Wolfram R. Zückert, Brandon L. Jutras, Alvaro M. Toledo and Sven Bergström
spirochaetes are often, but mistakenly, described as Gram-negative bacteria due to their diderm, i.e. double-membrane envelopes, a closer examination reveals significant differences in composition and architecture. Particularly striking is the lack of classical endotoxin/lipopolysaccharide. Instead, glycolipids and surface lipoproteins dominate the the host-pathogen interface, where they play important roles during transmission, persistence and ensuing pathogenic processes. A modified peptidoglycan cell wall is also emerging as a potent pathogenicity determinant, in addition to contributing to cell shape in concert with periplasmic flagella. While surface lipoproteins such as the Osps interact with a variety of ligands in different organ tissues, they are also targets of the immune response and several have emerged as vaccine candidates. Some of the identified periplasmic lipoproteins, i.e., the OppAs, are components of substrate transport complexes. Investigations into integral membrane proteins led to the identification of several Borrelia
porins: P13, whose structure and function is unknown, DipA, which is specific for dicarboxylates, and P66, which has a dual role as a pore-forming outer membrane protein and adhesin. Tol homologs BesA, -B, and -C appear to form a Type I 'channel' to export exogenous toxic agents such as antibiotics and maintain infectivity by an unknown mechanism. Initial studies on envelope biogenesis pathways and mechanisms based on diderm proteobacterial model organisms have revealed significant deviations from the Gram-negative norm, further bolstering the unique status of Borrelia
among microbial pathogens.