Spirochetes are a distinct group of bacteria characterized by their unique, long, helically coiled shape, which resembles a corkscrew. This phylum includes both free-living organisms found in water and soil, as well as species that cause serious diseases in humans and animals. Their spiral morphology, combined with a specialized internal motor, allows them to move with a characteristic spinning motion. This unique movement sets them apart from most other bacterial types and contributes to their ability to thrive as pathogens inside a host body.
Defining Physical Characteristics
The physical structure of a spirochete is unlike that of other bacteria, beginning with its distinctive spiral shape. These bacteria possess a double-membrane structure, classifying them as Gram-negative, though they are often difficult to stain with the traditional Gram method. The cell body is highly flexible and enclosed by a thin, protective outer membrane or sheath, giving the organism a smooth contour.
The most unique feature is the location of their flagella, which are not external but are housed within the periplasmic space, the region between the inner and outer bacterial membranes. These internal appendages are known as periplasmic flagella, or axial filaments, and they are anchored at opposite ends of the cell. The number of these filaments varies by species, ranging from as few as two to over a hundred.
The rotation of these enclosed flagella generates torque, causing the entire spiral cell body to twist and flex. This mechanism creates the powerful, corkscrew-like motion that allows spirochetes to bore through thick or viscous media. This unique motility is integral to their ability to invade host tissues.
Mechanisms of Infection and Pathogenicity
The unique spinning movement of spirochetes is a primary factor in their ability to cause infection, enabling them to penetrate highly viscous substances like connective tissue and mucous membranes. This allows them to quickly move away from the initial site of entry and disseminate throughout the body. Once inside a host, many pathogenic spirochetes exhibit a phase called spirochetemia, circulating widely in the blood and lymph fluid before settling in distant tissues.
Pathogenic spirochetes have developed sophisticated mechanisms for evading the host’s immune system. Their outer membrane is generally less complex than that of other Gram-negative bacteria, often lacking abundant surface proteins. This helps them remain less visible to immune defenses, and some species can alter the expression of these proteins to better colonize host tissues and persist.
The ability of these bacteria to travel deep into host tissues enables them to establish chronic or systemic infections that are challenging to eradicate. Certain spirochetes can cross protective barriers, such as the blood-brain barrier, leading to neurological complications in later stages of disease.
Major Spirochete-Related Diseases
The phylum Spirochaetota is responsible for several major human diseases, each caused by a different genus with unique transmission routes. Syphilis is one of the most historically significant spirochetal diseases, caused by the bacterium Treponema pallidum. Transmission occurs primarily through sexual contact, where the organism enters the body through the skin or mucous membranes.
Syphilis progresses through distinct stages. It begins with a localized, painless sore called a chancre at the site of infection. This is followed by a secondary stage characterized by a widespread rash and systemic symptoms as the bacteria disseminate. Without treatment, the infection can enter a latent phase and potentially progress to tertiary syphilis, which can cause severe damage to the heart, brain, and other organs.
Lyme disease represents another public health concern, caused mainly by Borrelia burgdorferi in the United States and related species elsewhere. This infection is transmitted through the bite of infected ticks, which introduce the spirochetes into the bloodstream. An early, characteristic sign is often a circular, expanding rash known as erythema migrans.
The Borrelia spirochetes can spread rapidly, leading to symptoms that affect the joints, nervous system, and heart if not treated early. Leptospirosis is a third major spirochetal disease, caused by pathogenic species of the genus Leptospira. This is a zoonotic infection, transmitted from animals, most commonly through contact with water or soil contaminated by the urine of infected rodents or other mammals.
Symptoms of leptospirosis can range from mild flu-like illness to a severe form called Weil’s disease, which involves liver damage, kidney failure, and jaundice. Because Leptospira can survive in freshwater environments, people involved in water sports, farming, or veterinary work face a higher risk of exposure.