Modes of Nutrient Transport Across the Chlamydia trachomatis Inclusion
Chlamydia trachomatis is a gram-negative obligate intracellular bacterium that has a significant public health toll. During infection, the bacteria alternate between a metabolically inert, infectious form (elementary body, EB), and a metabolically active, non-infectious form (reticulate body, RB). The process through which the bacteria alternate between the two forms, called the developmental cycle, occurs within the confines of a parasitophorous vacuole termed the inclusion. The inclusion represents an important aspect of chlamydial pathogenesis as it is the mediator between the host cell and the bacteria.
In order to establish and maintain infection, all pathogens must be able to acquire nutrients from the host. Vacuolar intracellular pathogens have developed three ways in which to acquire nutrients: (1) intercept vacuoles from the endosomal/lysosomal pathway or through the autophagosomal pathway, (2) have open channels that allow for free exchange between the host cytoplasm, and (3) possess a transport system. The inclusion of C. trachomatis does not acquire nutrients via fusion of host vesicles within the endosomal/lysosomal or autophagosomal pathways. It also does not possess an open channel as studies have shown the inclusion to exclude molecules larger than 520Da. These studies demonstrate that the two previously mentioned methods of nutrient acquisition are not utilized by C. trachomatis and suggest there exists a transport system within the chlamydial inclusion. No transport system within a bacterial vacuole has been reported, although it has been documented in parasitic vacuoles. It is intriguing to note that RBs align themselves along the inner inclusion membrane. Specifically there is a unique structural morphology indicative of a large macromolecular complex located at the junction between RB and integral to the inclusion membrane. This leads to the hypothesis that the chlamydial inclusion possesses a nutrient transport complex that links the RBs to the inclusion that is essential for nutrient acquisition.
Methionine and glucose analog localization was monitored in C. trachomatis infected HeLa cells. It was found that each analog exhibited a unique localization pattern that was dependent upon host and bacterial factors. Methionine was sequestered in the bacteria, while glucose was found in the lumen of the inclusion. This work was carried out in conjunction with the development of a novel protocol to isolate inclusions from the host cell. This would provide a means for direct manipulation of the inclusion, allowing for more detailed analysis of nutrient acquisition.