The genus Borrelia is included in the bacterial family Spirochaetaceae, and the order Spirochaetales. The spirochete isolated from ticks and humans was identified as Borrelia by W. Burgdorfer, et al. in 1982. In Europe, clinical signs of human borreliosis were described for the first time by Afzelius in 1910 and Lipschütz in 1913 in patients.
Besides the genus Borrelia the family Spirochaetaceae also contains the genera Spirochaeta, Treponema and Cristispira. The genus Borrelia is heterogeneous and includes a complex of species that can be functionally summarised as relapsing-fever borrelia, such as B. recurrentis (endemic louse-borne relapsing fever in humans), B. lonestari (southern tick-associated rash illness in humans) and B. anserina (avian spirochetosis in poultry) (Krupka and Straubinger, 2010). All borrelia that are transmitted by Ixodes ticks and are associated with Lyme borreliosis infections are functionally grouped in the B. burgdorferi sensu lato complex, which currently comprises 16 species. For several species in this group which have recently been identified after isolation from ticks in North America, the pathogenicity in people or animals has not been proved so far. For the other species causing Lyme borreliosis, worldwide distribution and the effect of the disease is a complicated issue (Krupka and Straubinger, 2010). In North America only the species B. burgdorferi sensu stricto (s.s.) has been found to be pathologic in humans and in dogs. In contrast, in central Europe, Scandinavia and parts of Asia, at least 3 species have been identified to cause clinical apparent Lyme borreliosis in humans: B. burgdorferi s.s., B. garinii, and B. afzelii. The experimental proof that natural infection with B. afzelii and B. garinii occurs in dogs is still missing, but DNA of these species was found in naturally infected dogs (Hovius et al., 1999; Speck et al., 2001) B. spielmanii and B. lusitaniae have also been isolated from skin lesions found on human patients and thus are believed to be causative agents of Lyme borreliosis.
Canine borreliosis is caused by B. burgdorferi s.l., which is transmitted by ticks of the Ixodes genus, like the Castor Bean tick Ixodes ricinus. In Northern Europe, the distribution of borreliosis is expanding due to the spread of its parasitic vector. Infected I. ricinus ticks are now commonly found in urban areas. Much of the published borreliosis research is related to the species B. burgdorferi s.s., which is the primary isolate in humans and dogs in the USA.
Borreliae are phylogenetic old bacteria that neither can be classified as Gram-negative nor as Gram-positive. Borrelia bacteria including Borrelia burgdorferi are flexible helical cells, 10-25 µm long by 0.2-0.3 µm in diameter, consisting of a protoplasmic cylinder, a lateral ridge containing a clear area, the periplasmic space, and 7 to 11 periplasmic flagella (also termed axial fibrils or endoflagella). The cells are surrounded by a cell membrane, a cell wall - an outer membrane that is loosely associated with the underlying structures - and a mucoid layer. Axial filaments just below the outer envelope extend throughout the length of the cells. The axial filaments are believed to be responsible for the characteristic corkscrew-like motion of the spirochetes.
Two of the outer surface membrane proteins, OspA and OspB (Osp=outer surface protein), are antigenic and are useful in differentiating these organisms from other spirochetal species.
The outer membrane of B. burgdorferi and other Borrelia is unique in the way that genes encoding these proteins are located on linear plasmids; these extrachromasomal genes determine the antigenic identity of these organisms and presumably help the bacteria adapt and survive in ticks and different mammalian hosts.
B. burgdorferi can be cultivated from their arthropod vectors or vertebrate hosts in a modified Kelly medium called Barbour-Stoenner-Kelly (BSK II)-medium. Borrelia from ticks and often skin biopsy samples, rarely from the blood or cerebrospinal fluid of Lyme disease patients have been successfully cultivated in BSK. BSK solidified with 1.3% agarose allows the production of colonies from single organisms. B. burgdorferi grows slowly as compared to most other bacteria. Each spirochete divides into two cells after 12 to 24 hours of elongation. Cultivation of Borrelia is very difficult, because of a long time needed for development (6-8 weeks) and a medium which also supports the growth of other bacteria and fungi, potentially reducing or even blocking Borrelia growth. Thus a sterile sampling technique is of major importance.
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