Rickettsia prowazekii

Rickettsia prowazekii is the pathogenic bacterium responsible for epidemic typhus fever, an infectious disease that has plagued humans ever since it was first recorded in Europe nearly 1000 years ago (Szybalski, 1999). More recently, R. prowazekii is estimated to have infected nearly 30 million humans following the First and Second World War (Andersson et al., 1998), and continues to pose a major health threat in certain parts of the world, despite the availability of antibiotics.

R. prowazekii is typically transmitted to humans by the body louse – an obligate ectoparasitic arthropod (Figure 1) (Baxter, 1996). In its vegetative state, the pleomorphic R. prowazekii is 0.3 μm (micrometres) to 0.5 μm by 0.8 μm to 2.0 μm in size (Walker, 1998), which makes this species too small to be clearly seen under an ordinary light microscope. R. prowazekii belongs to a large group of Gram-negative bacteria known as α-proteobacteria that multiply in eukaryotic cells. This suggests that they can only grow and reproduce within the living cells of their host. Although classified as Gram-negative bacteria, R. prowazekii are poorly stained by the Gram method and are better visualized using the Giemsa or Giménez stains (Raoult et al., 2004).

 Body louse

Figure 1. This image shows an adult female human body louse (Pediculus humanus humanus) along with two larval young.

Intracellular R. prowazekii often appear as short, paired or single, lanceolate-to-ovoid coccobacilli (Raoult et al., 2004). To undergo cellular respiration, this species requires oxygen as a terminal electron acceptor. Its envelope consists of three major layers: an innermost cytoplasmic membrane, a thin electron-dense rigid cell wall containing peptidoglycan, and an outer layer, which contains lipopolysaccharide endotoxin (Yu & Walker, 2005) and immunodominant surface-exposed proteins that provide structure or potentially contribute to host cell adhesion or other host cell interactions (Figure 2) (Raoult et al., 2004). Cells of R. prowazekii also possess intracytoplasmic invaginations of the plasma membrane – a morphological feature that resembles cristae found in the mitochondria. Unlike the mitochondria, however, members of this species possess a flagellum for motility – a feature not common among the family Rickettsiaceae (Yu & Walker, 2005).


Figure 2. A transmission electron micrograph revealing the cell envelop of R. prowazekii. The red arrow points to the outer layer, the yellow arrow points to the thin electron-dense rigid cell wall, and the blue arrow points to the innermost cytoplasmic membrane [196 000 X].

Since R. prowazekii requires an intracellular habitat, cells must be co-cultivated in tissue culture or yolk sac of developing chicken embryos (Figure 3, Figure 4); hence, a characteristic colonial morphology is inexistent. Under poor nutritional conditions, cells of R. prowazekii grow into long, 4 μm filaments and cease to multiply via transverse binary fission (Yu & Walker, 2005). However, when proper conditions are restored, they immediately divide into their ordinary short rod form and engage in extensive movements until released by the disruption of massively infected cells.

Figure 3. Rickettsia prowazekii bacteria growing inside human fibroblasts. The black arrows point to the bacteria and the 'n' indicates host cell nuclei.

 Figure 4. This transmission electron micrograph depicts a chicken embryo fibroblast infected with a large mass of R. prowazekii. The red arrows point to the bacteria, the blue arrow points to the fibroblast cell membrane, and the yellow arrow points to the nucleus of the fibroblast [1200 X].

The nutritional requirements of R. prowazekii, outside their host cell, are not known (Yu & Walker, 2005). What is known is that R. prowazekii acquires most of its amino acids from its host cell because only the genes associated with the biosynthesis of lysine and serine are present in its DNA (Yu & Walker, 2005). In addition, maximal growth of this species can only occur if there is a sufficient amount of host cell proline and serine or glycine (Yu & Walker, 2005). This implies that R. prowazekii has a highly permeable cytoplasmic membrane. Likewise, glycolytic intermediates and products, such as acetyl-CoA, must also be obtained by its host. Similar to the mitochondrion found in eukaryotic cells, this species produces its own physiological energy supply of ATP via the enzyme ATPase (Yu & Walker, 2005). This reason, along with the evidence suggesting that R. prowazekii shares a phylogenetic relationship with the mitochondria, is why some biologists speculate that this species may have given rise to the modern-day mitochondria. Altogether, to be able to identify this species for diagnostic purposes, samples containing bacterial cells must be cultivated in tissue culture at 35°C. Once nutrients are depleted, longer, more filamentous-like cells with prominent vacuoles will develop (Figure 5). Infected tissues stained with Giemsa should show bluish-purple organisms, whereas tissue stained with Giménez should show bright red organisms, with a decolorized background stain of pale greenish blue (Yu & Walker, 2005). Due to the inability to grow R. prowazekii on ordinary microbiological media, such as nutrient broth, diagnosis is often difficult (Ge et al., 2004).

Figure 5. This transmission electron micrograph shows a large number of free cytoplasmic R. prowazekii late in chicken embryo fibroblast infection. The red arrow shows an example of a single vacuole-like structure that appears when nutrients are low or depleted, and the yellow arrow points to bacteria dividing by binary fission  [13 600 X].

The entry of R. prowazekii into a human host cell (typically a phagocyte) involves three steps: attachment (via OmpA protein and a host cell receptor), internalization and escape from the phagosome, prior to lysosomal fusion (Figure 6) (Yu & Walker, 2005). By escaping phagosomal digestion, this evasion mechanism allows the pathogen to reside in the cytoplasm and escape MHC class II molecule-presentation. However, the pathogenic mechanism used particularly by R. prowazekii as a means to escape phagosomal entrapment is unclear, but may involve potential virulence factors, such as phospholipase A (Winkler and Miller, 1982), antigenic surface proteins, hemolysin, type IV secretion systems, or hydrolase enzymes (Ge et al., 2004).


Figure 6. This micrograph shows R. prowazekii in the process of escaping phagosomal entrapment. The red arrow points to a break in the phagosome membrane [31 350 X].

Once freely in the cytoplasm, bacteria replicate and spread cell-to-cell via the polymerization of host cell F-actin, seen as a comet-like tail (Figure 7). Propulsion by F-actin into long host cell projections known as filopedia precedes the release of R. prowazekii from the cell surface or their spread to adjacent endothelial cells (Figure 8). Growth of Rickettsiae in the endothelium results in damage to vascular integrity and thus the leakage of fluid into a vital organ such as the brain. The accumulation of fluid (edema) in the perivascular space may result in clinical encephalitis. Gamma interferon and tumor necrosis factor α, substances secreted by host immune cells, activate the infected endothelial cell to kill intracellular Rickettsiae via the creation of autophagosomes. Later, fusion of lysosomes with autophagosomes results in the digestion of dying R. prowazekii.

Figure 7. Rickettsiae are propelled through the host cell cytoplasm. The area points to the polymerization of actin.

Figure 8. Propulsion by F-actin into long host cell projections known as filopedia proceeds the release of R. prowazekii. The arrow points to a bacterium's point of release from an infected cell.

Body louse (Pediculus humanus humanus), the flying squirrel flea (Orchopeas howardii), and louse (Neohematopinus sciuropteri) are the most effective vectors in the spread of R. prowazekii-induced epidemic typhus fever. Once transmitted to the mammalian host through a bite via arthropod saliva, the bacteria are found principally in the endothelium of blood vessels, particularly in those of the brain, skin and heart. This, in turn, causes hyperplasia of endothelial cells and localized thrombus formation, resulting in petechial rash, fever – which may reach 39°C – and terminal shock. Finally, areas of poor hygiene, such as refugee camps during a major famine or natural disaster, are places where diseases such as typhus fever tend to flourish. In fact, due to recent outbreaks in parts of Europe, Africa, and South America, some experts support the notion that epidemic typhus is a reemerging public health problem (Ge et al., 2004).


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