These are Gram-negative, nonsporing non-acid fast short rods (generally 0.3-10.5µm x 0.8-2.0µm in size). They may be pleomorphic and are non-motile. They can be stained by the Giemsa stain. They are aerobes. They will not grow on standard bacteriological media and most either exclusively grow intracellularly or in close association with eukaryotic cells such as in yolk sacs of chicken embryos or on epithelial cells of insect or arachnid gut. An erythrocyte factor and increased CO2 pressure are required for growth by some species, such as R. quintana. Glucose is not utilised but acquired via uridine-5'-diphosphoglucose. Glutamate is the preferred substrate. They have an ATP/ADP translocase, with which they exchange an external molecule of ATP for an internal one of ADP. The reaction does act in reverse. This is their main source of energy. They can synthesize most of their proteins and RNA. The base composition of the DNA is 28-45 mol%GC. Rickettsia is the type genus and the other genera are Rochalimaea, Ehrlichia, Cowdria and Neorickettsia.

Genera

Rickettsia: These are currently divided into two groups and a possible third group:

Group 1: These strains comprise the Typhus group. The base composition of the DNA is 28.5-30.5 mol%GC.

The species include: R. canada, R. prowazekii and R. typhi.

Group 2: These strains comprise the Spotted fever group. The base composition of the DNA is 32.0-33.2 mol%GC.

The species include: R. conorii, R. japonica, R. montana, R. rhipicephali and R. cickettsii, and a number of as yet unclassified types.

Group 3: These strains comprise the Scrub Typhus group. The base composition of the DNA is 35±0.5 mol%GC. The species include R. tsutsugamushi.

Rochalimaea: The base composition of the DNA is 38.8-39.8 mol%GC.

Ehrlichia:

Cowdria:

Neorickettsia:

Species Profiles

Rickettsia

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