Rickettsia prowazekii - Classification, Morphology, Culture, Pathogenesis
Classification of Rickettsia prowazekii
The classification of Rickettsia prowazekii is done as follows:
Domain: Bacteria
Phylum: Pseudomonadota
Class: Alphaproteobacteria
Order: Rickettsiales
Family: Rickettsiaceae
Genus: Rickettsia
Species group: Typhus group
Species: prowazekii
Morphology of Rickettsia prowazekii
Morphologically, Rickettsia prowazekii is gram-negative bacilli that are pleomorphic, obligate, intracellular, fastidious bacteria which is of 0.3 x 1-2 μm in size. The bacterium has a small genome composed of 1-1.5 million bp.
They multiply by binary fission within the cytoplasm of eukaryotic cells. The target cells for the infection of the bacterium include the reticuloendothelial system which releases mature rickettsiae resulting in the lysis of host cells.
Rickettsiae are primary pathogens of arthropods (lice, fleas, ticks, mites) and are present in the intestinal tract- transmitted by arthropod vectors.
Moreover, Rickettsia prowazekii is non-capsulated and non-motile with the presence of a loose slime layer. It has an LPS and peptidoglycan layer while LPS shows weak endotoxic activity. Although they are poorly Gram-stained, they have stained well with Giemsa and Castaneda staining.
Culture of Rickettsia prowazekii
Rickettsia prowazekii fails to grow in cell-free media but can grow in the cell line, chick embryo, and animal inoculation. However, 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 the culture of Rickettsia prowazekii is around 32-35°C.
Rickettsia prowazekii is the causative agent of epidemic/louse-borne typhus and relapsing louse-borne typhus. It infects vascular endothelial cells, producing widespread vasculitis.
Pathogenesis of Rickettsia prowazekii
Rickettsia prowazekii is transmitted via contact with broken skin and feces of infected lice, and inhalation of aerosols of dried nouse faces, through the conjunctiva. It adheres to host cells via adhesins. Microorganisms are localized in endothelial cells of small arterial capillaries and venous vessels.
Phagocytosis occurs but microorganisms escape from the phagosome before phagolysosome formation to the cytoplasm. It multiplies by binary fission in the cytoplasm until the cell is packed and thin bursts in the endothelial cellular hyperplasia occur resulting in multiplying on vasculitis.
With increased vascular permeability with consequent edema, loss of blood volume, hypoalbuminemia, reduced OP, hypotension, thrombosis, and development of small nodules occur.