A genus of Gram-negative bacteria that
can exist in the form of cocci (0.1 μm in diameter), rods (1–4
μm long) or threads (10 μm long). Being obligatory intracellular organisms,
the Rickettsia survival depends (like viruses) on entry, growth,
and replication within the cytoplasm of eukaryotic host cells. Rickettsiae
are primarily parasites of arthropods, ticks
lice, fleas, and mites, but they can be transmitted
to the larger animals via insects mites. They are responsible for a number
of diseases in humans including typhus, scrub typhus, and Rocky
Mountain spotted fever; related organisms cause Q
fever and psittacosis. They are
sensitive to tetracyclines and cause
characteristic serological reactions cross-specific to Proteus bacteria.
| Rickettsia rickettsii.
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