2. Introduction
Spirochetes are Gram-negative, spiral-shaped bacteria of the phylum
Spirochaetes.
In nature, they may exist as free-living bacteria, symbionts, or as parasites
capable of causing diseases in animals.
As such, they are widely distributed in nature, exhibiting varying
characteristics in shape and size etc.
However, they are all characterized by a distinctive diderm (double-
membrane).
5. Ecology and Distribution
As a group of bacteria, spirochetes are widely distributed in nature and can be found in different
environments across the world.
Many of the species have been shown to exist as free-living organisms and can be found in
various habitats in water (surface water/freshwater), lakes, salt marsh sediments, mud,
sediments, and deep-sea vents among others.
Apart from the species found in these habitats, others form an association with various hosts
(termites, protozoa, mammals, etc) and are therefore found living within these hosts (in the
intestine).
While some of the species are beneficial, some are pathogenic and tend to cause diseases (e.g.
Lyme disease, dysentery, etc).
6. Ecology and Distribution
Because of the diversity between species and where they are found in nature, spirochetes have
also been classified based on their distribution.
Obligatory aerobic spirochetes can be found in water and soil as free-living organisms with
some species living as pathogens in their hosts.
Members of the genus Leptospira are classified as obligatory aerobic spirochetes.
Anaerobic and facultative anaerobic spirochetes exist as free-living forms and include
members of the genus Spirochaeta.
They are mostly found in different environments where they survive on a variety of organic matter
(disaccharides, pentoses, and hexoses, etc).
7. Characteristics of Spirochetes
Cell structure and Morphology
As the name suggests, spirochetes have a spiral morphology that has been used to classify them
based on morphology.
From early records, it's believed that spirochetes were first observed under the microscope by Van
Leeuwenhoek, who noted that in their motion, they "bent their body into curves in going forwards“
Today, it's well understood that this morphology is a result of the flexible peptidoglycan cell wall
wound by several axial fibrils.
Along with the cell wall, these fibrils are in turn covered by an outer membrane similar to the
membrane found in Gram-negative bacteria.
8. Cell structure and Morphology
The axial fibril, which has a similar structure to flagellum found in bacteria,
consists of a shaft and a covering sheath.
It also consists of an insertion apparatus that differentiate into a terminal
knob.
Depending on the species, the shaft has been shown to either have a
filamentous or globular substructure.
While it has similarities to the shaft found in bacterial flagella, it's located
between the inner and outer membranes (within the periplasmic space)
along the length of the organism.
9. Cell structure and Morphology
As already mentioned, the shaft (filament) of the axil fibril is covered by a
sheath (covering that encloses the shaft).
When the sheath is removed, the internal core is estimated to range
between 10 and 16nm in diameter.
Apart from giving the organism its shape, the axial fibrils (axial filaments)
have been shown to contribute to the movement of spirochetes through a
twisting motion.
The axial filaments/fibrils are also commonly referred to as the
endocellular flagella in spirochetes.
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12.
13. Cell Body
The protoplasmic cylinder is located beneath the outer
sheath.
This is the cell body and consists of a cell wall,
cytoplasmic membrane as well as cell contents
(cytoplasmic contents) enclosed within.
Like other bacteria, the internal environment of the cell
has a simple arrangement consisting of a nucleoid,
mesosomes, vacuoles, and ribosome.
The cell-wall-cell membrane complex is referred to as
parietocytoplasmic membrane.
14. Outer Structural Features
Like a number of other bacteria, the cell body of spirochetes is enclosed within several layers.
These include the outer and inner membrane, the peptidoglycan layer as well as the cytoplasmic
membrane.
Some of the species (e.g. Treponema pallidum) have been shown to contain a slime layer covering
the cell.
In these species, the layer has been associated with serological non-reactivity.
All species, however, have an outer membrane/envelope that surrounds the entire cell.
This is particularly important for the survival of the cell given that damage to this membrane results
in a loss of intracellular components and the consequent death of the cell.
15. Outer Structural Features
This membrane is particularly fragile to various adverse conditions during given stages of cell
development.
When exposed to hypertonic conditions, the outer membrane/envelope of Leptospira bacteria has
been shown to separate from the protoplasmic cylinder producing a spherical shape.
Here, it's worth noting that unlike other cell membranes, the outer membrane does not contain a
phospholipid bilayer.
According to a number of studies, lipids make up about 20 percent of the total dry weight.
This layer also consists of a number of proteins including lipoproteins and B-barrel proteins.
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17. Spirochete Peptidoglycan
The peptidoglycan has been shown to make up about 1 percent of the total cell dry weight and has a
similar composition to the peptidoglycan found in gram-negative bacteria.
While the axil filaments are responsible for the general spiral shape of spirochetes, the peptidoglycan
layer has been shown to help maintain this shape.
This layer consists of units of disaccharide N-acetyl glucosamine-N-actyl muramic acid that cross-links
with pentapeptide chains.
Some of the other components of this peptidoglycan in spirochetes include:
Glutamic acid
Muramic acid
L-ornithine
Based on a number of studies, it has been demonstrated that in spirochetes, the peptidoglycan layer is
associated with the inner membrane rather than the outer sheath.
18. Free-Living Spirochetes
The majority of free-living spirochetes are found in the genus Spirochaeta.
They can be found in various habitats including freshwater and seawater mud
environments that contain hydrogen sulfide.
A good example of free-living spirochete is the Spirochaeta isovalerica.
Along with S. litoralis, Spirochaeta isovalerica are obligate anaerobes that survive by
fermenting carbohydrates to produce acetate, ethanol, carbon dioxide and hydrogen.
Free-living species can also be found in such genus as Leptospira and live in soil and
various aquatic environments.
19. Host-Associated Spirochetes (non-pathogenic)
Through their association with hosts, some spirochetes become pathogenic and cause various diseases.
However, there are many non-pathogenic species that co-exist peacefully with their hosts.
Good examples of these are the spirochetes found in the intestine of termites where they digest wood
material.
On the other hand, Cristispira, found in the digestive tract of various marine and freshwater hosts (e.g.
mollusks) contributes to the digestion process.
In turn, they are protected from the external environment where they can only survive for a short period of
time.
This relationship has also been observed among nitrogen-fixing free-living spirochetes.
These spirochetes are commonly found in the hind-gut of termites (wood-feeding termites).
In these hosts, spirochetes have been shown to supply as much as 60 percent of the nitrogen in termite
biomass
20. Pathogenic Spirochetes
Unlike symbiotic spirochetes that live as commensals in their association with the host without
causing any harm, some spirochetes cause harm to the host through this association.
Treponemes which cause human treponematoses are a good example of pathogenic spirochetes.
Given that they cause disease in human beings, they are distributed in different parts of the world
where they can be spread through sexual contact.
Some of the other diseases caused by spirochetes include Lyme diseases, yaws, and relapsing
fever.
