Electron Microscopic Observations of Orientia tsutsugamushi in Salivary Gland Cells of Naturally Infected Leptotrombidium pallidum Larvae during Feeding

Abstract

We performed a detailed electron microscopic observation on the escaping process of Orientia tsutsugamushi from the salivary gland cells of naturally infected trombiculid larvae into the acinar lumen of the gland during feeding on mice. In unfed larvae, many O. tsutsugamushi were intermingled with secretory granules in the cytoplasm of the salivary gland cell. O. tsutsugamushi was neither found in the acinar lumen nor observed escaping from the apical surface of the gland cell. In contrast, in the larvae fed on mice, many O. tsutsugamushi were observable in the acinar lumen. They were enveloped with the host glandular cell membrane. In salivary gland cells, secretory granules changed the distribution and accumulated in the apical region. In such cells, the majority of O. tsutsugamushi were found at the base of the cell. Some O. tsutsugamushi were pushing the glandular cell membrane outward in various degrees, showing different stages of escape. These findings suggest that larval feeding induced O. tsutsugamushi escape from salivary gland cells, that the escape was by budding, during which O. tsutsugamushi were enveloped in the host cell membrane, and that O. tsutsugamushi would be injected into the mouse skin as a mixture with mite saliva. The study also revealed the presence of many small vesicles that had the same cell wall structure as O. tsutsugamushi in the cytoplasm of the salivary gland cell. Most of them seemed to be products from degenerated Orientia.

Orientia tsutsugamushi in Human Scrub Typhus Eschars Shows Tropism for Dendritic Cells and Monocytes Rather than Endothelium

“…These microparticles stained strongly positive with the monoclonal antibody targeting the surface type specific antigen p56. Similar microbial protuberances leading to shedding of microparticles have been described in salivary gland cells of the transmitting vectors (trombiculid mites) in EM studies, with an apparently similar membrane structure [33]. They may represent particles of degenerating O. tsutsugamushi .…”

Diagnosis of scrub typhus: recent advancements and challenges

“…Scrub typhus in humans is caused by the larval bite of trombiculid mite that harvests the bacteria ( Orientia tsutsugamushi) in its salivary gland (Kadosaka and Kimura 2003 ). In this, infection and activation of endothelial cells assist in the morbid physiology of Scrub typhus and involved in infection of several organs including heart, kidney, skin, pancreas, and brain.…”

“…Some elements were recognized that assist in bacterial ingress process including transmembrane glycoproteins like α5β1 co-localized Orientia by activating integrin-activated signal transduction pathways (Fig. 1 ) (Kadosaka and Kimura, 2003 ). There are different defense mechanisms involved in the diminishing infection of an intracellular bacterium.…”

An Update on Host-Pathogen Interplay and Modulation of Immune Responses during Orientia tsutsugamushi Infection

“…Although the physiopathology of this disease is not fully understood, immunemediated processes are thought to contribute, along with direct bacterial damage, to cause local or disseminated vasculitis that drives the pathological mechanism (20,35). Humans acquire the pathogen during the feeding of trombiculid mite larvae (chiggers), which host Orientia tsutsugamushi in salivary glands (36). Humans are incidental hosts of Leptotrombidium species and dead-end hosts of O. tsutsugamushi.…”