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.
