Ehrlichia and Anaplasma are two genera of Gram-negative pleomorphic bacteria, which means they can take different shapes - round like a coccus, or coccobacillary, which means somewhere between a spherical coccus and a rod-like bacillus.
The most common species that cause disease in humans are Ehrlichia chaffeensis, which causes a disease called human monocytic ehrlichiosis, or HME, and Anaplasma phagocytophilum, which causes a disease called human granulocytic anaplasmosis, or HGA.
Now, Ehrlichia and Anaplasma have a thin peptidoglycan layer, so they don’t retain the crystal violet dye during Gram staining.
Instead, like any other Gram-negative bacteria, they stain pink with safranin dye.
Both are non-motile, non-spore forming, and obligate intracellular which means they can survive only inside cells.
Finally, Ehrlichia and Anaplasma don’t grow on routine culture media and they need to be cultivated in vitro in different cell lines.
So, Ehrlichia chaffeensis can be isolated in DH82 canine histiocytic cell line and Anaplasma phagocytophilum can be isolated in promyelocytic leukemia HL-60 cell line.
Now, Ehrlichia and Anaplasma enter circulation following a tick bite and once inside the body, they infect circulating leukocytes.
Ehrlichia primarily targets monocytes and macrophages, and it infects them using tandem repeat proteins, or TRP.
These bacterial proteins bind to proteins found on the surface of the cell, and they induce phagocytosis - so basically, they make the cell gobble up the bacteria.
Anaplasma, on the other hand, primarily targets neutrophils, and infects them with the help of a P-selectin glycoprotein which binds on the P-selectin glycoprotein ligand-1, or PSGL-1 found on the surface of neutrophils.
This activates an intracellular pathway that leads to reorganization of cellular actin which leads to phagocytosis, allowing Anaplasma to enter the cell.
Once inside the cell, both Ehrlichia and Anaplasma live in an early endosome, which normally merge with lysosomes to kill invading bacteria.