The periplasmic flagella (PFs), located in the periplasmic space between the cytoplasm-peptidoglycan layer and the outer membrane, have an essential role in cell morphology and motility (Sadziene et al., 1991; Goldstein et al., 1994, 1996; Motaleb et al., 2000). The distinct cell morphology of this bacterium is caused by the flagella that are wrapped around the protoplasmic cylinder (Burgdorfer et al., 1982; Hovind Hougen, 1984; Barbour and Hayes, 1986;
Goldstein et al., 1996). Each PF is attached at only one end and in Borrelia spp. they are long enough to overlap in the centre of the cell (Johnson et al., 1984). The genes encoding the flagellar proteins are flaA (Ge and Charon, 1997a), flaB (Wallich et al, 1990), flgE (Jwang et al, 1995), fliH and fliI (Ge et al, 1996), and the flgK operons (Ge et al., 1997), and are located on the chromosome. These genes have homologies to various genes in the flagellar apparatus of both animal and plant pathogens. This suggests that the flagellar apparatus and its protein export pathways are well conserved. Swimming B. burgdorferi s.l. cells have a flattened wave-like shape similar to that of eukaryotic flagella (Goldstein et al., 1994) A striking feature of B. burgdorferi s.l. is its capacity to swim efficiently in a viscous medium such as connective tissue where other bacteria are slowed down or immobilized (Kimsey and Spielman, 1990; Goldstein et al., 1994). Motility in B. burgdorferi s.l. requires an environment similar to interstitial fluid (e.g. pH 7.6, 0.15 M NaCl and viscous) (Shi et al., 1998). The relative immobility of B. burgdorferi s.l. spirochaetes in less viscous media suggests active dissemination via the skin, as optimal mobility of borreliae is seen in collagen-like viscosity (Kimsey and Spielman, 1990). Furthermore, virulence studies on a spontaneously occurring non-flagellated, non-motile, mutant strain indicate that motility is important for the pathogenesis of this organism (Sadziene et al., 1991, 1996).
The spirochaete motility apparatus is similar to that of other bacteria in that it has a filament, hook and basal body (Holt, 1978; Hovind Hougen, 1984; Barbour et al., 1986; Brahamsha and Greenberg, 1988; Charon et al., 1992). However, the PFs of spirochaetes differ from the flagella of most other bacteria as they are composed of two classes of proteins instead of one class (Cockayne et al., 1987; Norris et al., 1988; Koopman et al., 1992; Trueba et al., 1992; Rosey et al., 1995; Ge et al., 1998). These two classes are the flagellar outer sheath protein FlaA and the core protein FlaB. FlaA is unique to spirochaetes whereas FlaB shows homology to flagellar proteins of other bacteria (Magnarelli et al., 1987; Wallich et al., 1990). Until recently it was a general belief that the PFs of B. burgdorferi differed from those of other spirochaetes since the flagellar core protein FlaB was not thought to be surrounded by an outer sheath protein. However, using a modified extraction technique, Ge and Charon identified a FlaA homologue, showing 54-58% similarity to FlaA proteins from other spirochaetes (Ge and Charon, 1997a; Ge et al., 1998). In other spirochaetes, the FlaA and FlaB proteins are among the most abundant cell proteins (Koopman et al., 1992; Li et al., 1993; Norris, 1993). Depending on species, the PFs consist of one to two different FlaA proteins and two to four different FlaB proteins (Brahamsha and Greenberg, 1988; Norris et al., 1988; Charon et al., 1992; Koopman et al., 1992; Trueba et al., 1992; Li et al., 1993; Ge et al., 1998). In B. burgdorferi, the PFs are composed of only FlaA (39 kDa) and FlaB (41 kDa), where FlaB is one of the most abundant proteins and FlaA is expressed in lower amounts.
Chemotaxis studies have revealed that rabbit serum acts as an attractant while ethanol and butanol act as repellents (Shi et al., 1998). Interestingly, B. burgdor-feri does not exhibit chemotaxis towards common sugars or amino acids (Shi et al., 1998). It has also been noted that spirochaetes grown in tissue chambers implanted in rats (host-adapted bacteria) show more vigorous motility compared with spirochaetes grown in vitro (Akins et al., 1998). The chemotaxis genes cheA, cheE (Trueba et al., 1997), cheW and cheY (Fraser et al., 1997; Ge and Charon, 1997b; Trueba et al., 1997) have been identified in B. burgdorferi s.l. In E. coli, chemotaxis is achieved by regulating the direction of rotation of the flagellar bundle. In B. burgdorferi and other spirochaetes, this is not easily achieved since they have two flagellar bundles, one at each cell end. In order to switch the direction of swimming, they have to rotate in different directions and then switch synchronously. This type of motility is observed as smooth swimming, used when an attractant is present. Another type of observed swimming pattern, a flexing movement, is observed in the presence of repellents. The flexing movement is the result of two bundles rotating in the same direction (Charon et al., 1992; Goldstein et al., 1994; Shi et al., 1998).
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