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 the basal body (embedded in the cell envelope, acting as a rotary motor powered by the proton-motive force), the hook (a curved connector), and the filament (the long external structure, made of repeating subunits of the protein flagellin, also the H (flagellar) antigen used in bacterial serotyping).
Arrangement patterns: monotrichous (single polar flagellum, e.g., Vibrio cholerae), lophotrichous (tuft at one pole), amphitrichous (flagella at both poles), peritrichous (flagella all over the cell surface, e.g., Salmonella, E. coli).
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 or growth pattern in semisolid motility agar.
Significance: contributes to virulence by enabling chemotaxis and penetration through mucus layers (e.g., H. pylori in gastric mucus); the H antigen is used for serotyping and outbreak investigation (e.g., E. coli O157:H7).