable cause of blindness in the world. Chlamydophila pneumoniae and Chlamydophila psittaci are respiratory pathogens. Our knowledge of biology and pathogenesis of these bacteria is based primarily on the study of C trachomatis.



Chlamydia trachomatis are round cells between 0.3 and 1 μm in diameter depending on the stage in the replicative cycle (see below). Their envelope is ...

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