A spirochete is a type of bacteria within the phylum Spirochetes. The spirochetes are so called because they are very long, thin shape and helically coiled, hence they have a spiraling corkscrew shape. The helical shape is one of three morphological categories of the prokaryotes.
The spirochetes are a diverse phylum, occupying a wide range of ecological niches; some are free-living in aquatic environments, while some can only survive by parasitizing the cells of other organisms. Most are anaerobic (can sustain themselves without the presence of oxygen), although certain species are aerobic. They are very difficult to culture and are gram-negative, meaning they are not easily visible.
Although some play an important role as symbionts within the stomach of other animals, many members of the spirochete class are responsible for common diseases such as Lyme disease and syphilis. Some spirochetes have also been found present within marine bivalves although with no apparent positive or negative effects; this neutral relationship is called a commensalism.
Spirochetes are distinguishable from other bacteria in that they move with unique endoflagella. The flagella are tightly wound around the corkscrew shape of the bacteria, between the outer membrane and the cell wall, within the periplasm. Together, the endoflagella make up a structure called the axial filament. The axial filament is rotated by the rotation of the flagella, causing the spirochete to move with a twisting motion. This method of motility is unique to the spirochetes and—much like the way a corkscrew can penetrate the cork of a wine bottle—allows them to move through viscous materials such as mucus, blood, mud and host connective tissues including cartilage and dental plaque; external flagella do not allow effective movement through these mediums.
Additionally, the presence of endoflagella allows the spirochetes to move backward and forward with equal ease, this allows them to move freely in their environment and target the best site of host attachment or resources.
The image shows the spiral structure of the spirochete form (top) and a cross section (bottom) with the following labeled: 1-Bacterial Cell Envelope; 2-Cytoplasm; 3-Flagella; 4-The Attachment Point of the Flagella.
Spirochetes as Parasites
Spirochetes are best known by their prevalence as causative agents of disease. Three genera contain pathogenic species:
- Treponema: The Treponema palidum species causes the sexually transmitted disease, syphilis. Closely related to this is are three other species which cause yaws, pinta and bejel; these are diseases transmitted either sexually or through skin contact and result in symptoms of mouth and skin sores, lumps in the bone, growths and thick discolored patches of skin. These bacteria can be killed with a penicillin injection.
- Leptospira: Certain species within this genus cause a disease called Leptospirosis (also known as Weil’s disease). The symptoms of this are flu-like; including fever, headaches, and muscle pains, although can become as severe as meningitis and can cause bleeding into the lungs.
- Borrelia: There are 52 species of the Borrelia genus, which transmit through the bites of ticks and lice. Of the 52 species, 21 are responsible for causing borreliosis (also known as Lyme disease) while 29 cause Relapsing Fever. Relapsing fever victims experience fever, chills, headaches, nausea and rashes. If left untreated, the symptoms often subside and then return several weeks later.
Lyme disease is a common disease, transmitted by tick bites after they have been attached for at least 24 hours. Usually a small rash appears on the body, which expands over several days. As the disease progresses, symptoms can include severe headaches, neck stiffness, loss of muscle use in the face, pains and numbing of the hands and feet, palpitations and irregular heartbeat, arthritis, and inflammation of the brain and spiral chord. In some cases Lyme disease is fatal.
Lyme disease is hard to cure unless it is caught immediately after infection and symptoms often do not occur for several days or even weeks after transmission. This is because of evolved features in the Lyme-causing spirochetes, which provide a high chance of transmission and the ability to evade the immune system of their host. Firstly, when the tick feeds, a surface protein of the Borrelia binds to a protein within the saliva of the tick. This protein coats the spirochete and makes it undetectable to the immune system. Because antibodies are not produced to fight the intruder, the spirochete is free to establish itself before it is recognized. In the following weeks the bacterium spreads throughout the body—it is able to do so effectively because of the motility provided by the endoflagella system.
The movement of the flagella stimulates the immune system, however the spirochete is able to alter the expression of proteins of its outer cell wall in order to ‘disguise’ it. This makes it extremely difficult for the immune system to produce the correct antibodies to attack the bacteria. The immune system responds by producing multiple types of antibody, which fail to kill the bacteria due to the changing protein structure but have a toxic effect on the host, causing inflammation and damage to tissues throughout the body
The spirochetes are also able to ‘cloak’ themselves within the extracellular matrix of the host cells, making it easier still to avoid detection. They have also been found to enter the lymph nodes, where antibodies are produced, and alter the normal function of antibody production.
Finally, the Borrelia have been found to protect themselves using a biofilm—a slimy film substance, which they generate themselves, containing the bacteria and other microorganisms. The biofilm protects the bacteria from the harsh conditions created by antibiotics and allows the invader to lie dormant for long periods of time until the conditions are more favorable and they can begin the attack again.
Spirochetes as Symbionts
Not all spirochetes are parasitic, in fact many of them hold symbiotic relationships or mutualisms with their host; in this case, both organisms depend on the other for their survival.
Symbiosis with Termites
Within the species of termite Mastotermes darwiniensis is a wood eating flagellate called “Mixotricha“. The surface of the mixotricha is covered by Treponema spirochetes, giving the appearance of cilia. The spirochetes use coordinated undulation to propel the symbionts through the intestinal fluid of the termite, while the anterior flagella of the mixotricha is used for steering.
The mixotricha helps to break down the cellulose within the wood that the termite has eaten into sugars, hydrogen, carbon dioxide and acetate. It is thought that the spirochetes oxidize the acetate, and use it to support the respiration requirements of the termite, whilst deriving nutrition from the other products produced by the mixotricha.
Symbiosis with Ruminants
Ruminants such as cows, deer, sheep, giraffes and elk have four chambers within their stomachs in which to break down hard-to-digest cellulose. The first chamber of the stomach is the ’rumen’. This chamber contains a diverse range of fungi, bacteria and protozoans, which secrete enzymes that the ruminant cannot produce independently, in order to digest the cellulose in the plant matter. Treponema spirochetes make up a small percentage of the organisms found within the rumen and, in a similar way to those found within termite intestines, aid and enhance the breakdown of cellulose, while using the by-products produced by the other bacteria for their growth.
The three other chambers of the stomach, the ‘reticulum’, the ‘obassum’ and the ‘abomasum’ perform functions such as absorption of liquids into the body and true digestion with gastric acids.