Question
Bacterial flagella.
Answer
Bacterial flagella are long, thin, whip-like proteinaceous appendages extending from the bacterial cell surface, responsible for motility — they rotate like a propeller to drive the bacterium through liquid media.
Structure: composed of three parts — the basal body (embedded in the cell envelope, containing a rod and rings that anchor the flagellum and act as a rotary motor, powered by the proton-motive force), the hook (a short curved connector), and the filament (the long external helical structure, made of repeating subunits of the protein flagellin, also the H (flagellar) antigen used in bacterial serotyping).
Arrangement patterns (used in classification): monotrichous (single polar flagellum, e.g., Vibrio cholerae, Pseudomonas aeruginosa), lophotrichous (tuft at one pole), amphitrichous (flagella at both poles), peritrichous (flagella all over the cell surface, e.g., Salmonella, E. coli, Proteus).
Detection: too thin for ordinary light microscopy without special staining (silver impregnation/Leifson’s flagellar stain); motility can be inferred functionally by hanging-drop preparation (direct visualization) or growth pattern in semisolid motility agar (motile organisms show diffuse turbid growth away from the stab line, versus a sharp line for non-motile organisms).
Significance: contributes to virulence by enabling chemotaxis toward favourable environments and penetration through mucus layers to reach epithelial surfaces (e.g., H. pylori in gastric mucus); the H antigen is used for serotyping and outbreak investigation (e.g., E. coli O157:H7).