Abstract
Chlamydia trachomatis manipulates host cellular pathways to ensure its proliferation and survival. Translocation of host materials into the pathogenic vacuole (termed ‘inclusion’) may facilitate nutrient acquisition and various organelles have been observed within the inclusion, including lipid droplets, peroxisomes, multivesicular body components, and membranes of the endoplasmic reticulum (ER). However, few of these processes have been documented in living cells. Here, we survey the localization of a broad panel of subcellular elements and find ER, mitochondria, and inclusion membranes within the inclusion lumen of fixed cells. However, we see little evidence of intraluminal localization of these organelles in live inclusions. Using time-lapse video microscopy we document ER marker translocation into the inclusion lumen during chemical fixation. These intra-inclusion ER elements resist a variety of post-fixation manipulations and are detectable via immunofluorescence microscopy. We speculate that the localization of a subset of organelles may be exaggerated during fixation. Finally, we find similar structures within the pathogenic vacuole of Coxiella burnetti infected cells, suggesting that fixation-induced translocation of cellular materials may occur into the vacuole of a range of intracellular pathogens.
Highlights
Chlamydia trachomatis is a human pathogen of global significance–it is the most common sexually-transmitted bacterial pathogen and the leading cause of preventable blindness worldwide [1]
We report that fluorescently tagged markers of the endoplasmic reticulum (ER), mitochondria, and inclusion membranes are readily observed within the lumen of fixed Chlamydia trachomatis inclusions
These internalized structures resist many common immunostaining procedures and were detected by immunofluorescence microscopy, indicating that select cellular materials detected within the inclusions of fixed cells may overestimate the true localization in living cells
Summary
Chlamydia trachomatis is a human pathogen of global significance–it is the most common sexually-transmitted bacterial pathogen and the leading cause of preventable blindness worldwide [1]. During infection of mammalian cells, bacteria remain sequestered within a membranebound compartment termed an inclusion. The inclusion is fundamental to the intracellular lifestyle of C. trachomatis as it represents a first line of defense against immune surveillance and anti-microbial effectors [2]. As an obligate intracellular pathogen, C. trachomatis requires vital nutrients obtained from its infected host cell (reviewed in [3]) and interactions between the inclusion and host cellular components play an important role in this process (reviewed in [4]). Many subcellular organelles reside in close proximity to the inclusion, some of which have been reported to interact intimately with inclusions (reviewed in [4]). In a well-characterized example, endoplasmic reticulum (ER) tubules closely appose to C. trachomatis inclusion
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