spirillum

This term describes a specific morphology of bacteria characterized by a helical or corkscrew shape. It is distinguished from spirochetes by having a more rigid cell wall and external flagella, which allow it to move through viscous environments with a twisting motion. As a biological term derived from Latin, the plural form is typically spirilla. This follows the classical second-declension neuter pattern where the -um ending shifts to -a in the plural.