Pathogenic Clostridia, including Botulism and Tetanus (page 4)
(This chapter has 4 pages)
© Kenneth Todar, PhD
C. botulinum is a large anaerobic bacillus that forms
endospores. It is widely distributed in soil, sediments of lakes and
and decaying vegetation. Hence, the intestinal tracts of birds, mammals
and fish may occasionally contain the organism as a transient. Seven
types of the organism exist, each producing an immunologically distinct
form of botulinum toxin. The toxins are designated A, B, C1, D, E, F,
G). In the U.S., type A is the most significant cause of
involved in 62% of the cases. Not all strains of C. botulinum
the botulinum toxin. Lysogenic phages encode toxin serotypes C and D,
non lysogenized bacteria (which exist in nature) do not produce the
Type G toxin is thought to be plasmid encoded.
Pathogenesis of Botulism
In food-borne botulism, the botulinum toxin is ingested with food in
spores have germinated and the organism has grown. The toxin is
by the upper part of the GI tract in the duodenum and jejunum and
into the blood stream by which it reaches the peripheral neuromuscular
synapses. The toxin binds to the presynaptic stimulatory terminals and
blocks the release of the neurotransmitter acetylcholine which is
for a nerve to simulate the muscle.
Food-borne botulism is not an infection but an intoxication
it results from the ingestion of foods that contain the preformed
toxin. In this respect, it resembles staphylococcal or Bacillus cereus food poisoning.
results from eating uncooked foods in which contaminating spores have
and produced the toxin. C. botulinum spores are relatively heat
resistant and may survive the sterilizing process of improper canning
The anaerobic environment produced by the canning process may further
the outgrowth of spores. The organisms grow best in neutral or "low
Clinical symptoms of botulism begin 18-36 hours after toxin
with weakness, dizziness and dryness of the mouth. Nausea and vomiting
may occur. Neurologic features soon develop, including blurred vision,
to swallow, difficulty in speech, descending weakness of skeletal
and respiratory paralysis.
Botulinum toxin may be transported within nerves in a manner
to tetanospasmin, and can thereby gain access to the CNS. However,
CNS involvement is rare.
Infant botulism is due to infection caused by C. botulinum.
The disease occurs in infants 5 - 20 weeks of age that have been
to solid foods, presumably the source of infection (spores). It is
by constipation and weak sucking ability and generalized weakness. C.
botulinum can apparently establish itself in the bowel of infants
a critical age before the establishment of competing intestinal
microbiota. Production of toxin by bacteria in the GI tract induces
symptoms. This "infection-intoxication" is restricted to infants. C.
botulinum organisms, as well as toxin, can be found in the feces of
infected infants. Almost all known cases of the disease have recovered.
The possible role of infant botulism in "sudden infant death
has been suggested but remains unproven. C. botulinum,
toxin, or both have been found in the bowel contents of several infants
who have died suddenly and unexpectedly.
The Botulinum Toxins
The botulinum toxins are very similar in structure and function to the
tetanus toxin, but differ dramatically in their clinical effects
they target different cells in the nervous system. Botulinum neurotoxins
predominantly affect the peripheral nervous system reflecting a
preference of the toxin for stimulatory motor neurons at a
junction. The primary symptom is weakness or flaccid paralysis.
Tetanus toxin can affect the same system, but the tetanospasmin shows a
tropism for inhibitory motor neurons of the central nervous system, and
its effects are primarily rigidity and spastic paralysis.
Botulinum toxin is synthesized as a single polypeptide chain with a
molecular weight around 150 kDa. In this form, the toxin has a
low potency. The toxin is nicked by a bacterial protease (or possibly
gastric proteases) to produce two chains: a light chain (the A
with a molecular weight of 50 kDa; and a heavy chain (the B fragment),
with a mw of 100kDa. As with tetanospasmin, the chains remain connected
by a disulfide bond. The A fragment of the nicked toxin, on a molecular
weight basis, becomes the most potent toxin found in nature.
Structure of the botulinum
The botulinum toxin is specific for peripheral nerve endings at
the point where a motor neuron stimulates a muscle. The toxin binds to
the neuron and prevents the release of acetylcholine across the
The heavy chain of the toxin mediates binding to presynaptic
The nature of these receptors is uncertain; different toxin types seem
to utilize slightly different receptors. The binding region of the
molecule is located near the carboxy terminus of the heavy chain. The
terminus of the heavy chain is thought to form a channel through the
of the neuron allowing the light chain to enter. The toxin (A fragment)
enters the cell by receptor mediated endocytosis. Once inside a neuron,
different toxin types probably differ in mechanisms by which they
release, but a mechanism similar to or identical to tetanospasmin has
reported (i.e., proteolytic cleavage of synaptobrevin II). The affected
cells fail to release a neurotransmitter, thus producing paralysis of
motor system. Once damaged, the synapse is rendered permanently
The recovery of function requires sprouting of a new presynaptic axon
the subsequent formation of a new synapse.
As stated above, the mechanism by which acetylcholine release is
is not known. However, recent evidence suggests that both botulinum
as well as tetanus toxin are zinc-dependent endopeptidases that
cleave specific proteins that are involved in excretion of
Both toxins cleave a set of proteins called synaptobrevins.
are found in synaptic vesicle of neurons, the
responsible for release of neurotransmitters. Presumably, proteolytic
of synaptobrevin II would interfere with vesicle function and release
On the average there are about 25 cases of botulism annually in the
Prior to the advent of critical care, the case fatality rate exceeded
but currently it is about 20%. The first (or only) patient in an
has a 25% chance of death, whereas subsequent cases which are diagnosed
and treated more quickly, carry only a 4% risk.
Each od the toxins that cause botulism is specifically neutralized
its antitoxin. Botulinum toxins can be toxoided and make good
for inducing protective antibody. As with tetanus, immunity to botulism
does not develop, even with severe disease, because the amount of toxin
necessary to induce an immune response is lethal. Repeated occurrences
botulism has been reported.
Once the botulinum toxin has bound to nerve endings, its activity is
unaffected by antitoxin. Any circulating ("unfixed") toxin can be
by intravenous injection of antitoxin. Therefore, individuals known to
food with botulism should be treated immediately with antiserum.
A multivalent toxoid evokes good protective antibody
but its use is unjustified due to the infrequency of the disease. An
vaccine exists for laboratory workers.
The most important aspect of botulism prevention is proper food
and preparation. The spores of C. botulinum can survive boiling
(100oC at 1 atm) for more than one hour, although they are
by autoclaving. Because the toxin is heat-labile, boiling or intense
(cooking) of contaminated food will inactivate the toxin. Food
that bulge may contain gas produced by C. botulinum and should
be opened or tasted. Other foods that appear to be spoiled should not
Botulism and Bioterrorism
Botulinum Toxin in Biowarfare......of course,
it has been thought of .......botulinum toxin is the most potent poison
known for humans; 10 grams is a lethal dose for the human population of
Los Angeles. Below is an interesting anecdote that appeared in JAMA
285, No. 21, June 6, 2001
To the Editor:
A historical incident
a number of features of botulinum toxin not discussed in the review of
bioweaponry by Dr. Arnon and colleagues.
During World War II, the US
of Strategic Services (OSS) developed a plan for Chinese prostitutes to
assassinate high-ranking Japanese officers with whom they sometimes
in occupied Chinese cities. Concealing traditional weapons on the women
at the appropriate time would obviously be difficult. Therefore, under
the direction of Stanley Lovell, the OSS prepared gelatin capsules
than the size of the head of a common pin" containing a lethal dose of
botulinum toxin. Wetted, a capsule could be stuck behind the ear or in
scalp hair, later to be detached and slipped into the officer's food or
drink. The OSS recognized that the normal background of botulism cases
would deflect suspicion from the women.
The capsules were shipped
Chunking, China. The Navy detachment there, taking nothing for granted,
tested the capsules on stray donkeys. The donkeys lived. Lovell was
that the capsules were faulty, and the project was abandoned. Much
Lovell learned of the donkey test with, one imagines, some
since "donkeys are one of the few living creatures immune to botulism."
This incident has been
in other publications. No source for the donkey-resistance information
is ever given. More recent experience shows that botulism can occur in
mules and donkeys (R. H. Whitlock, DVM, PhD, written communication,
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