Full text loading...
Chapter 9 : Motility and Chemotaxis
Category: Environmental Microbiology
Ebook: Choose a downloadable PDF or ePub file. Chapter is a downloadable PDF file. File must be downloaded within 48 hours of purchase
To possess a functioning flagellar motility system, three things are requisite: a propeller, a motor, and a system of navigation. In this chapter, the swimming and swarming systems are considered separately. There is a central processing system that is shared by both flagellar systems, i.e., chemotaxis, and it allows the bacteria to detect signals in their environment and respond by modifying their movement. Various types of polar flagellar genes and polar flagellar and linked chemotaxis genes have been provided in the chapter. Chemotaxis integrates environmental signaling to modulate behavior by biasing movement toward more favorable conditions or away from unfavorable environments. Chemotaxis integrates environmental signaling to modulate behavior by biasing movement toward more favorable conditions or away from unfavorable environments. One consequence is that not just motility, but also chemotaxis is important for survival and colonization. In Vibrio cholerae, a number of studies show differential expression of various motility and chemotaxis genes under in vivo and in vitro conditions. Bacterial chemotaxis has been most extensively studied in organisms with a few (~6 to 10) peritrichously arranged flagella, e.g., Escherichia coli, Salmonella enterica serovar Typhimurium, and Bacillus subtilis. In swimmer cells, the methyl-accepting chemotaxis proteins (MCPs) localize to both poles; in the swarmer cells, MCPs are found at the poles and at intervals along the cell body. The result is that polar flagellar function influences expression of cell surface polysaccharide, which has important consequences for biofilm formation and host colonization.
Flagellation patterns of liquid- and plategrown V. parahaemolyticus. The V. parahaemolyticus swimmer cells (left panel) possess single, sheathed polar flagella. This electron micrograph is of planktonically grown cells, stained with uranyl acetate. Cell proportions are ~1 × 2 μm. The swarmer cells (right panels) are elongated and display numerous lateral flagella in addition to the single, sheathed polar flagellum. V. parahaemolyticus swarmer cells were harvested from a plate and stained with phosphotungstic acid. The arrows point to the partially dissolved sheath of polar flagellum. The diameter of the sheathed polar flagellum is ~30 nm, and the diameter of the unsheathed lateral flagellum is ~15 nm.
Regulatory hierarchy and morphogenetic pathway for polar flagellar biogenesis. Flagellar assembly is an ordered process, initiating with integral membrane proteins forming a scaffold for the export and assembly of the remainder of the organelle and culminating with polymerization of the filament ( reviewed by Macnab, 2004 ). Gene expression is strictly regulated and tied into morphogenesis of the organelle (reviewed by Aldridge and Hughes, 2002). Early Vibrio polar gene expression requires σ54 and late gene expression σ28. The scheme of control seems conserved for the Vibrio sp. in which it has been studied, and this figure, which depicts a general pathway for gene expression and organelle biogenesis, represents a summary of what is currently known (Kim and McCarter, 2000; Prouty et al., 2001; Millikan and Ruby, 2003, 2004). The master flagellar regulator FlrA (a.k.a. FlaK) directs transcription of class 2 genes, which encode the export and assembly apparatus and other regulators that act sequentially. FlhF and FlhG regulate flagellar number and placement by an unknown mechanism, but one that also influences transcription ( Correa et al., 2005 ). Expression of class 3 genes is controlled by the activity of a two-component regulator, FlrC (a.k.a. FlaM) and results in the assembly of the hook and basal body. Completion of the hook allows export of the anti-sigma factor FlgM, which frees the σ28 to direct transcription of class 4 genes (Correa et al., 2004).
Regulatory hierarchy and morphogenetic pathway for lateral flagellar biogenesis. Although it is a distinct flagellar system, with its own complete set of flagellar genes, the lateral flagellar hierarchy of V. parahaemolyticus resembles the polar hierarchy in that a σ54−dependent transcriptional regulator directs expression of intermediate genes, and σ28 controls late flagellar gene expression (Stewart and McCarter, 2003). The master lateral regulator LafK has also been shown to affect polar gene transcription (Kim and McCarter, 2004).
Flagellation patterns of Vibrio species a
Polar flagellar and linked chemotaxis genes
V. parahaemolyticus lateral flagellar genes a
Multiple copies of potential chemotaxis genes are encoded in Vibrio genomes