Rickettsia - Classification, Morphology, Culture, Medically important species

Classification of Rickettsia

Rickettsia can be classified as follows:

Domain: Bacteria

Phylum: Pseudomonadota

Class: Alphaproteobacteria

Order: Rickettsiales

Family: Rickettsiaceae

Tribe: Rickettsieae

Genus: Rickettsia

Morphology of Rickettsia

Morphologically, Rickettsia spp. is gram-negative bacilli that are obligate, intracellular, fastidious, and pleomorphic, with a size of 0.3 x 1-2 μm. They are non-capsulated, non-motile with a loose slime layer. The bacteria also contain LPS and peptidoglycan later while LPS shows weak endotoxic activity. They are poorly Gram-stained but stained well with Giemsa and Castaneda staining.

Rickettsia multiply by binary fission within the cytoplasm of eukaryotic cells. The target cells are the reticuloendothelial system and the release of mature rickettsiae results in the lysis of host cells.

Rickettsia has a very small genome composed of 1-1.5 million bp.

Rickettsiae are primary pathogens of arthropods (lice, fleas, ticks, mites) and are present in the intestinal tract- transmitted by arthropod vectors.

Culture of Rickettsia

Rickettsia spp. fails to grow in cell-free media. It can also grow in the cell line, chick embryo, and animal inoculation.

Cell lines such as HeLa, Hep 2, are used to maintain culture for primary isolation. Culture in yolk sack done for vaccine preparation. Animal inoculation is done in guinea pigs and mice.

The optimum temperature for culture is around 32-35°C.

Medically important Rickettsia