Borrelia is a genus of spirochetes, which are long, thin, spiral shaped bacteria, that spin or twist to move around.
The genus has two major groups. The first one is associated with Lyme disease and the most common species is Borrelia burgdorferi.
The second group is associated with relapsing fever and the most common species are Borrelia hermsii, Borrelia turicatae, Borrelia parkeri and Borrelia recurrentis.
Of the second group, the first three species cause tick-borne relapsing fever, while Borrelia recurrentis causes louse-borne relapsing fever.
By the way, you may know this already, but “louse” is actually singular for “lice” - so body lice are the vector that transmits Borrelia recurrentis.
Now, all Borrelia species have an outer membrane that contains a substance similar to lipopolysaccharides, an inner membrane, and a layer of peptidoglycan in a periplasmic space, which is the space between the inner and the outer membrane.
Since the peptidoglycan layer is thin, it doesn’t retain the purple dye used during Gram staining, so they’re classified as Gram-negative.
However, they can’t be easily visualized with Gram staining. Instead, they’re best seen with the Wright or Giemsa stain, which make the bacteria appear purple, and with dark-field microscopy, which make the bacteria appear white against a dark background.
Now, Borrelia species are motile, which means they can move around using long, thin filaments called endoflagella, which are located in their periplasmic space.
The filaments rotate in this space, between the outer membrane and the peptidoglycan layer, propelling the bacterium forward in a corkscrew-like motion.
Finally, the bacteria can be cultivated on Barbour-Stoenner-Kelly medium or by intraperitoneal inoculation of immature laboratory mice, but these tests are not widely available.
Now, Borrelia can enter the body one of two ways, depending on the type of relapsing fever.
So, in tick-borne relapsing fever, or TBRF, the bacteria enters the bloodstream through the bite of an infected tick.