Borrelia burgdorferi

Borrelia burgdorferi
Name	Borrelia burgdorferi
Domain	Bacteria
Phylum	Spirochaetota
Class	Spirochaetia
Order	Spirochaetales
Family	Borreliaceae
Genus	Borrelia
Species	B. burgdorferi
Binomial	Borrelia burgdorferi
Binomial authority	Burgdorfer et al., 1984

Borrelia burgdorferi is a spirochetal bacterium and the primary etiological agent of Lyme disease in North America. It was first isolated in 1982 by Willy Burgdorfer from the black-legged tick in Shelter Island, New York. This pathogen is maintained in an enzootic cycle involving Ixodes ticks and various mammalian and avian reservoir hosts, most notably the white-footed mouse.

Biology and morphology

Borrelia burgdorferi is a member of the Spirochaetales order, characterized by its distinctive helical shape and highly motile nature due to endoflagella located within its periplasmic space. The bacterium's outer membrane lacks lipopolysaccharide with typical endotoxic activity but contains numerous surface-exposed lipoproteins, such as OspA and OspC, which are critical for its life cycle. Its morphology and motility are essential for disseminating through dense tissues like the dermis and synovial fluid, facilitating its pathogenic spread. The organism is microaerophilic and has complex nutritional requirements, making it difficult to culture in standard laboratory media, a trait that historically complicated its discovery and study.

Genomics and genetics

The genome of Borrelia burgdorferi strain B31 was completely sequenced in 1997, revealing a highly segmented and redundant genetic structure. It consists of a linear chromosome of approximately 910 kilobase pairs and a multitude of both linear and circular plasmids, some of which are essential for infectivity and survival in different hosts. Key genetic elements, like the vlsE locus on plasmid lp28-1, enable antigenic variation through recombination, allowing the bacterium to evade the host's adaptive immune system. The genome exhibits a high degree of genetic plasticity, with significant variation observed between strains isolated from different geographical regions, such as those in Europe compared to North America.

Pathogenesis and disease

Infection typically begins following the bite of an infected Ixodes tick, with the bacterium migrating from the dermis to disseminate via the bloodstream and lymphatic system. The primary clinical manifestation is erythema migrans, an expanding skin rash, though untreated infection can lead to more severe sequelae involving the joints, heart, and nervous system, a condition often referred to as Lyme neuroborreliosis. Pathogenesis is mediated by the bacterium's ability to adhere to host extracellular matrix components and its induction of a potent inflammatory response, driven largely by its lipoproteins interacting with toll-like receptor 2 on immune cells. Persistent infection is associated with the pathogen's ability to resist complement system killing and to colonize immune-privileged sites.

Epidemiology and transmission

The geographic distribution of Lyme disease caused by this bacterium is tightly linked to the range of its primary tick vectors, Ixodes scapularis in the northeastern and north-central United States and Ixodes pacificus in the Pacific Northwest. The enzootic cycle is maintained in nature between these ticks and reservoir hosts like the white-footed mouse and certain species of birds. Human incidence peaks during the summer months when nymphal tick activity is highest. Notable endemic areas include regions of New England, the Mid-Atlantic states, Wisconsin, and Minnesota, with thousands of cases reported annually to the Centers for Disease Control and Prevention.

Diagnosis and detection

Clinical diagnosis is often supported by a two-tiered serological testing algorithm recommended by the Centers for Disease Control and Prevention, involving an initial enzyme-linked immunosorbent assay followed by a confirmatory Western blot. Direct detection methods, such as polymerase chain reaction on samples from synovial fluid or cerebrospinal fluid, can be useful in specific disseminated cases but lack sensitivity for early disease. The characteristic erythema migrans rash is considered diagnostic in endemic areas, though its absence complicates identification. Challenges in diagnosis stem from the slow and variable humoral immune response and potential cross-reactivity with other spirochetal diseases like syphilis.

Treatment and prevention

The standard treatment for early localized Lyme disease is a course of oral antibiotics, such as doxycycline, amoxicillin, or cefuroxime axetil. For more severe manifestations like Lyme carditis or neuroborreliosis, intravenous regimens with ceftriaxone or penicillin G are often employed. Primary prevention focuses on personal protective measures against tick bites, including the use of DEET repellents, permethrin-treated clothing, and prompt tick checks after outdoor activities in endemic areas. Environmental strategies involve landscaping practices to reduce tick habitats, and there is a recombinant OspA vaccine (LYMErix) that was previously marketed but is no longer available.

Category:Spirochaetes Category:Bacteria described in 1984