American Water Works Water Treatment Plant Operator (WTPO) Practice Test

Spirilla describe bacteria that have a distinct spiral or helical shape. In bacterial shape classifications, rod-shaped corresponds to bacilli, spherical to cocci, and filamentous to long threadlike forms, while spirilla are the spiral-shaped category. Spirilla are typically rigid and move with external flagella, giving a recognizable helical appearance under the microscope. This makes spiral-shaped the best description for spirilla, as opposed to the straight rods, spheres, or threadlike forms of the other shapes.