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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 of the Gram-negative type including an outer membrane that contains lipopolysaccharide and proteins. A major difference is that chlamydiae lack the thin peptidoglycan layer between the outer membrane and the plasma membrane. Although there is no detectable peptidoglycan in chlamydial cells, genomic studies have demonstrated an almost complete set of genes for peptidoglycan synthesis. The outer membrane includes a major outer membrane protein (MOMP) which is immunogenic. Chlamydia are obligate intracellular parasites because they rely on the host cell for key amino acids and energy generating metabolites like ATP. Among bacteria only the mycoplasmas have a smaller genome.

Envelope has no peptidoglycan layer between membranes

Obligate intracellular growth requires metabolites from host cell

DNA homology between C trachomatis, C psittaci, and C pneumoniae is less than 30%, although rRNA sequence analysis suggests they share a common origin. The three species share a common group antigen. Their major differential features are shown in Table 39–1. Chlamydia trachomatis has three each with a different tissue tropism. Biovars A-C infect ocular epithelial cell and cause trachoma; biovars D-K target urogenital epithelial cells and cause nongonococcal urethritis (NGU), mucopurulent cervicitis, and inclusion conjunctivitis; and biovars L1-L3 infect genital colorectal tissues causing lymphogranuloma venereum (LGV).

TABLE 39–1Features of Human Chlamydia and Chlamydophila Infection


The replicative cycle of chlamydiae is illustrated in Figure 39–1. It involves two major forms of the organism: a small, hardy infectious form termed the elementary body (EB), and a larger fragile intracellular ...

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