Rickettsia: obligate intracellular bacteria and their medical significance
Rickettsia is a genus of small, obligate intracellular bacteria transmitted by arthropods. They cause several human febrile illnesses and require living cells for growth; treatment relies on specific antibiotics.
Overview
Rickettsia is a genus of small, obligate intracellular bacteria that live and multiply inside the cells of animals and arthropod vectors. Although they share some lifestyle features with viruses, rickettsiae are true bacteria with bacterial ribosomes and metabolic machinery. For information on their taxonomic placement see classification resources, and for contrasts with viruses consult comparative material.
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Biology and distinguishing features
Members of the genus are gram-negative–type organisms with a cell envelope resembling that of other proteobacteria, but they often stain poorly in routine preparations. Because they are adapted to life inside host cells they lack some pathways for independent survival and must obtain nutrients from their hosts. Their dependence on cells for replication differentiates them from free-living bacteria; for more on bacterial distinctions see bacterial references and on cell structure see cell wall descriptions.
Diseases, vectors and examples
Rickettsia species cause important human illnesses transmitted by ticks, lice, fleas and mites. Notable examples include:
- Rocky Mountain spotted fever (Rickettsia rickettsii)
- Epidemic typhus (Rickettsia prowazekii)
- Murine (endemic) typhus (Rickettsia typhi)
Clinical presentations typically include fever, headache and sometimes a characteristic rash. Prevention emphasizes control of the arthropod vectors and avoidance of bites.
Diagnosis and treatment
Diagnosis combines clinical suspicion with laboratory tests such as serology, PCR and specialized culture methods. Because rickettsiae are sensitive to certain antibiotics, early empiric therapy with tetracycline-class agents (for example doxycycline) is commonly recommended for suspected cases. For information about antibiotic therapy see antibiotic guidance.
History and laboratory cultivation
Rickettsial organisms were historically difficult to cultivate because they require living host cells. A key advance was the development of techniques to grow them in the yolk sac of embryonated chicken eggs; this method, pioneered in the early 20th century, helped researchers identify species and study disease mechanisms. One notable contribution to these cultivation methods came from investigators at Vanderbilt University; see a historical reference at Vanderbilt research.
Notable distinctions and research notes
Rickettsia are members of the order Rickettsiales within the Alphaproteobacteria; some related organisms (for example Orientia) were reclassified as knowledge of genetics and biology advanced. Modern research continues to explore host interactions, vector ecology and vaccine development, while public-health measures remain focused on vector control and rapid treatment.
Properties
Rickettsiae are Gram-negative, highly polymorphic (polymorphic, pleomorphic) organisms that do not form spores. They are often round (cocci) to oval bacteria with a diameter of 0.1 µm; they may also appear as rods (1-4 μm long) or filamentous (10 μm long). Occasionally they form chains, but usually they occur singly or in pairs. The survival of obligate intracellular rickettsiae depends entirely on their eukaryotic host cell (usually endothelial cells), into whose cytoplasm they must invade to be protected from the host defense system. Reproduction by transverse division also takes place inside the host cell. The bacteria are subsequently released by strangulation from the cell membrane (exocytosis) or by lysis, which destroys the host cell. At least R. conorii is capable of movement within the host cell.
Due to their dependence on the host cell, the bacteria cannot be kept in artificial culture media in the laboratory. They are therefore grown either in biological tissues or embryo cultures (typically chicken embryos are used). Because of their cell dependency and their reduced metabolism, rickettsiae were in the past often classified as microorganisms somewhere between viruses and the larger true bacteria, as an "intermediate species". For a long time, they were also referred to as "large viruses".
Mechanism of cell invasion
How rickettsiae manage to penetrate eukaryotic cells has been a mystery until now. At the end of 2005, scientists at the Institut Pasteur in Paris succeeded in identifying two key proteins involved in the penetration process using Rickettsia conorii. These are the bacterial protein rOmpB and the mammalian protein Ku70, which is normally found in the nucleus of mammalian cells. Apparently, however, it can also migrate to the cell membrane, where it is captured by Rickettsia's own rOmpB and used to enter the cell. The scientists also referred to Ku70 as the "molecular stooge" of the rickettsiae because of this "treacherous" property.
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AlegsaOnline.com Rickettsia: obligate intracellular bacteria and their medical significance Leandro Alegsa
URL: https://en.alegsaonline.com/art/82805