What structure in spirochetes enables corkscrew motility?

The structure that enables corkscrew motility in spirochetes is indeed flagella bundles, which are located within the periplasmic space between the inner and outer membranes of the bacteria. This unique arrangement of flagella is crucial for the distinctive movement of spirochetes, as the flagella wrap around the cell body and rotate. When the flagella turn, they generate torque that causes the spirochete to twist and propel itself in a corkscrew-like motion through its environment. This form of motility is highly effective for navigating through viscous environments, such as mucus, which is often encountered by spirochetes within host organisms.

In contrast, pseudopodia are extensions of the cell membrane used mostly by amoeboid cells for movement and feeding, while cilia are short, hair-like structures that provide movement to many eukaryotic microorganisms but are not a feature of spirochetes. Fibriae, on the other hand, are primarily involved in adhesion to surfaces and do not facilitate motility. Thus, the function of flagella bundles in spirochetes is specifically adapted for their unique corkscrew movement.