Structure of the outer membrane

Since B. burgdorferi s.l. possesses both an outer membrane and a cytoplasmic membrane, the surface of this bacterium is analogous to enteric Gram-negative bacteria. However, its membrane composition differs from that of Gram-negative bacteria and has several distinct features (Fig. 3.2). For example, B. burgdorferi s.l. has an extraordinarily high abundance of membrane proteins covalently modified with lipids (Brandt et al., 1990; Fraser et al., 1997). Another characteristic is the absence of phosphatidylethanolamine (Belisle et al., 1994) and lipopolysaccharide (LPS) (Takayama et al., 1987), and the presence of glycolipid antigens other than LPS (Eiffert et al., 1991; Wheeler et al., 1993; Belisle et al., 1994). Glycolipids represent about 50% of the total lipids and comprise only galactose as monosaccharide constituents (Berg, 2001; Hossain et al., 2001). The existence of LPS on the surface of B. burgdorferi s.l. has been controversial, although most evidence currently points to its absence (Beck et al., 1985; Takayama et al., 1987; Cinco et al., 1991; Schoenfeld et al., 1992; de Souza et

B. burgdorferi

E. coli

Fig. 3.2. Comparison of Borrelia burgdorferi sensu lato and Escherichia coli cell envelopes. IM, inner membrane; LPS, lipopolysaccharide; OM, outer membrane; P, protein.

al., 1992). The B. burgdorferi outer membrane contains a relatively low density of transmembrane-spanning proteins as determined by freeze-fracture electron microscopy studies (Walker et al., 1991; Radolf et al., 1994; Jones et al., 1995). This may explain why B. burgdorferi is more susceptible to disruption by routine physical manipulations such as centrifugation and resuspension and is more susceptible to detergents compared with Gram-negative bacteria. It is very likely that the unusual surface of this bacterium contributes to the unique properties that B. burgdorferi s.l. possesses, e.g. the ability to survive in both the mammalian host and the tick and to evade the mammalian immune system. Thus, a comprehensive analysis of the membrane architecture and constituents would yield insight into the unique physiological mechanisms by which the bacterium survives in diverse environments and may also elucidate pathogenic mechanisms operative during LB.

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