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

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.