Borrelia burgdorferi

Appearance

''B. burgdorferi'' resembles other spirochetes in that it has an outer membrane and inner membrane with a thin layer of peptidoglycan in between. However, the outer membrane lacks lipopolysaccharide. Its shape is a flat wave. It is about 0.3 μm wide and 5 to 20 μm in length.

''B. burgdorferi'' is a microaerobic, motile spirochete with seven to 11 bundled perisplasmic flagella set at each end that allow the bacterium to move in low- and high-viscosity media alike, which is related to its high virulence factor.

Behavior

''Borrelia burgdorferi'' is named after the researcher Willy Burgdorfer, who first isolated the bacterium in 1982. ''Borrelia'' species is the species complex known to cause Lyme disease are collectively called ''Borrelia burgdorferi''.''B. burgdorferi'' circulates between ''Ixodes'' ticks and a vertebrate host in an enzootic cycle. ''B. burgdorferi'' living in a tick cannot be passed to its offspring: it is acquired through blood meals. Once a tick is infected, it will then transmit ''B. burgdorferi'' by feeding on another vertebrate to complete the cycle. Ticks can transmit ''B. burgdorferi'' to humans, but humans are dead-end hosts, unlikely to continue the life cycle of the spirochete. Nymphs molt into adult ticks, which usually feed on larger mammals that are not able to support the survival of ''B. burgdorferi''.

Evolution

Genetically diverse ''B. burgdorferi'' strains, as defined by the sequence of ''ospC'', are maintained within the Northeastern United States. Balancing selection may act upon ''ospC'' or a nearby sequence to maintain the genetic variety of ''B. burgdorferi''. Balancing selection is the process by which multiple versions of a gene are kept within the gene pool at unexpectedly high frequencies. Two major models that control the selection balance of ''B.burgdorferi'' is negative frequency-dependent selection and multiple-niche polymorphism. These models may explain how ''B. burgdorferi'' have diversified, and how selection may have affected the distribution of the ''B. burgdorferi'' variants, or the variation of specific traits of the species, in certain environments.