Anaerobic bacteria
lect:2
Dr. Khalid Waleed
M.B.ch.B., Msc., PhD. Immunology
Objectives
The objectives of this lecture are to know
1. Diseases related to Clostridium tetani infections.
2. Diseases related to Clostridium botulinum infections.
3. The laboratory diagnosis of Clostridium tetani and Clostridium botulinum.
Clostridium tetani
Clostridium tetani is an anaerobic, gram-positive, spore-forming rod
whose spores are highly resilient and can survive readily in the
environment throughout the world. Spores resist boiling and many
disinfectants. In addition, very low concentrations of this highly potent
toxin can result in tetanus (minimal lethal human dose, 2.5 ng/kg).
The disease is tetanus
Transmission
Spores are widespread in soil. Tetanus occurs when spores in the
environment enter the skin through puncture wounds. Germination
of spores is favored by necrotic tissue and poor blood supply in the
wound.
Neonatal tetanus, in which the organism enters through a
contaminated umbilicus.
Pathogenesis
The spores germinate into vegetative cells that produce
tetanospasmin. Symptoms usually appear approximately 7 days after the
injury, but the incubation period has been reported to range from 3 to 21
days. The length of incubation is related to the distance from the injury to
the central nervous system.
Tetanus toxin (tetanospasmin) is an exotoxin produced by vegetative
cells at the wound site. This polypeptide toxin is carried intra-axonally
(retrograde) to the central nervous system, where it binds to ganglioside
receptors and blocks release of inhibitory mediators (e.g., glycine and γ-
aminobutyric acid [GABA]) at spinal synapses. It is proteases that cleave
the proteins involved in mediator release.
Clinical findings
Tetanus is characterized by strong muscle spasms (spastic paralysis,
tetany). Specific clinical features include lockjaw (trismus) due to rigid
contraction of the jaw muscles, which prevents the mouth from opening;
a characteristic grimace known as risus sardonicus; and exaggerated
reflexes.
Opisthotonos, a pronounced arching of the back due to spasm of
the strong extensor muscles of the back, is often seen. Respiratory failure
ensues. A high mortality rate is associated with this disease. The
diagnosis of tetanus is based on clinical findings. Organisms are rarely
isolated from the wound site.
Diagnosis
Clostridium tetani produces a terminal spore (i.e., a spore at the
end of the rod). This gives the organism the characteristic appearance of
a “tennis racket.” Culture of C. tetani from a wound provides supportive
evidence. Serum anti-tetanus immunoglobulin G also may be measured
in a sample taken before the administration of antitoxin or
immunoglobulin; levels >0.1 IU/mL are protective and do not support the
diagnosis of tetanus.
Treatment and prevention
Tetanus immune globulin (tetanus antitoxin) is used to neutralize the
toxin. Tetanus is prevented by immunization with tetanus toxoid. The
WHO guidelines for tetanus vaccination consist of a primary course of
three doses in infancy, boosters at 4–7 and 12–15 years of age, and one
booster in adulthood. Standard WHO recommendations for prevention of
maternal and neonatal tetanus: administration of two doses of tetanus
toxoid at least 4 weeks apart to previously unimmunized pregnant
women.
Clostridium botulinum
Disease: C. botulinum causes botulism.
Transmission
Spores, widespread in soil, contaminate vegetables and meats. When
these foods are canned or vacuum-packed without adequate sterilization,
spores survive and germinate in the anaerobic environment. Just 1 gram
of botulinum toxin could kill over 1 million people. Two kilograms could
kill the entire human population of Earth.
Toxin is produced within the canned food and ingested preformed. The
highest-risk foods are (1) alkaline vegetables such as green beans,
peppers, and mushrooms and (2) smoked fish. The toxin is relatively
heat-labile; it is inactivated by boiling for several minutes. Thus, disease
can be prevented by sufficient cooking.
Pathogenesis
The botulinum toxin is absorbed from the gut into the blood stream. It
leaves the circulatory system at the point where an neuron joins a muscle.
The toxin prevents the fusion of synaptic vesicles of acetylcholine. There
is no release of acetylcholine at the neuromuscular junction, with
consequent paralysis of the muscle fiber. There are eight immunologic
types of toxin; types A, B, and E are the most common in human illness.
Botox is a commercial preparation of exotoxin A used to remove
wrinkles on the face.
Clinical findings
Descending weakness and paralysis, including diplopia, dysphagia,
and respiratory muscle failure, are seen. No fever.
Four special clinical forms occur:
1. Foodborne botulism: Results from ingestion of preformed toxin.
2. Infant botulism: Occurs when the organism elaborates the toxin
after it has colonized the gastrointestinal tract of infants.
3. Wound botulism: C. botulinum produces the toxin from an infected
wound site.
4. Intestinal botulism: after Intestinal colonization has been (with
surgery and administration of antibiotics).
Laboratory Diagnosis
The organism is usually not cultured. Botulinum toxin is
demonstrable in uneaten food and the patient’s serum by mouse
protection tests. Mice are inoculated with a sample of the clinical
specimen and will die unless protected by antitoxin.
Treatment
Trivalent antitoxin (types A, B, and E) is given, along with
respiratory support. The antitoxin is made in horses, and serum sickness
occurs in about 15% of antiserum recipients.
Prevention
Proper sterilization of all canned and vacuum-packed foods is
essential. Food must be adequately cooked to inactivate the toxin.
Swollen cans must be discarded.