Rickettsiae Microbiology

 

Rickettsiae

Rickettsiae are obligate, intracellular, very small (0.3 x 1-2 µm), Gram-negative bacilli. They multiply within the cytoplasm of eukaryotic cells. They have a very small genome. Due to their small size, they were initially mistaken for viruses. However, they are classified as bacteria because:

o   They possess typical Gram-negative cell walls.

o   They contain both DNA and RNA, enzymes for the Krebs cycle, and ribosomes for protein synthesis.

o   They multiply by binary fission.

o   They are susceptible to antibiotics.

Rickettsiae are primarily pathogens of arthropods like lice, fleas, ticks, and mites, residing in their intestinal tracts. They are typically transmitted to humans by arthropod vectors (lice, mites, ticks). Coxiella burnetii, the cause of Q fever, is an exception and is usually transmitted by airborne droplets. Infected humans harbor rickettsiae in their reticuloendothelial cells and vascular endothelium. They are positioned phylogenetically between bacteria and viruses.

Classification

Rickettsiae belong to the order Rickettsiales, tribe Rickettsiae, and family Rickettsiaceae. The family Rickettsiaceae includes three genera: Rickettsia, Orientia, and Ehrlichia. The genus Coxiella was previously included but is now excluded because, unlike other genera in Rickettsiaceae, Coxiella are not primarily obligate intracellular parasites and can grow in cell-free media.

Properties of Rickettsiae

• Morphology: Small Gram-negative coccobacilli (0.3-0.6 to 0.8-2 µm), nonmotile and noncapsulated. They stain poorly with Gram stain but stain well with Machiavello, Gimenez, Giemsa, and Castaneda stains.
• Culture: Rickettsiae fail to grow on cell-free media. They usually grow inside cells, primarily in the cytoplasm (most Rickettsia and Orientia) or in cytoplasmic vacuoles (Coxiella and Ehrlichia).
• They can be grown in various cell lines (HeLa, Hep2, Detroit-6, mouse fibroblasts), but this is mainly for maintenance, not primary isolation.
• They grow well in the yolk sac of developing chick embryos (5-6 days old) incubated at 32-35°C (33°C for spotted fever group). The yolk sac is used for preparing antigens and vaccines. They show poor growth on the chorioallantoic membrane.
• Guinea pigs and mice are commonly used laboratory animals for isolation.
• Metabolism: They synthesize proteins and produce adenosine diphosphate via the tricarboxylic acid cycle. They are suggested to be energy parasites, utilizing ATP from the host cell. Some Chlamydia strains also depend on the host for amino acids.
• Cell Wall: Typical Gram-negative structure with lipopolysaccharide (LPS) and peptidoglycan. They lack flagella and are surrounded by a loose slime layer.
• Sensitivity to Physical and Chemical Agents: Extracellular rickettsiae are delicate and are rapidly killed by heating at 56°C and at room temperature. They are destroyed by common antiseptics (hypochlorite, ethanol, formaldehyde, hydrogen peroxide). They are preserved at -70°C or in a lyophilized state, and better in SPG medium or skimmed milk.

Antigenic Structure

Rickettsiae possess three types of antigens:

• Group-specific antigen: A soluble surface antigen.
• Species- or strain-specific antigen: Present in the cell wall.
• Alkali-stable polysaccharide antigen: A surface antigen found in some Rickettsia species and some Proteus species (OX19, OX2, OXK). This cross-reactivity forms the basis of the Weil-Felix test.

Pathogenesis of Rickettsial Infections

• After inoculation, rickettsiae reach the circulation, multiply, causing rickettsemia.
• They localize in the endothelial cells of small blood vessels (arterial, capillary, and venous).
• Intracellular multiplication leads to endothelial cellular hyperplasia, resulting in multiorgan vasculitis.
• This process can lead to thrombosis and the development of small nodules. Thrombosis of blood vessels can cause gangrene of the extremities.
• Vasculitis increases vascular permeability, leading to edema, loss of blood volume, hypoalbuminemia, and reduced osmotic pressure.
• Adhesins (outer membrane proteins) are important virulence factors that facilitate entry into host cells.

Laboratory Diagnosis

• Diagnosis involves isolation of rickettsiae (high safety precautions needed) or serological tests.
• Culture: Isolation is carried out by intraperitoneal inoculation of clinical specimens into male guinea pigs or mice. Animal response varies depending on the species. Rickettsiae can also be grown in cell cultures (Vero cells), identified by immunofluorescence. Primary isolation in eggs is not common.
• Serodiagnosis:
• Weil-Felix test: A heterologous agglutination test detecting antirickettsial antibodies that cross-react with Proteus O antigens (OX19, OX2, OXK). It becomes positive 10-20 days after infection. Different rickettsial diseases show varying reactions with these Proteus strains (Table 48-3). It may be negative or weakly reactive in Brill-Zinsser disease.
• Direct detection of rickettsial antigen can be done on skin biopsies.
• Molecular Diagnosis: PCR is used to detect R. rickettsiae DNA in skin biopsy specimens.

Important Human Diseases Caused by Rickettsiae and Related Genera

• Genus Rickettsia:
• Rickettsia prowazekii: Epidemic or louse-borne typhus, relapsing louse-borne typhus (Brill-Zinsser's disease). Transmitted by the body louse.
• Rickettsia typhi: Endemic or flea-borne murine typhus. Transmitted by the rat flea or cat flea.
• Rickettsia rickettsiae: Rocky Mountain spotted fever. Transmitted by wood tick, American dog tick, Lone star tick.
• Rickettsia akari: Rickettsial pox. Transmitted by the mouse mite.
• Rickettsia conori: Boutonneuse fever (Kenya tick bite fever, African tick typhus, Mediterranean spotted fever, Indian tick typhus, Marseilles fever). Transmitted by ticks.
• Rickettsia australis: Queensland tick typhus. Transmitted by ticks.
• Rickettsia sibirica: North Asian tick typhus. Transmitted by ticks.
• Genus Orientia:
• Orientia tsutsugamushi: Scrub typhus. Transmission involves mites.
• Genus Ehrlichia:
• Ehrlichia sennetsu: Sennetsu fever. No tick vector; possibly ingestion of raw fish.
• Ehrlichia chaffeensis: Monocytic ehrlichiosis. Transmitted by the Lone star tick.
• Ehrlichia phagocytophila: Human granulocytic ehrlichiosis. Transmitted by Ixodes ticks.
• Genus Coxiella:
• Coxiella burnetii: Q fever. Usually transmitted by airborne droplets from infected animals (farm animals, cats, dogs, rabbits). Ticks maintain infection in animals but do not typically transmit to humans.

Treatment

• Tetracyclines, chloramphenicol, and fluoroquinolones (like ciprofloxacin) are effective against R. rickettsiae.
• Doxycycline or chloramphenicol are highly effective for rickettsial pox.
• Doxycycline is the drug of choice for human ehrlichiosis.

 Prevention and Control

• Preventive measures include using protective clothing and insect repellents, and avoiding tick-infested areas.
• No vaccine is available against Rocky Mountain spotted fever or ehrlichiosis.

 Reference

Parija, S. C. (2012). Textbook of Microbiology & Immunology (2nd ed.). Elsevier, a division of Reed Elsevier India Private Limited.