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

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From: PLoS ONE(Vol. 7, Issue 5)
Publisher: Public Library of Science
Document Type: Article
Length: 6,924 words
Lexile Measure: 1500L

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Author(s): Sarah E. Dedonder 1 , Chuanmin Cheng 1 , Lloyd H. Willard 1 , Daniel L. Boyle 2 , Roman R. Ganta 1 , *

Introduction

Ehrlichia chaffeensis is a Gram negative obligate intracellular pathogen that is transmitted via the bite of an infected Amblyomma americanum tick to humans and several other vertebrate hosts [1]-[3]. This organism is responsible for an emerging disease, human monocytic ehrlichiosis (HME) [4], [5]. HME is characterized by an acute onset of febrile illness which can sometimes be a fatal disease. Clinical symptoms of the disease may resemble flu-like illness which may include malaise, headache, myalgia and persistent fever. Laboratory findings may include leukopenia, thrombocytopenia, and elevated liver transaminases [4]-[6].

E. chaffeensis and other related tick transmitted rickettsial pathogens are capable of persisting in both vertebrate and tick hosts [7]-[13]. The pathogens may have evolved unique strategies to establish infections in both invertebrate and vertebrate hosts in order to successfully complete their lifecycle in dual hosts. Persistent infection in ticks is also important as the organism cannot be transovarially transmitted. Our recent molecular and proteomic studies revealed global differences in the expressed proteins of E. chaffeensis within different host cell environments [13]-[16]. The pathogen's growth in different host cell environments is also a major contributor for its dual host adaptation and persistence [11]. The host cell-specific differences in the expressed proteins support the hypothesis that E. chaffeensis employs novel strategies to adapt and persist in both types of hosts, however, the exact mechanism of adaptation remains to be established.

In this study, we investigated ultrastructural differences in E. chaffeensis replicating in vertebrate and tick cells by employing transmission electron microscopy (TEM) analysis to assess if the organism differs in its progression. Specimens for TEM were prepared and observed under various magnifications ranging from 2,000x to 70,000 x. The pathogen progression stages described here included the attachment of the organism to the host cell membrane, its engulfment, replication within a morula by binary fission, and release of the organisms from infected host cells by complete host cell lysis or by exocytosis. We found evidence for unique host cell-specific differences in the organism's progression within phagosomes. In addition, our novel data suggest that E. chaffeensis enters into host nuclei.

Results

Morphological forms of E. chaffeensis

Transmission electron microscopic (TEM) analysis aided the visualization of diversity in the size and number of morulae containing E. chaffeensis organisms within invertebrate and vertebrate host cells. Two morphologically distinct forms (reticulate and dense core cells) were identified within the phagosomes of infected tick cells and macrophages. Although the two morphological forms observed for the first time for E. chaffeens is infection in tick cells, they are similar to the TEM data reported earlier for the organism in macrophage cultures [17]-[20]. The reticulate bodies had an even distribution of cytoplasmic structures, while the dense core cells contained condensed material considered to contain ribosomes and nucleoid material [17], [18]. E. chaffeensis in macrophages was relatively more synchronized compared to infected tick cells...

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Gale Document Number: GALE|A477083177