Spirochete chemotaxis, motility, and the structure of the spirochetal periplasmic flagella

Spirochetes have a unique motility system that is characterized by flagellar filaments contained within the outer membrane sheath. Direct evidence using video microscopy has recently been obtained which indicates that these periplasmic flagella (PF) rotate in several spirochetal species. This rotation generates thrust. As shown for one spirochete, Spirochaeta aurantia, motility is driven by a proton motive force. Spirochete chemotaxis has been most thoroughly studied in S. aurantia. This spirochete exhibits three distinct behaviours, runs of smooth swimming, reversals and flexing. These behaviours are modulated by addition of attractants such that S. aurantia swims towards higher concentrations of attractants in a spatial gradient. Unlike the prototypical bacterium, Escherichia coli, chemotaxis in S. aurantia involves fluctuations in membrane potential. The PF of a number of spirochetes have been examined in considerable detail. For most species, the PF filaments are complex, consisting of an assembly of several different polypeptides. There are several antigenically related core polypeptides surrounded by an outer layer consisting of a different polypeptide. Borrelia burgdorferi and Spirochaeta zuelzerae represent exceptions where the filaments are composed of a single major polypeptide species. The genes encoding the filament polypeptides from several spirochete species have been cloned and analysed. Apparently, the outer layer polypeptides of S. aurantia, Treponema pallidum and Serpulina hyodysenteriae are transcribed from sigma-70-like promoters, whereas the core polypeptide genes are transcribed from sigma-28-like promoters. A gene encoding the hook polypeptide in Treponema phagedenis has been cloned and analysed. The product of this gene shows significant similarity to the E. coli hook protein, FlgE, and homologs have been identified in T. pallidum and B. burgdorferi.(ABSTRACT TRUNCATED AT 250 WORDS)