What type of cell has a helical shape, similar to a screw?

The correct choice is the cell type known as the spirochete. Spirochetes are characterized by their distinct helical shape, which resembles a flexible screw. This shape allows them to move in a corkscrew motion, which is advantageous for navigating through viscous environments, such as mucus or tissues in host organisms. Their unique structure and movement make spirochetes quite different from other bacterial shapes.

Cocci are spherical, bacilli are rod-shaped, and spirilla, while also helical, are generally more rigid than spirochetes. Understanding these differences in shapes helps with the identification of bacteria and their classification, which is crucial in microbiological studies and applications.