Bacterial morphology, or the shape of a cell, is a fundamental characteristic used by scientists to classify and identify different types of microorganisms. While many bacteria adopt simple forms like spheres (cocci) or rods (bacilli), a significant group exhibits a helical or spiral architecture. The precise structural details of these spiral forms, such as rigidity and number of twists, allow for specific identification. Examining the physical structure of a bacterium provides valuable insight into its lifestyle, including where it lives and how it interacts with its environment.
Defining the Shape: The Spirillum
The descriptor for a bacterium that is a rigid helical cell, often appearing twisted once or twice, is a Spirillum (plural: Spirilla). These microorganisms are distinguished by their fixed, corkscrew-like or curved shape that does not easily flex or bend. The reference to being “twisted twice” describes a common appearance where the cell body forms a short, thick, S-shaped rod.
The overall helical body of a Spirillum cell can range from one to five complete turns. Some species are quite large; for instance, a cell can measure between 1.4 and 1.7 micrometers in diameter and reach lengths up to 60 micrometers. This spiral form is permanently maintained by the cell’s underlying structure, giving the bacterium its characteristic rigid appearance.
The rigid, helical shape is coupled with external, tufted flagella used for propulsion. This combination of a fixed spiral body and external flagella is the defining feature that sets the Spirillum apart from other spiral-shaped bacteria. The specific arrangement of these flagella allow the Spirillum to move in a rapid, darting, and spinning fashion through liquid environments.
Structural Basis of Rigidity and Movement
The fixed, helical shape of the Spirillum is a direct result of its robust cell wall structure. Spirilla are classified as Gram-negative bacteria, meaning their cell wall includes a thin layer of peptidoglycan sandwiched between two membranes. While the peptidoglycan layer is thin (typically 7 to 8 nanometers), it is the structural component that maintains the cell’s fixed shape against internal osmotic pressure.
The overall cell envelope, which includes the inner membrane, the peptidoglycan layer, and the outer membrane, provides the necessary structural integrity for the rigid helix. The outer membrane contains lipopolysaccharide (LPS), which contributes to the stability of the Gram-negative cell. This multi-layered envelope ensures that the corkscrew form is fixed and cannot be easily deformed by external forces, which is crucial for its locomotion.
Movement is achieved by the rotation of flagella located at one or both ends of the cell, often in tufts. This specific arrangement is known as lophotrichous or amphitrichous flagellation, where multiple flagella aggregate to form bundles. The rapid, high-speed rotation of these external, polar flagellar tufts propels the rigid cell body forward.
This mechanism creates a corkscrew-like motion for the entire cell, allowing the Spirillum to move quickly through water. The rotation of the flagellar bundles pushes the cell in a straight line, which is mechanically distinct from the flexing or bending movement seen in other spiral bacteria.
Comparing Helical Forms: Spirilla vs. Spirochetes
While both Spirilla and Spirochetes are spiral-shaped bacteria, they represent fundamentally different structural and motile strategies. The defining difference lies in their cell wall rigidity: Spirilla possess a rigid cell wall that fixes their helical shape, whereas Spirochetes have a highly flexible, wavy cell body that can twist and bend.
The mechanism for movement provides the second major contrast, revolving around the location of their flagella. Spirilla utilize external polar flagella that project out from the cell ends and rapidly rotate to push the cell. Spirochetes, conversely, possess a unique internal structure called the axial filament, which is a bundle of endoflagella located within the periplasmic space between the inner and outer cell membranes.
This internal axial filament rotates, causing the entire flexible cell body to writhe, flex, and move with a characteristic corkscrew burrowing motion. The movement of a Spirillum is a spinning, darting run through liquid, while the Spirochete movement is better suited for navigating viscous environments or penetrating tissue. The fixed form and external propulsion of a Spirillum contrasts sharply with the flexible form and internal rotation of a Spirochete.
Ecological Roles and Notable Examples
Most members of the genus Spirillum are free-living organisms commonly found in aquatic habitats, particularly stagnant freshwater rich in organic matter. These bacteria are often microaerophilic, meaning they require a low concentration of oxygen (typically between 1% and 9%) to survive. They play a role in aquatic ecosystems by cycling nutrients, though their exact ecological contribution is still being explored.
One of the largest and most historically studied species is Spirillum volutans, which can be up to 60 micrometers long and 8 micrometers wide, making it easily observable under a microscope. This species is known for storing reserves of polyhydroxybutyrate (PHB) granules, which are internal carbon and energy storage compounds.
While the majority of Spirilla are non-pathogenic, the species Spirillum minus is of medical significance, as it is associated with sodoku, a form of rat-bite fever. This organism is typically shorter and thicker than other Spirilla, and it is transmitted to humans through the bite of an infected rodent. Its rigid, coiled rod shape fits the general morphology of the Spirillum group.