Fluorescent Microscopy Techniques to Study Hook Length Control and Flagella Formation.

The bacterial flagellum is a sophisticated motility device made of about 30 different proteins and consists of three main structural parts: (1) a membrane-embedded basal body, (2) a flexible linking structure (the hook) that connects the basal body to, (3) the rigid filament that extends up to 10 μm from the cell surface. In Salmonella enterica serovar Typhimurium, the hook structure is controlled to a length of 55 nm by a molecular ruler protein, FliK. Only upon hook completion, FliK induces a switch in substrate specificity of the flagellar export apparatus, which allows secretion of filament-type substrates, such as flagellin. Up to 20,000 subunits of flagellin assemble one flagellar filament that extends several micrometers beyond the cell surface. The formation of hook and filament structures as hallmarks of the hook length control mechanism can be monitored by immunofluorescence microscopy as described in this chapter.