Classification and Ecological Distribution of Spirochetes
Spirochete is also spelt as spirochaete, which refers to any of a group of spiral-shaped bacteria. Some of them are serious pathogens for humans, that cause diseases like syphilis, yaws, Lyme disease, and also relapsing fever. Examples of spirochetes are Spirochaeta, Treponema, Borrelia, and Leptospira.
What is Spirochete?
Spirochetes are gram-negative, motile, and spiral bacteria. Their length is from 3 to 500 m long. Spirochetes are very different and they have endocellular flagella also known as axial fibrils, or axial filaments, which is between 2 and more than 100 per organism, depending upon the species.
Each axial fibril of spirochetes attaches at an opposite end and then winds around the cell body that is enclosed by an envelope. Spirochetes are characteristically found in a liquid environment such as in mud and water, blood, and lymph.
Spirochetes Classification
Domain - Bacteria:
A large group of single-celled prokaryotes that lacks membrane-bound organelles found in just about any environment across the world that are soil, water, air, hot streams, etc. They may exist as parasites, or as free-living organisms, or even as symbionts with some of the species being very beneficial to man.
Phylum - Spirochaetes:
Bacterial cells that are characterized by a unique diderm (double-membrane) which gives them their gram-negative characteristic. They are usually thin with a spiral-shaped appearance and also possess flagella which are commonly known as axial filaments.
Order - Spirochaetales:
These helically shaped bacteria have the capability of locomotion. They display significant variation in their physiology and also in their distribution.
In nature, they might exist as facultative anaerobes, obligate anaerobes, or obligate aerobes. This order is further divided into three major phylogenetic families that include Spirochaetaceae, Brachyspiraceae, and Leptospiraceae.
Ecology and Distribution
As a group of bacteria, the spirochetes are distributed in nature. They can be found in different environments across the whole world. Many of the species on this planet have shown to exist as free-living organisms and also can be found in different habitats in water such as surface water or freshwater, or in lakes, salt marsh sediments, or even in mud, sediments, and also deep-sea vents among others.
Apart from the species found in these habitats, the other species form an association with various hosts that are termites, protozoa, mammals, etc., and are hence found living within these hosts such as in the intestine. While some of the species are also very beneficial, whereas some are pathogenic and they tend to cause diseases such as Lyme disease, dysentery, etc.
Because of the diversity between species and also where they are found in nature, the spirochetes are classified based on their distribution. Obligatory Aerobic Spirochetes that need oxygen for metabolism might be found in water and sometimes also in the soil as free-living organisms with some species living as pathogens in their hosts.
Members of the genus Leptospira have been classified as Obligatory Aerobic Spirochetes. The Anaerobic and the facultative anaerobic spirochetes exist as free-living forms and also include members of the genus Spirochaeta. They are usually found in various environments where they can survive on a variety of organic matter like disaccharides, pentoses, and hexoses, etc.
Some of the other habitats in which spirochetes can be found are in High salinity ponds or hot/boiling water springs.
FAQs on Spirochete: Definition, Structure, and Importance
1. What is a spirochete?
A spirochete is a type of bacterium belonging to the phylum Spirochaetota. They are distinguished by their long, helically coiled or corkscrew-like shape. Unlike most bacteria, their flagella, known as axial filaments or endoflagella, are located inside the cell body within the periplasmic space, which gives them their characteristic twisting motion.
2. What are the main structural components of a spirochete?
The structure of a spirochete is unique among bacteria. Key components include:
Outer Sheath/Membrane: A flexible outer membrane that encloses the entire cell.
Periplasmic Space: The gel-like space between the outer sheath and the inner membrane. This is where the axial filaments are located.
Axial Filaments (Endoflagella): These are flagella-like structures that run lengthwise within the periplasmic space. They are anchored at the poles of the cell and are responsible for its motility.
Protoplasmic Cylinder: This is the main cell body, which contains the cytoplasm, DNA (nucleoid), and ribosomes, all enclosed by a peptidoglycan cell wall and an inner cytoplasmic membrane.
3. How does the structure of a spirochete's axial filament cause its unique motility?
The corkscrew motility of a spirochete is a direct result of its internal axial filaments. These filaments are anchored at each end of the bacterium and rotate within the periplasmic space. Because they are contained within the outer sheath, their rotation causes the entire flexible protoplasmic cylinder and the outer sheath to twist and flex. This creates a corkscrew-like propulsion that allows the spirochete to move effectively through viscous environments like mud, water, and even host tissues, an advantage over bacteria with external flagella.
4. What is the primary difference between the flagella of a spirochete and other common bacteria?
The main difference lies in their location. In common bacteria like E. coli, the flagella are external appendages that extend from the cell surface into the surrounding environment and propel the cell by rotating like a propeller. In contrast, a spirochete's flagella, called axial filaments or endoflagella, are located internally within the periplasmic space, between the inner and outer membranes. This internal location is the defining feature that drives their unique twisting motility.
5. What is the medical importance of spirochetes? Can you provide some examples of diseases they cause?
While many spirochetes are harmless, several genera are significant human pathogens due to their invasive nature. Their ability to burrow through tissues makes them particularly effective at causing systemic infections. Notable examples include:
Syphilis: Caused by Treponema pallidum.
Lyme Disease: Caused by Borrelia burgdorferi and transmitted by ticks.
Leptospirosis: Caused by species of the genus Leptospira, often contracted from water contaminated with animal urine.
Relapsing Fever: Caused by other species of Borrelia.
6. Are all spirochetes pathogenic? What is their ecological role?
No, not all spirochetes are pathogenic. In fact, the majority are free-living or commensal, playing important ecological roles. Many are found in aquatic environments like ponds and marshes, where they contribute to the decomposition of organic matter. Some spirochetes live as symbionts in the digestive tracts of animals, such as in the gut of termites, where they play a crucial role in digesting cellulose from wood. This highlights their diverse metabolic capabilities and importance beyond just causing disease.
7. How are spirochetes classified?
Spirochetes are classified under the phylum Spirochaetota (or Spirochaetes). Within this phylum, they are placed in the order Spirochaetales. This order is further divided into families based on phylogenetic groupings. The three major families that contain medically and ecologically important genera are:
Spirochaetaceae (includes genera like Spirochaeta, Borrelia, and Treponema)
Leptospiraceae (includes the genus Leptospira)
Brachyspiraceae (includes the genus Brachyspira)
