The Genus Borrelia
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The Genus Borrelia MELISSA CAIMANO
Phylogeny The inability to cultivate many spirochetes, including the borreliae, at one time significantly hindered the placement of these organisms into a taxonomic scheme based on traditional biochemical methodologies. As a result, these organisms were initially grouped based primarily on their common helical or spiral shape (Fig. 1). The advent of contemporary ribosomal RNA cataloging has facilitated the division of the spirochetes into three phylogenetic families, the Spirochaetaceae (including the genera Borrelia, Brevinema, Cristispira, Spirochaeta, Spironema and Treponema), the Brachyspiraceae (including the genus Brachyspira [Serpulina]), and the Leptospiraceae (including the genera Leptospira and Leptonema; Thomas et al., 2001). All species in the genus Borrelia are transmitted to vertebrates by hematophagous (blood-feeding) arthropods. In most cases, the differentiation of borreliae species was based on 1) identification of the specific vector that transmits the spirochete, 2) the vertebrate host (humans, animals or birds), and 3) the variable infectivity of isolated borreliae for different species of laboratory animals (Davis, 1956; Felsenfeld, 1971; Burgdorfer, 1976a; Table 1). To date, thirty species of Borrelia have been recognized (Thomas et al., 2001). The principal species of Borrelia and their primary vectors are listed in Table 1. Additional information on B. burgdorferi sensu lato strains is also available: (http:// www.pasteur.fr/recherche/borrelia/Bb_ strains_ alphabetic.html.) With the exception of louseborne relapsing fever, all of the borrelioses are zoonoses. Borreliae, like most spirochetes, were historically considered to be Gram negative because of their double-membrane structure (Fig. 6), but as stated above, genetic analyses have placed them, along with the other spirochetes, into a separate eubacterial phylum (Paster et al., 1991). Ultrastructural (Borrelia Molecular Architecture), molecular, and biochemical studies also have emphasized the wide taxonomic gap between
spirochetes and Gram-negative bacteria. Most notably, Borrelia burgdorferi, and presumably all Borrelia spp., does not contain lipopolysaccharide (LPS; Takayama et al., 1987). In addition to lacking LPS, borrelial outer membranes differ from those of Gram-negative bacteria in that they 1) exhibit considerably greater fluidity, 2) are more susceptible to physical manipulations (i.e., centrifugation, washing and resuspension; Brusca and Radolf, 1994; Cox et al., 1994; Cox et al., 1996), 3) are more easily solubilized in very dilute detergents than are the membranes of Gram-negative bacteria (Barbour and Hayes, 1986b; Brusca and Radolf, 1994; Cox et al., 1994; Cox et al., 1996), and 4) have a much lower density of transmembrane proteins (Walker et al., 1991; Radolf et al., 1994b; Radolf et al., 1995b). (For excellent reviews of Borrelia morphology and ultrastructure, see Hovind-Hougen, 1976, Barbour, 1989, Hayes and Burgdorfer, 1993, and Radolf, 1994a.)
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