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Journal of Clinical Microbiology, August 1999, p. 2518-2524, Vol. 37, No. 8
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Invasion and Intracellular Development of the Human Granulocytic Ehrlichiosis Agent in Tick Cell Culture

Ulrike G. Munderloh,1,* Steven D. Jauron,1 Volker Fingerle,2 Lorenz Leitritz,2 S. Fred Hayes,3 Joan M. Hautman,1 Curtis M. Nelson,4 Brent W. Huberty,4 Timothy J. Kurtti,1 Gilbert G. Ahlstrand,5 Barbara Greig,6 Martha A. Mellencamp,7 and Jesse L. Goodman4

Departments of Entomology,1 Plant Pathology,5 and Clinical and Population Sciences7 and Veterinary Diagnostic Laboratories,6 University of Minnesota, St. Paul, Minnesota, Rocky Mountain Laboratory, National Institutes of Health, Hamilton, Montana,3 and Division of Infectious Diseases, Department of Medicine,4 and Max von Pettenkofer Institut,2 Universität München, Munich, Germany

Received 19 January 1999/Returned for modification 7 April 1999/Accepted 29 April 1999

Human granulocytotropic ehrlichias are tick-borne bacterial pathogens that cause an acute, life-threatening illness, human granulocytic ehrlichiosis (HGE). Ehrlichias within neutrophil granulocytes that invade tick bite sites are likely ingested by the vector, to be transmitted to another mammalian host during the tick's next blood meal. Thus, the cycle of replication and development in the vector is prerequisite to mammalian infection, and yet these events have not been described. We report tick cell culture isolation of two strains of the HGE agent directly from an infected horse and a dog and have also established a human isolate from HL60 culture in tick cells, proving that the blood stages of the HGE agent are infectious for tick cells, as are those replicating in the human cell line HL60. This required changes to the culture system, including a new tick cell line. In tick cell layers, the HGE agent induced foci of infection that caused necrotic plaques and eventual destruction of the culture. Using the human isolate and electron microscopy, we monitored adhesion, internalization, and replication in vector tick cells. Both electron-lucent and -dense forms adhered to and entered cells by a mechanism reminiscent of phagocytosis. Ehrlichial cell division was initiated soon after, resulting in endosomes filled with numerous ehrlichias. During early development, pale ehrlichias with a tight cell wall dominated, but by day 2, individual bacteria condensed into dark forms with a rippled membrane. These may become compacted into clumps where individual organisms are barely discernible. Whether these are part of an ehrlichia life cycle or are degenerating is unknown.

* Corresponding author. Mailing address: Department of Entomology, University of Minnesota, 219 Hodson Hall, 1980 Folwell Ave., St. Paul, MN 55108. Phone: (612) 624-3688. Fax: (612) 625-5299. E-mail: munde001{at}maroon.tc.umn.edu.

Journal of Clinical Microbiology, August 1999, p. 2518-2524, Vol. 37, No. 8
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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