Spirillum refers to both a genus of Gram-negative bacteria and a general morphological classification for any bacterium possessing a helical or spiral shape.
[[Etymology]] The term "spirillum" originates from the Latin word spirillum, which is a diminutive of spira, meaning "coil" or "twist."
[[Definition and Morphology]] As a general morphological descriptor, a spirillum is characterized by its rigid, wave-like, or corkscrew shape. Unlike spirochetes, which are flexible and have internal flagella (endoflagella), bacteria classified morphologically as spirilla typically possess a rigid cell wall and external flagella, often appearing as tufts at one or both poles (lophotrichous flagella), enabling their distinctive spiraling motility. They are generally larger than cocci (spherical bacteria) and bacilli (rod-shaped bacteria).
[[Habitat and Metabolism]] Members of the genus Spirillum and other spiral-shaped bacteria are commonly found in aquatic environments, including fresh and saltwater, where many exist as free-living organisms. Many species are microaerophilic, meaning they thrive in environments with low, but not absent, oxygen concentrations.
[[Clinical Significance and Related Genera]] While the genus Spirillum itself is not as diverse in its medical impact as some other bacterial groups, Spirillum minus is a notable species that causes one form of rat-bite fever, known as Sodoku.
The broader morphological classification of "spirillum" encompasses other important genera of spiral or curved bacteria, distinct from the genus Spirillum itself. These include:
- Campylobacter: A genus of spiral-shaped bacteria known to cause gastroenteritis in humans.
- Helicobacter: Another genus of spiral-shaped bacteria, most famously Helicobacter pylori, which is a primary cause of stomach ulcers and is implicated in stomach cancer.
- Vibrio: While typically described as comma-shaped (a curved rod), Vibrio species are sometimes discussed in conjunction with other spiral forms due to their non-straight morphology. Vibrio cholerae is the causative agent of cholera.
- Spirochetes: Though distinct, spirochetes (e.g., Treponema, Borrelia, Leptospira) are also spiral-shaped bacteria but are characterized by their flexible bodies and unique internal flagella (axial filaments) that allow for a characteristic corkscrew-like movement through viscous media.