Borrelia

Lore & Background

Borrelia species are members of the family Spirochaetaceae and present the characteristic spirochete spiral shape. Most species are obligate anaerobes, although some are aerotolerant. They have an outer membrane containing a substance similar to lipopolysaccharides, an inner membrane, and a layer of peptidoglycan in a periplasmic space, classifying them as Gram-negative, though this result is not easily visualized using Gram staining. They are typically 20–30 μm long and 0.2–0.3 μm wide. Spirochetes move using axial filaments called endoflagella in their periplasmic space, rotating between the outer membrane and the peptidoglycan layer to propel the bacterium forward in a corkscrew-like motion. The outer membrane contains outer surface proteins (Osp) that play a role in their virulence. Of 52 known species of Borrelia, 20 are members of the Lyme disease group (with an additional 3 proposed), 29 belong to the relapsing fever group, and two are members of a genetically distinct third group typically found in reptiles. A proposal has been made to split the Lyme disease group based on genetic diversity and move them to their own genus, Borelliella, but this change is not widely accepted. A few Borrelia species such as 'Candidatus Borrelia mahuryensis' harbor intermediate genetic features between Lyme disease and relapsing fever Borrelia. Hard ticks of the family Ixodidae are common vectors for Lyme disease Borrelia, but not all Borrelia species are transmitted by hard ticks; relapsing fever Borrelia are transmitted by soft ticks (e.g., Ornithodoros) or lice. Lice that feed on infected humans acquire Borrelia organisms, but the U.S. Centers for Disease Control and Prevention reported that no credible evidence shows that lice can carry Borrelia.

Reader's Guide

Borrelia is significant as the bacterial genus responsible for two major human diseases: Lyme disease and relapsing fever. Lyme disease, caused by species such as Borrelia burgdorferi, Borrelia afzelii, and Borrelia garinii, is a widespread zoonotic illness transmitted by hard ticks. Relapsing fever, caused by species like Borrelia recurrentis and Borrelia hermsii, can be spread by lice or soft ticks and often involves severe bacteremia. The genus comprises 52 known species, with ongoing taxonomic debate about splitting the Lyme disease group into a separate genus, Borelliella. Diagnosis relies on two-tiered serological testing, including immunoassays and immunoblotting, as well as direct culture and polymerase chain reaction. The bacteria's unique spiral shape and periplasmic flagella enable corkscrew-like motility, aiding their transmission and virulence. Understanding Borrelia is critical for public health, as tick-borne diseases continue to emerge globally, and the bacteria's complex interactions with vectors and hosts pose ongoing challenges for prevention and treatment.

Did You Know?

• Borrelia species must be viewed using dark-field microscopy, which makes the cells appear white against a dark background.
• Of 52 known species, 20 belong to the Lyme disease group, 29 to the relapsing fever group, and two to a genetically distinct third group found in reptiles.
• For Lyme disease Borrelia, ticks must have around 36 to 48 hours of contact with a host to successfully transmit the bacteria; however, relapsing fever Borrelia can be transmitted in minutes.
• A proposal to move Lyme disease group species to a new genus, Borelliella, is not widely accepted.