Structure, Function and Biogenesis of the Borrelia Cell Envelope
Sven Bergström and Wolfram R. Zückert
from: Borrelia: Molecular Biology, Host Interaction and Pathogenesis (Edited by: D. Scott Samuels and Justin D. Radolf). Caister Academic Press, U.K. (2010)
Over the past two decades, modern molecular tools have been crucial in shaping our current concepts of the Borrelia cell envelope. Although Borrelia spirochetes 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. Probably most striking is the lack of LPS, the presence of major surface lipoproteins at the host-pathogen interface during transmission, persistence and ensuing pathogenic processes and the additional function of periplasmic flagella in defining cell shape. 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 (Oms66), which has a dual role as a pore-forming outer membrane protein with an extremely high single channel conductance and an adhesin for β3-integrin. The recently identified Tol homologs BesA, -B and -C appear to form a Type I 'channel' to export exogenous toxic agents such as antibiotics and to maintain infectivity by an unknown mechanism. Initial studies on envelope biogenesis pathways based on diderm proteobacterial model organisms already revealed significant deviations from the norm. This further bolsters the unique status of Borrelia among microbial pathogens read more ...