Borrelia

Borrelia is a genus of helical, motile spirochetes within the phylum Spirochaetes. They are Gram-negative in nature but stain poorly with routine Gram methods and are typically visualized by dark-field or fluorescence microscopy. Borrelia cells are slender and long, measuring roughly 0.2–0.3 μm in diameter and several micrometers in length, with flagella located in the periplasm that confer axial motility.

Genomics and taxonomy: The genus includes the Lyme borreliae (such as B. burgdorferi sensu lato complex, including B. burgdorferi sensu stricto, B. afzelii, B. garinii) and relapsing fever borreliae (e.g., B. recurrentis, B. duttonii). Their genomes commonly consist of a relatively small linear chromosome along with numerous linear and circular plasmids, enabling extensive antigenic variation.

Ecology and transmission: Lyme borreliae are transmitted primarily by Ixodes ticks; relapsing fever borreliae are transmitted by body lice or soft ticks, depending on the species. The bacteria survive in a vertebrate host–tick or host–lice cycle, adapting to different vector environments through stage-specific gene expression and antigenic variation.

Clinical manifestations vary by group. Lyme borreliosis often begins with erythema migrans and flu-like symptoms, followed by disseminated infection affecting the skin, joints, nervous system, and heart. Relapsing fever presents with recurring febrile episodes linked to periodic antigenic variation of surface proteins, leading to immune evasion.

Diagnosis and treatment: Diagnosis relies on serology (ELISA or Western blot) for Lyme disease, supported by PCR or culture in certain contexts. Treatment typically uses doxycycline or amoxicillin for early disease, with IV ceftriaxone for neurologic or cardiac involvement; relapsing fever is treated with tetracyclines or penicillins. Preventive measures include tick avoidance, proper clothing, repellents, prompt tick removal, and, where applicable, lice control. Vaccines for humans have not been widely used in recent years.