Chlamydia trachomatis is the causative agent of a several human diseases including trachoma, the leading cause of infectious blindness worldwide, and is the most common form of sexually transmitted disease in the U.S. and developed countries. Chlamydiae are obligate intracellular bacterial that undergo their life cycles entirely within an intracellular vesicle that is isolated from established routes of intracellular trafficking. Although chlamydiae obviously acquire essential nutrients from the host cell, the mechanisms for obtaining these across the inclusion membrane are also unknown. Whereas the majority of intracellular parasites are thought to block maturation of the endocytic vesicle to a lysosome, chlamydiae dissociate themselves from this pathway and establish a functional interaction with an exocytic pathway which delivers sphingolipids from the Golgi apparatus to the plasma membrane. One of the initial events in chlamydial infection is the expression of a chlamydial gene product(s) that effectively isolates the inclusion from the endosomal/lysosomal pathway and initiates fusion competence with a subset of exocytic vesicles. Surprisingly, although chlamydiae intercept sphingolipids in transit to the plasma membrane, the processing and export of cellular glycoprotiens is not inhibited. Thus, chlamydiae interrupt a specific cellular pathway with minimal interference of normal cellular function. Collectively, the data suggest that the chlamydial inclusion occupies a site distal to the trans-Golgi apparatus with properties of an exocytic vesicle in which fusion with the plasma membrane is inhibited or delayed. These results show potential not only for defining the interactions of chlamydiae with the host cell but will serve as a model system for other obligate intracellular pathogens which occupy vacuoles that do not fuse with lysosomes. Understanding the initial events in chlamydial differentiation including the transition in properties of the endocytic vesicle to one which intersects an exocytic pathway remains a significant challenges in understanding the pathogenic mechanisms of chlamydiae.
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