Morphology and Physiology
In terms of cell wall structure, Rickettsia do not differ from typical Gram-negative bacteria (containing peptidoglycan and lipopolysaccharides [LPS]), but they tend to be pleomorphic. They lack flagella, but possess pili and fimbriae for adhesion.
Unlike Chlamydia, Rickettsia are not strict "energy parasites." They possess an independent metabolism and are capable of synthesizing ATP. The primary substrate for energy production is glutamate, which participates in the Krebs cycle.
Due to their structural features, they stain poorly with standard bacterial stains. Specific microscopy methods are used for visualization:
- Macchiavello or Zdrodovsky stain (organisms stain red, while infected cells stain blue);
- Giemsa stain (bacteria acquire a bluish-purple hue);
- Gimenez stain (red bacteria against a green background).
Life Cycle and Pathogenesis
Rickettsia invade highly specialized host cells (primarily vascular endothelium). Adhesion is mediated by surface proteins, and for entry, the bacteria use their own enzyme—phospholipase A2—which degrades cell membrane lipids.
Replication occurs via binary division in the cytoplasm (and in some species, such as R. rickettsii, also within the cell nucleus). One division cycle takes 6 to 14 hours. Within a few days, up to 1,000 bacteria can accumulate in a single cell.
The release of the pathogen is accompanied by host cell death resulting from the lysis of an overfilled vacuole or via budding. Endothelial destruction triggers a cascade of pathological reactions: release of arachidonic acid, alteration of vascular tone, development of panvasculitis (inflammation of the vessel wall), microthrombosis, and tissue edema.
Classification and Ecology
Depending on the vector and clinical manifestations, pathogenic species are divided into several groups:
- Typhu group (associated with lice and fleas)
- Rickettsia prowazekii — the causative agent of epidemic typhus. Transmitted by body lice. Humans are the reservoir.
- Rickettsia typhi — causes endemic (murine) typhus. Reservoirs are rodents, vectors are fleas.
- Spotted fever group (associated with ticks)
- Rickettsia rickettsii — causes severe Rocky Mountain spotted fever.
- Rickettsia sibirica — the causative agent of North Asian tick typhus.
- Rickettsia conorii — Mediterranean spotted fever.
Most tick-borne rickettsioses are zoonoses, where ticks act not only as vectors but also as natural reservoirs due to the mechanism of transovarial transmission (from female to offspring).
Cultivation and Diagnostics
Because Rickettsia are obligate parasites, they do not grow on artificial nutrient media. Laboratory cultivation requires:
- Cell cultures (fibroblasts, umbilical vein endothelium);
- Chicken embryos (inoculation into the yolk sac);
- Laboratory animals (guinea pigs, mice).
In routine medical practice, the primary diagnostic method is serological testing (indirect hemagglutination assay [IHA], indirect immunofluorescence assay [IFA]) aimed at detecting specific antibodies in the patient's serum. Tetracycline-class antibiotics are traditionally used to treat rickettsioses, with doxycycline being the drug of choice.