Transmission Electron Microscopy Reveals Distinct Macrophage- and Tick Cell-Specific Morphological Stages of Ehrlichia chaffeensis

Abstract

Background Ehrlichia chaffeensis is an emerging tick-borne rickettsial pathogen responsible for human monocytic ehrlichiosis. Despite the induction of an active host immune response, the pathogen has evolved to persist in its vertebrate and tick hosts. Understanding how the organism progresses in tick and vertebrate host cells is critical in identifying effective strategies to block the pathogen transmission. Our recent molecular and proteomic studies revealed differences in numerous expressed proteins of the organism during its growth in different host environments.Methodology/Principal FindingsTransmission electron microscopy analysis was performed to assess morphological changes in the bacterium within macrophages and tick cells. The stages of pathogen progression observed included the attachment of the organism to the host cells, its engulfment and replication within a morulae by binary fission and release of the organisms from infected host cells by complete host cell lysis or by exocytosis. E. chaffeensis grown in tick cells was highly pleomorphic and appears to replicate by both binary fission and filamentous type cell divisions. The presence of Ehrlichia-like inclusions was also observed within the nucleus of both macrophages and tick cells. This observation was confirmed by confocal microscopy and immunoblot analysis.Conclusions/SignificanceMorphological differences in the pathogen’s progression, replication, and processing within macrophages and tick cells provide further evidence that E. chaffeensis employs unique host-cell specific strategies in support of adaptation to vertebrate and tick cell environments.

Ehrlichia minasensis sp. nov., isolated from the tick Rhipicephalus microplus

“…2). Similarly, non-synchronized maturation was observed in E. chaffeensis replicating in the tick cell line ISE6 (Dedonder et al, 2012). Rarely, vacuoles were opened to the extracellular space.…”

Identification of Critical Host Mitochondrion-Associated Genes during Ehrlichia chaffeensis Infections

“…It is possible that Apop1 is needed to help in the release of bacteria at the end of the replicative phase. E. chaffeensis organisms disseminate by lysing host cells or by exocytosis in order to spread to uninfected cells (17). More empirical analyses will be needed to support this hypothesis.…”

“…Transmission electron microscopy (TEM) analysis of uninfected and E. chaffeensis-infected DH82 cultures was performed as described previously (17).…”

Ehrlichia moonlighting effectors and interkingdom interactions with the mononuclear phagocyte

“…The ehrlichial vacuole does not fuse with lysosomes, phenotypically resembles an early-endosomal vesicle, and contains vacuolar (H+) ATPase, transferrin, transferrin receptor, and major histocompatibility molecules [19]. Within an hour of endocytosis, the DC cell transitions to RC form and divides via binary fission, doubling every 8 h for the next 48 h [8, 20], The resulting microcolony (morula) within the endosomal-like compartment, which contains as many as 400 individual bacterium [19]. By 72 h post infection, the RC transitions into the DC morphology, and the bacteria exit the host cell through undefined mechanisms including direct cell lysis, exocytosis, or cell-cell transfer via filopodia [21].…”