Spirochetes, unique bacteria in the phylum Spirochaetes, are gram-negative, motile, tightly coiled, slender, and flexible. They inhabit aquatic sediments and animals, with some causing diseases like syphilis. Spirochetes are classified into eight genera based on habitat, pathogenicity, phylogeny, and characteristics.
Their distinctive motility arises from endoflagella, located within the cell’s periplasm. These endoflagella anchor at the cell poles and extend along the cell length, encased by a flexible outer sheath. When the endoflagella rotates, they create torsion, causing the cell to flex and move in a corkscrew-like motion, aiding penetration through viscous materials or tissues.
Spirochetes are often mistaken for spirilla, helical, rod-shaped cells with polar flagella. Unlike spirochetes, spirilla lack an outer sheath, endoflagella, and corkscrew motility. Spirilla are more rigid and larger, while spirochetes are thin (around 0.5 μm) and highly flexible.
The genus Spirochaeta includes free-living, anaerobic, and facultatively aerobic spirochetes found in freshwater, sediments, and oceans. Spirochaeta plicatilis inhabits sulfidic freshwater and marine habitats, with bundles of endoflagella around its coiled protoplasmic cylinder. Spirochaeta stenostrepta, an obligate anaerobe in H₂S-rich muds, ferments sugars into ethanol, acetate, lactate, CO₂, and H₂.
The genus Cristispira, unique to the crystalline style of mollusks like clams and oysters, aids in food processing in their digestive tracts. Found in both freshwater and marine mollusks, Cristispira has not been cultured, and its specific adaptation to this habitat remains unclear.
Anaerobic or microaerophilic spirochetes in the genus Treponema are commensals or pathogens of humans and animals. Treponema pallidum, the causative agent of syphilis, is unusually thin (0.2 μm) with a flat, wavy morphology. Dark-field microscopy is commonly used to detect it in syphilitic lesions.
Other species include T. denticola, which is found in the human oral cavity and is associated with gum disease. It ferments amino acids to produce acetate, CO₂, NH₃, and H₂S. T. saccharophilum resides in the rumen of ruminants, fermenting plant polysaccharides, while T. primitia and T. azotonutricium inhabit termite guts, aiding in cellulose fermentation, acetogenesis, and nitrogen fixation.
The genus Borrelia includes pathogens like B. burgdorferi, the cause of Lyme disease, which is transmitted by ticks. Unique for its linear chromosome, B. burgdorferi infects humans and animals. Other Borrelia species cause diseases in livestock and birds, typically transmitted via tick bites.
The genera Leptospira and Leptonema consist of strictly aerobic spirochetes that oxidize long-chain fatty acids, like oleic acid, for growth. Leptospira species are thin, finely coiled, and typically bent into a hook at each end. The group includes free-living (L. biflexa) and parasitic (L. interrogans) species.
L. interrogans strains infect humans and animals, with rodents being the primary hosts, though dogs and pigs are also carriers. In humans, leptospirosis is the most common condition caused by Leptospira, leading to kidney and liver damage, nephritis, jaundice, and potentially renal failure or death. The bacteria enter through mucous membranes or skin breaks during contact with infected animals. Domestic animals, like dogs, are vaccinated against leptospirosis as part of a combined distemper–leptospira–hepatitis vaccine.
Spirochetes are Gram-negative, coiled bacteria classified in the phylum Spirochaetes.
Endoflagella in spirochetes produce corkscrew-like motion, enabling movement through viscous materials or tissues.
The Spirochaeta genus includes anaerobic and facultatively aerobic spirochetes that are free-living in freshwater, sediment, and oceans.
Notable species include Spirochaeta plicatilis from sulfidic freshwater and marine environments.
Cristispira is a genus of spirochetes that are found in the crystalline style of many bivalve mollusks like clams, mussels, and oysters.
The genus Treponema includes anaerobic or microaerophilic spirochetes that are commensals or pathogens of humans and animals, such as Treponema denticola, common in the human oral cavity.
Many species in the genus Borrelia are pathogenic. Borrelia burgdorferi, for example, causes Lyme disease, which is transmitted by ticks.
The genera Leptospira and Leptonema are strictly aerobic spirochetes that oxidize long-chain fatty acids, like oleic acid, from their host's tissues or the environment to support their growth. Leptospira interrogans is parasitic in humans and causes leptospirosis.
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