Rickettsiae

Morphology and Physiology

In terms of cell wall structure, rickettsiae do not differ from typical Gram-negative bacteria (containing peptidoglycan and LPS), although they are prone to polymorphism. They lack flagella, but possess pili and fimbriae for adhesion.

Unlike chlamydiae, rickettsiae are not strict "energy parasites". They have their own independent metabolism and are capable of synthesizing ATP. The main substrate for energy production is glutamate, which participates in the Krebs cycle.

Due to their structural features, they stain poorly with standard bacterial dyes. Specific microscopy methods are used for their visualization:

• Macchiavello–Zdrodovsky stain (the pathogen stains red, and infected cells stain blue);
• Romanowsky–Giemsa stain (bacteria acquire a bluish-purple hue);
• Gimenez stain (red bacteria against a green background).

Life cycle and pathogenesis

Rickettsiae invade highly specialized host cells (primarily the vascular endothelium). Adhesion is mediated by surface proteins, and for cellular entry, the bacteria use their own enzyme, phospholipase A2, which degrades the lipids of the cell membrane.

Reproduction occurs via binary division in the cytoplasm (and in some species, such as R. rickettsii, also in the cell nucleus). One division cycle takes from 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 the death of the host cell as a result of lysis of the overcrowded vacuole or by "budding". The destruction of the endothelium triggers a cascade of pathological reactions: release of arachidonic acid, changes in 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:

1. Typhus group (associated with lice and fleas)
2. Rickettsia prowazekii — the causative agent of epidemic typhus. Transmitted by body lice. The reservoir is humans.
3. Rickettsia typhi — causes endemic (murine) typhus. The reservoir is rodents, vectors are fleas.

1. Spotted fever group (associated with ticks)
2. Rickettsia rickettsii — causes severe Rocky Mountain spotted fever.
3. Rickettsia sibirica — the causative agent of North Asian tick typhus.
4. Rickettsia conorii — Mediterranean spotted fever (boutonneuse 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 the female to her offspring).

Cultivation and diagnostics

Since rickettsiae are obligate parasites, they do not grow on artificial nutrient media. Laboratory cultivation utilizes:

• Cell cultures (fibroblasts, umbilical vein endothelium);
• Chicken embryos (inoculation into the yolk sac);
• Laboratory animals (guinea pigs, mice).

In routine medical practice, the main diagnostic method is serological tests (IHA, IFA) aimed at detecting specific antibodies in the patient's blood serum. Tetracycline antibiotics are traditionally used to treat rickettsioses, with doxycycline being the drug of choice.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Characteristics of antigenic structure: the role of OmpA and OmpB proteins in species differentiation.
• Mechanisms of chronic rickettsioses and the pathogenesis of Brill–Zinsser disease.
• Cell wall structural features of bacteria of the genus Orientia (the causative agent of tsutsugamushi disease).
• Specifics of the immune response and coagulation system imbalance in rickettsial panvasculitis.

Other topics in the section "Morphology and Classification of Microbes"

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