The axial fibrils of spirochetes most closely resemble which bacterial structure?

• A. Cytoplasmic membrane
• B. Flagellum
• C. Pilus
• D. Sporangium

Answer: (B)

Axial fibrils in spirochetes are periplasmic flagella, located between the inner and outer membranes. They rotate to twist the entire cell, producing the distinctive corkscrew motility that spirochetes use to move through viscous environments. Like a traditional flagellum, these filaments are long, helical structures powered by motor proteins that generate propulsion. The difference is their location inside the cell envelope rather than external, but the function and structure underpinning motility are the same in principle. Cytoplasmic membranes are just the inner barrier, pili are involved in adhesion and genetic exchange, and sporangia are spore-containing structures. Thus, the axial fibrils resemble a flagellum most closely.