Genus Rickettsia: Properties of the Bacteria

Genus Rickettsia

Rickettsia organisms cause a wide variety of diseases varyingconsiderably in severity from self-limiting illness to fulminat-ing, life-threatening infection.

Properties of the Bacteria

◗ Morphology

They are small, Gram-negative coccobacilli varying from 0.3–0.6 to 0.8–2 mm in size. They are nonmotile and noncapsu-lated. They are stained poorly with Gram stain but are stained well with the following: deep red with Machiavello and Gimenez stain and bluish purple with Giemsa and Castaneda stain.

◗ Culture

Rickettsiae fail to grow on cell-free media. They usually grow inside the cell, usually in the cytoplasm (most Rickettsia) or in the nucleus of the cell (Rickettsia causing spotted fever).

      They grow well at optimum temperature of 32–358C. They grow in various cell lines, in the developing chick embryo, and also in many laboratory animals.

Cell lines: They grow on HeLa, Hep2, Detriot-6, mouse fibro-blasts, and other continuous cells lines. Cultures in the cell lines are used primarily for the maintenance of Rickettsia but are not useful for primary isolation of Rickettsia from clinical specimens.

Chick embryo: In the developing chick embryo 5–6 days old,Rickettsia spp. grow well in the yolk sac. The inoculated eggs areincubated at 358C for most Rickettsia spp. and at 338C for spot-ted group. The yolk sac is widely used as a source of Rickettsia for preparation of rickettsial antigens and vaccines. Rickettsia shows poor growth on chorioallantoic membrane.

Laboratory animals: Guinea pigs and mice are the commonlyused laboratory animals for isolation of Rickettsia organisms from animal specimens.

◗ Other properties

Susceptibility to physical and chemical agents: The extra-cellular Rickettsia organisms are very delicate microorganisms. They are rapidly killed by heating at 568C and also at room temperature. They are destroyed by usual strength of antiseptics, such as hypochlorite, 1% ethanol, 2% formaldehyde, 5% hydro-gen peroxide, and 70% ethanol. They are preserved at2708C or in a lyophilized state. They are preserved better in a special medium known as SPG medium containing sucrose, potassium phosphate, and glutamate, and also in the skimmed milk.