Spirilli

Spirilli, or spirillum bacteria, are a group of spiral-shaped, Gram-negative bacteria known for their rigid, helical cells. They are typically motile, using external flagella to propel the cell through aqueous environments. Unlike the more flexible spirochaetes, spirilla maintain a relatively stiff contour as they swim.

Most spirilla inhabit freshwater and soil, where they act as saprophytes and participate in microbial communities within biofilms. Some species occur in marine environments as well. They are generally non-pathogenic to humans, though rare opportunistic infections have been reported in some contexts.

Morphologically, spirilla are larger than many other bacteria and possess a Gram-negative cell envelope. Reproduction occurs by binary fission. Their shape and flagellar arrangement—often distributed around the cell surface or at one or both ends—facilitate movement in viscous media.

Spirilla are distinct from spirochaetes, another group of helical bacteria. Spirochaetes are typically slender and highly flexible, moving by axial filaments called endoflagella that run lengthwise between membranes. This produces a twisting motion different from the more rigid, externally flagellated spirilla.

Notable species include Spirillum volutans, commonly cited in freshwater microbiology. The term spirilli generally refers to these rigid, spiral-shaped bacteria, characterized by their morphology and motility rather than a single uniform taxonomic group.