Borrelia is a genus of bacteria of the spirochete phylum.[1] Several species cause Lyme disease, also called Lyme borreliosis, a zoonotic, vector-borne disease transmitted by ticks. Other species of Borrelia cause relapsing fever, and are transmitted by ticks or lice, depending on the species of bacteria.[2] A few Borrelia species as Candidatus Borrelia mahuryensis harbor intermediate genetic features between Lyme disease and relapsing fever Borrelia.[3] The genus is named after French biologist Amédée Borrel (1867–1936), who first documented the distinction between a species of Borrelia, B. anserina, and the other known type of spirochete at the time, Treponema pallidum.[4] This bacterium must be viewed using dark-field microscopy,[5] which make the cells appear white against a dark background. Borrelia species are grown in Barbour-Stoenner-Kelly medium.[5] Of 52 known species of Borrelia, 20 are members of the Lyme disease group (with an additional 3 proposed),[6] 29 belong to the relapsing fever group, and two are members of a genetically distinct third group typically found in reptiles.[4] A proposal has been made to split the Lyme disease group based on genetic diversity and move them to their own genus, Borelliella,[7] but this change is not widely accepted.[4] This bacterium uses hard and soft ticks and lice as vectors.[8][9] Testing for the presence of the bacteria in a human includes two-tiered serological testing, including immunoassays and immunoblotting.[10]
Borrelia species are members of the family Spirochaetaceae, so present the characteristic spirochete (spiral) shape. Most species are obligate anaerobes, although some are aerotolerant.[11] 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 classifies them as Gram-negative.[5] However, this result is not easily visualized using Gram staining.[5] They are typically 20–30 μm long and 0.2–0.3 μm wide.[5]
Spirochetes move using axial filaments called endoflagella in their periplasmic space.[5] The filaments rotate in this space, between the outer membrane and the peptidoglycan layer, propelling the bacterium forward in a corkscrew-like motion.[5] The outer membrane of Borrelia species contains outer surface proteins (Osp) that play a role in their virulence.[5]
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)[12] and National Center for Biotechnology Information (NCBI).[13]