The bacterial world shows immense diversity in physical form. A bacterium’s shape is a primary characteristic, with spheres, rods, and spirals as the most common arrangements. These structural differences are linked to a bacterium’s lifestyle, environment, and method of movement. Specific descriptions of shape can act as a guide to identity, raising the question: what is a rigid helical cell twisted at least twice along its axis?
Identifying the Twisted Bacterium: The Spirochete
The organism described as a helical cell twisted multiple times on its axis is a spirochete. These bacteria have long, slender, spiral-shaped bodies that give them a corkscrew-like appearance. The term “rigid” can be somewhat misleading, as spirochetes are flexible, capable of bending as they move. However, they maintain a consistent and stable helical structure, which provides structural integrity that can be interpreted as rigidity.
This structure differentiates spirochetes from another group of spiral bacteria called spirilla. While both are helical, spirilla are wider, possess a more rigid cell wall that prevents flexing, and use a different mechanism for movement. Spirochetes are defined by their flexible and tightly coiled bodies.
Unique Mechanism of Spirochete Twisting and Motility
The twisting of a spirochete is caused by a structure known as the endoflagellum, or axial filament. Unlike other motile bacteria, a spirochete’s flagella are contained within its periplasmic space, between the inner cell membrane and the outer sheath of the bacterium. These internal flagella are anchored at each end of the cell and extend towards the middle, overlapping in the center.
When these endoflagella rotate, they exert force on the flexible, protoplasmic cylinder of the cell body. This internal rotation causes the entire bacterium to twist and contort, creating the signature corkscrew motion. This method of propulsion is highly effective for moving through viscous substances, such as mucus, mud, or host tissues, allowing the spirochete to essentially drill its way through its environment.
Significance of Spirochetes in Health and Environment
Spirochetes play significant and varied roles in the world, with some species being notable pathogens. For example, Treponema pallidum is the bacterium responsible for syphilis, a complex multi-stage infection. Another pathogenic spirochete is Borrelia burgdorferi, which is transmitted by ticks and is the causative agent of Lyme disease. Species of the genus Leptospira are responsible for leptospirosis, contracted through contact with contaminated water or soil.
However, not all spirochetes are associated with disease. Many are harmless inhabitants of diverse ecosystems. For instance, certain spirochete species are found as commensal organisms in the human mouth and digestive tract. In the environment, free-living spirochetes thrive in aquatic sediments and waterlogged soils. Some have formed symbiotic relationships, such as those living in the guts of termites, where they digest cellulose from the wood consumed by their hosts.