Biology of ehrlichiae
- Daniel J Sexton, MD
Daniel J Sexton, MD
- Editor-in-Chief — Infectious Diseases
- Section Editor — Bacterial Infections
- Professor of Medicine
- Duke University Medical Center
Although ehrlichiae have been known to cause disease of interest to veterinarians for over 50 years, their role as agents of human disease was not recognized until 1987. Since then, the number of ehrlichiae that cause human disease has rapidly expanded, as has our knowledge about the epidemiology, clinical characteristics, and treatment of the diseases caused by these organisms. (See "Human ehrlichiosis and anaplasmosis".)
Ehrlichiae are obligate intracellular bacteria that grow within membrane-bound vacuoles in human and animal leukocytes. In humans, either monocytes (human monocytic ehrlichiosis; HME) or granulocytes (human granulocytic ehrlichiosis; HGE, the organism is now called Anaplasma phagocytophilum, and the disease anaplasmosis, HGA) may be infected. Ehrlichiae replicate within the phagosomes of the host cell, unlike rickettsiae, which grow in the cytoplasm of infected cells. A microcolony of ehrlichiae within a vacuole is called a morula (picture 1A-B). (See "Biology of Rickettsia rickettsii infection".)
The cell walls of ehrlichiae contain glycoproteins but do not have peptidoglycans or lipopolysaccharides . Ehrlichiae have distinct ribosomes that are homogeneously distributed in the cytoplasm and are surrounded by thin bileaflet outer and inner membranes.
Ehrlichiae are difficult to detect by light microscopy when they exist as single organisms; however, morulae are easily visualized when present within white blood cells. Ehrlichiae can be stained with Gram's, Giemsa, Wright, and silver stains, and by the Macchiavello method. Individual organisms are generally round, but may be pleomorphic when visualized in tissue culture cells . Thus far, ehrlichiae have only been cultured in eukaryotic cells and in yolk sacs.
The gene for a 120-kD protein in Ehrlichiae chaffeensis contains a series of tandem repeat units, the number of which differs among isolates of E. chaffeensis . As an example, the Arkansas and Jax strains contain four and the Sapulpa and St. Vincent strains contain three repeat units .
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