Sense and sensibility: flagellum-mediated gene regulation

Abstract

The flagellum, a rotary engine required for motility in many bacteria, plays key roles in gene expression. It has been known for some time that flagellar substructures serve as checkpoints that coordinate flagellar gene expression with assembly. Less well understood, however, are other more global effects on gene expression. For instance, the flagellum acts as a 'wetness' sensor in Salmonella typhimurium and as a mechanosensor in other bacteria. Additionally, it has been implicated in a variety of bacterial processes, including biofilm formation, pathogenesis and symbiosis. Although for many of these processes it may be simply that motility is required, for other cases it seems that the flagellum plays an underappreciated role in regulating gene expression. Roles of bacterial flagella in microbial processesThe bacterial flagellum is a complex rotary engine comprised of three basic parts: basal body, hook and filament [1] (Figure 1). The basal body anchors the flagellum in the cell envelope, and also contains the motor and a specialized type III secretion system (T3SS) for exporting flagellar proteins. The motor consists of a motile part, or rotor, that is connected to the hook via the rod, and a stationary part, or stator, which converts the potential energy of an electrochemical gradient to kinetic motion of the rotor. The hook is a flexible coupling that transfers torque from motor to the propeller-like filament In addition to motility, flagella have roles in other microbial processes, such as adherence to host cells, host cell invasion, protein secretion, autoagglutination (i.e. clumping or selfadherence of bacterial cells) and biofilm formation [2][3][4][5][6]. For some of these processes a direct involvement of flagella is clear. For example, Campylobacter jejuni flagella are responsible for secretion of several non-flagellar proteins [7][8][9]. Interestingly, some obligate intracellular symbionts of insects (e.g. Buchnera aphidicola) have lost many of the genes needed for assembly of the flagellum and appear to use the remaining flagellar assembly gene products for export of bacterial proteins to the host [10][11][12].For other processes, such as biofilm formation, it is difficult to ascertain the specific role of flagella, which can be quite different among bacteria. For example, flagella contribute to biofilm formation by acting as adhesins to promote attachment to surfaces in Aeromonas spp.[13], but not in Escherichia coli [14]. Additionally, studies with non-motile (Mot − ) mutants that make but are unable to rotate flagella, although useful, do not distinguish if a particular trait of a mutant is due to defects in locomotion, chemotaxis or just flagellar rotation (independently of the first two processes). Indeed, recent studies suggest a role for flagellar © 2009 Elsevier Ltd. All rights reserved.Corresponding author: Hoover, T.R. (trhoover@uga.edu). Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we ar...

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