Rickettsia rickettsii (Rocky Mountain spotted fever) and other Rickettsia species

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Rickettsia rickettsii (Rocky Mountain spotted fever) and other Rickettsia species


Introduction to bacteria

Bacterial structure and functions


Rickettsia rickettsii (Rocky Mountain spotted fever) and other Rickettsia species


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Rickettsia rickettsii (Rocky Mountain spotted fever) and other Rickettsia species

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A 16-year-old boy is brought to the clinic to evaluate for flu-like symptoms and rash.  The rash started two days ago and was initially on the trunk. It has now spread over the entire body, sparing the palms and soles. The patient also had malaise, myalgia, vomiting, and watery diarrhea two days before the rash eruption. The patient owns a cat and was recently bit by a mouse. Temperature is 102.5 ºF (39.2 ºC), pulse is 110/min, blood pressure is 120/80 mmHg, and respirations are 22/min. On examination, there is a maculopapular rash on the trunk and extremities, sparing the palms and soles. Several cat scratches are noted on the forearms. Which of the following is the most likely source of transmission of this disease?  

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Rickettsia typhi p. , 147, 148


Content Reviewers

The Rickettsiae are a genus of Gram-negative coccobacilli, which includes two major groups of bacteria.

First, there’s the spotted-fever group, the main species in this group is Rickettsia rickettsii, which causes a disease called Rocky Mountain spotted fever.

Second, there’s the typhus group of Rickettsia species - which cause different forms of typhus.

This group includes Rickettsia prowazekii, which causes a disease called epidemic typhus, and Rickettsia typhi, which causes murine typhus, also called endemic typhus.

Now, Rickettsiae are small bacteria, measuring only 0.7 to 2 micrometers in diameter.

They have a plasma membrane that’s surrounded by a microcapsule.

And inside the bacteria, there’s cytosol, which contains ribosomes and a single circular chromosome.

Also, these bacteria have a thin wall, that doesn’t retain the crystal violet dye during gram staining, so they’re classically considered Gram-negative bacteria.

However, they are very weak Gram-negative bacteria, so special staining methods are needed to visualize them, such as Giemsa, Gimenez or Machiavello.

So, on Giemsa staining, the bacteria appear bluish-purple, on Gimenez staining they look red, on a bluish-green background and on Machiavello staining they look bright red, on a blue background.

Finally, they’re non-motile and obligate intracellular which means they can survive only inside cells and this is because it can’t make two important energetic compounds, NAD+ and coenzyme A, by itself, and instead it gets them from eukaryotic cells.

So, they can be grown in vitro in the yolk sac of developing chicken embryos, but they are more conveniently cultured on cell culture monolayers, such as chicken embryo fibroblasts, mouse L cells, and golden hamster cells.


Rickettsiae are a genus of Gram-negative coccobacilli, which includes Rickettsia rickettsii known to cause Rocky Mountain spotted fever (RMSF); and Rickettsia prowazekii that causes epidemic typhus; and Rickettsia typhi, which is known to cause endemic typhus. Rickettsia rickettsii is transmitted through ticks; Rickettsia prowazekii through lice; whereas Rickettsia typhi is transmitted through rat fleas. Symptoms of Rickettsiae infections depend on the type but may include fever, headache, malaise, myalgias, arthralgias, and skin rashes. Rickettsiae can be diagnosed with serologic tests such as indirect immunofluorescence or with special stains on a skin biopsy and they can be treated with antibiotics like doxycycline or chloramphenicol.


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