flagellum

**Bacterial flagellum (Microbiology)** - Structure: filament (polymerized flagellin protein), hook, and basal body (rings anchored in the cell membrane and wall; L and P rings in gram-negatives, MS and C rings in both). Motor rotates, powered by proton motive force (H+ gradient), NOT ATP. - Function: motility and chemotaxis (run = counterclockwise rotation, bundled flagella; tumble = clockwise rotation). - Antigenicity: flagellar protein = **H antigen** (e.g., E. coli O157:H7; O = LPS somatic antigen, H = flagellar). Flagellin is recognized by **TLR5** and triggers innate immune responses. - Arrangements: - Monotrichous: single polar flagellum (Vibrio cholerae, Pseudomonas aeruginosa). - Lophotrichous: tuft at one pole (Helicobacter pylori). - Amphitrichous: one or more at both poles (Spirillum). - Peritrichous: all around the cell (E. coli, Proteus mirabilis, Salmonella). - Clinical associations: - Proteus: peritrichous flagella give "swarming" motility on agar and help ascend the urinary tract (urease → struvite stones). - H. pylori: flagella help it penetrate the gastric mucus. - Listeria: tumbling motility at 25°C (flagella expressed at room temp, not 37°C); also actin-rod "rocket" motility intracellularly. - Vibrio cholerae: darting motility. - Campylobacter jejuni: flagella aid colonization; Guillain-Barré association after infection via molecular mimicry. - Spirochetes (Borrelia, Treponema, Leptospira): axial filaments (endoflagella) in the periplasmic space produce corkscrew motility. - Non-motile: Shigella, Klebsiella (and Streptococcus/Staphylococcus), so no H antigen. - Lab: Too thin for light microscopy, so visualized with special flagella stains or electron microscopy; motility tested in semisolid (soft agar) medium or by hanging-drop.

**Eukaryotic flagellum (Cell biology/Histology)** - Structure: **axoneme** with a "9+2" arrangement (9 peripheral microtubule doublets + 2 central singlets), anchored by a basal body (9 triplets). Covered by plasma membrane (cell-membrane-enclosed, unlike bacterial flagella). - Motor protein: **dynein** (ATPase) generates sliding of doublets, producing bending. Dynein arms link adjacent doublets. - Human example: **sperm tail** (single flagellum), with mitochondria in the midpiece supplying ATP. - Clinical: **Primary ciliary dyskinesia / Kartagener syndrome**: autosomal recessive dynein arm defect (DNAI1, DNAH5) → immotile cilia/flagella → situs inversus, chronic sinusitis, bronchiectasis, and infertility (immotile sperm in men; ectopic pregnancy risk/reduced fertility in women). - Microtubule-targeting drugs (e.g., paclitaxel, vincristine, colchicine) disrupt microtubule function but are not specific to flagella.

**Key comparison:** Bacterial flagellum = flagellin, rotates, proton-motive-force driven, outside the membrane. Eukaryotic flagellum = tubulin/dynein, undulating whip-like motion, ATP-driven, membrane-covered.

**High-yield board points:** H antigen = flagellin; TLR5 recognizes flagellin; spirochetes use endoflagella; Kartagener = dynein defect; sperm motility depends on dynein ATPase.