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 across the cell membrane), the hook (a short curved connector), and the filament (the long external helical structure, made of repeating subunits of the protein flagellin, which is also the H (flagellar) antigen used in bacterial serotyping, e.g., in Salmonella).
Arrangement patterns (used in bacterial classification/identification):
- Monotrichous — a single flagellum at one pole (e.g., Vibrio cholerae, Pseudomonas aeruginosa).
- Lophotrichous — a tuft of multiple flagella at one pole.
- Amphitrichous — flagella at both poles.
- Peritrichous — flagella distributed all over the cell surface (e.g., Salmonella, E. coli, Proteus).
Detection: flagella are too thin to be seen by ordinary light microscopy without special staining techniques (e.g., silver impregnation/Leifson’s flagellar stain, which coats the flagellum to increase its apparent thickness); motility can be inferred functionally using hanging-drop preparation (direct visualization of movement) or growth pattern in semisolid motility agar (motile organisms diffuse away from the inoculation stab line, producing diffuse turbid growth, versus a sharp line of growth for non-motile organisms).
Clinical/biological significance: flagella contribute to virulence by enabling the organism to move toward favourable environments (chemotaxis) and to penetrate mucus layers to reach and colonize epithelial surfaces (e.g., H. pylori in gastric mucus, V. cholerae in the small intestine); the H antigen is used for serotyping and outbreak investigation (e.g., E. coli O157:H7, Salmonella Typhi’s characteristic H antigen “d”).