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Bacteriology at UW-Madison |
Electron micrograph of Clostridium
tetani endospores are
formed at the tips of the cell and they cause the sporangium (cell that
contains a spore) to swell, which imparts a characteristic "drumstick"
appearance to the cells
Clostridium tetani
Clostridium tetani is the causative agent of tetanus. The bacterium is relatively large, Gram-positive, rod-shaped, and characteristically produces endospores. The organism is found in soil, especially heavily-manured soils, and in the intestinal tracts and feces of various animals. Carrier rates in humans vary from 0 to 25%, and the organism is thought to be a transient member of the intestinal flora whose presence depends upon ingestion.
Clostridium tetani produces terminal spores within a swollen sporangium giving it a distinctive drumstick appearance. Although the bacterium has a typical Gram-positive cell wall, it may stain Gram-negative or Gram-variable, especially in older cells.
The bacterium has a strictly anaerobic type of metabolism and will not grow under aerobic conditions. In fact, vegetative cells are killed by exposure to O2, although their spores are able to survive long periods of exposure to air and other deleterious conditions.
C. tetani Gram
stain. The bacterium is Gram-positive, but like many older
cultures of clostridia, there is a tendency towards Gram variability.
Vegetative
cells are relatively large rod-shaped cells. Endospores are
formed intracellularly at the tips of the sporangium and cause a
characteristic swelling.
Tetanus
Tetanus is a highly fatal disease of humans. Mortality rates reported vary from 40% to 78%. The disease stems not from invasive infection but from a potent neurotoxin (tetanus toxin or tetanospasmin) produced when spores germinate and vegetative cells grow after gaining access to wounds. The organism multiplies locally but symptoms appear remote from the infection site.
Because of the widespread use of the tetanus toxoid for prophylactic immunization (routinely as part of the DPT vaccine), fewer than 100 cases occur annually in the U.S., but the disease is a significant problem world-wide where there are more than 300,000 cases annually. Most cases in the U.S. occur in individuals over age 60, which is taken to mean that waning immunity is a significant risk factor.
Below. Two
surveillance reports from CDC regarding tetanus. Figure 1 indicates
that between 1947 1nd 1997, the number of deaths in the U.S. declined
nearly
100 fold, coincidental to implementation of mass vaccination. Figure 2
shows the number of cases by state between 1995 1nd 1997. There are no
apparent regional correlation with disease other than population
density.
Most cases of tetanus result from small puncture wounds or lacerations which become contaminated with C. tetani spores that germinate and produce toxin. The infection remains localized often with only minimal inflammatory damage. The toxin is produced during cell growth and sporulation at the site of inoculation. It migrates along neural paths from a local wound to sites of action in the central nervous system. The clinical pattern of generalized tetanus consists of severe painful spasms and rigidity of the voluntary muscles. The characteristic symptom of "lockjaw" involves spasms of the masseter muscle. It is an early symptom which is followed by progressive rigidity and violent spasms of the trunk and limb muscles. Spasms of the pharyngeal muscles cause difficulty in swallowing. Death usually results from interference with the mechanics of respiration.
Neonatal tetanus accounts for about half of the tetanus deaths in developing countries. In a study of neonatal mortality in Bangladesh, 112 of 330 infant deaths were due to tetanus. Neonatal tetanus follows infection of the umbilical stump in infants born to nonimmune mothers (therefore, the infant has not acquired passive immunity). It usually results from a failure of aseptic technique during the birthing, but certain cultural practices may contribute to infection.
Tetanus Toxin
There have been 11 strains of C. tetani identified. They differ in their ability to produce tetanus toxin , but all strains produce a toxin which is identical in its immunological and pharmacological properties. The toxin gene is located on a plasmid which is present in all strains that produce the toxin. Tetanus toxin is one of the three most poisonous substances known, the other two being the toxins of botulism and diphtheria.
The bacterium synthesizes the tetanus toxin as a single 150kDa polypeptide chain (called the progenitor toxin), that can be separated into two polypeptide domains or fragments. The B domain is responsible for binding of the toxin to host cell neurons, and the A fragment is responsible for the neurotoxicity of the toxin. The A fragment has toxic activity after the B fragment secures its entry. Binding appears to be an irreversible event. Recovery depends on sprouting a new axon terminal.
Tetanus toxin is produced in vitro in amounts up to 5 to 10% of the bacterial weight. Because the toxin has a specific affinity for nervous tissue, it is referred to as a neurotoxin. The toxin has no known useful function to C. tetani. Why the toxin has a specific action on nervous tissue, to which the organism naturally has no access, may be an anomaly of nature. The toxin is heat labile, being destroyed at 56 degrees C in 5 minutes, and is O2 labile. The purified toxin rapidly converts to toxoid at 0 degrees C in the presence of formalin.
Mechanism of Action of the Toxin
The tetanus toxin initially binds to peripheral nerve terminals. It
is transported within the axon and across synaptic junctions until it
reaches
the central nervous system. There it becomes rapidly fixed to
gangliosides
at the presynaptic inhibitory motor nerve endings, and is taken up into
the axon by endocytosis. The effect of the toxin is to block the
release
of inhibitory neurotransmitters (glycine and gamma-amino butyric acid)
across the synaptic cleft, which is required to check the nervous
impulse.
If nervous impulses cannot be checked by normal inhibitory mechanisms,
it produces the generalized muscular spasms characteristic of tetanus.
The toxin appears to act by selective cleavage of a protein component
of
synaptic vesicles, synaptobrevin II, and this prevents the
release
of neurotransmitters by the cells.
A soldier dying from tetanus.
Painting by Charles Bell in the Royal College of Surgeons, Edinburgh.
Immunity
Unlike other diseases, such as diphtheria, recovery from the natural disease usually does not confer immunity, since even a lethal dose of tetanus toxin is insufficient to provoke an immune response.
Prophylactic immunization is accomplished with tetanus toxoid, as part of the DPT (DTP) vaccine or the DT (TD) vaccine. Three injections are given in the first year of life, and a booster is given about a year later, and again on the entrance into elementary school.
Whenever a previously-immunized individual sustains a potentially dangerous wound, a booster of toxoid should be injected. Wherever employed, intensive programs of immunization with toxoid have led to a striking reduction in the incidence of the disease.
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