These microscopic life forms exhibit a wide array of shapes, sizes, and metabolic capabilities, allowing them to adapt to diverse ecological niches. Among the various bacterial morphologies, the spiral shape is particularly distinctive. This unique form characterizes a specific group of bacteria known as spirilla, which possess a rigid, twisted structure.
Defining Spirilla
Spirilla are bacteria primarily defined by their characteristic spiral or helical shape. This morphological category distinguishes them from other common bacterial forms, such as spherical cocci or rod-shaped bacilli. Unlike spirochetes, another group of spiral bacteria, spirilla possess a rigid cell wall that maintains their corkscrew-like structure. This rigidity is a key feature, preventing the cell from flexing or bending significantly.
The term “spirilla” encompasses a variety of bacterial species that share this distinct shape rather than representing a single taxonomic lineage. Individual spirilla typically range in size, with diameters from 0.1 to 1.7 micrometers and lengths that can extend up to 60 micrometers.
Unique Structural Features
Their distinct corkscrew or wavy appearance is maintained by a rigid cell wall, which is characteristic of Gram-negative bacteria. This means they possess a thin peptidoglycan layer that does not retain the crystal violet stain used in the Gram staining process. Many spirilla are also microaerophilic, thriving in environments with low oxygen concentrations, typically between 1% and 9%.
Movement for spirilla is facilitated by flagella, which are whip-like appendages extending from the cell body. These flagella are often arranged in polar tufts, meaning they emerge from one or both ends of the bacterium. The rotation of these flagella allows spirilla to propel themselves with a characteristic corkscrew-like motion, enabling efficient navigation through viscous fluids.
Examples of Spirilla and Their Impact
Several notable examples illustrate the diversity and impact of spirilla in various environments. Spirillum minus is a Gram-negative bacterium recognized as an etiological agent of rat-bite fever, specifically a form known as sodoku. This bacterium is typically found in the oral cavity of wild rats and can be transmitted to humans through bites. Infection with Spirillum minus often presents with fever, inflammation at the bite site, and a rash, usually appearing one to four weeks after exposure.
Another significant spirillum is Campylobacter, which commonly appears as curved, S-shaped, or comma-shaped rods. These Gram-negative bacteria are highly motile due to their unipolar or bipolar flagella. Campylobacter is a leading cause of bacterial foodborne illness worldwide, primarily causing gastroenteritis with symptoms such as diarrhea, abdominal pain, and fever. It is frequently found in raw poultry and other domestic animals, highlighting its relevance in food safety.
Helicobacter pylori is a widely recognized helical or S-shaped bacterium that inhabits the human stomach. This Gram-negative, microaerophilic organism possesses tufts of polar sheathed flagella, typically five to seven in number, which aid its movement. Its unique spiral shape and flagellar motility enable it to burrow through the stomach’s protective mucus layer, allowing it to colonize the gastric lining. While over half of the global population may be infected, Helicobacter pylori is a primary cause of chronic gastritis and peptic ulcers, and it is also identified as a risk factor for stomach cancer.