It is the causative organism of anthrax. It derives its name from the Greek word for coal, “anthrakis,” because of the black eschars seen in cutaneous anthrax. It has been used since World War II as an agent of biological warfare or bioterrorism. Between 2001 and 2002, after the attacks on the World Trade Center, letters laced with spores of B. anthracis were sent to multiple people, causing pulmonary and cutaneous anthrax in 22 cases and resulting in 5 deaths.
It is a large, gram-positive, spore-forming, rod-shaped bacillus that is frequently present in short chains in infected tissues. It has a capsule composed of D glutamate; it is the only bacterium with this type of capsule, because capsules of other bacteria are made of polysaccharides. Spore formation is not seen in host tissues because enough nutrients are available.
For epidemiological purposes, it can be divided into 5 categories on the basis of a variable number of tandem repeats in the VrrA gene region.
Exotoxins are important for the pathogenesis of anthrax. These are edema factor and lethal factor, which are encoded on plasmids. Each exists as a binary A-B unit:
The capsule itself is antiphagocytic.
Infection occurs after exposure to spores in one of three main ways:
It is an occupational hazard for factory workers processing wool and animal hides (called “Woolsorter’s disease”) and for slaughterhouse workers, who are at risk for cutaneous anthrax. The incubation period may be as long as 2 months. Ciprofloxacin and doxycycline are recommended for prophylaxis and treatment.
Cutaneous anthrax: Begins as a small papule, followed by ulceration and local edema, which forms the classic black, painless eschar. An eschar is dry, dead, necrotic tissue.
Gastrointestinal anthrax: Classical eschars are seen anywhere in the gastrointestinal tract, most commonly in the wall of the terminal ileum and caecum. It presents as nausea, vomiting, abdominal pain, hematemesis, and bloody diarrhoea. It may cause septicemia and death.
Pulmonary anthrax: Starts with flu-like symptoms and a dry cough, followed by rapid onset of respiratory distress, cyanosis, septicemia, and shock, with a 95% mortality rate. Hemorrhagic mediastinitis is seen. Chest X ray shows classic mediastinal widening and adenopathy.
Anthrax meningitis: It is a fatal form of meningitis with the presence of blood in CSF.
A presumptive diagnosis can be made based on classic clinical features, history of exposure, and Gram stain of skin eschar or papules, blood, or CSF showing “box car” shaped, non-motile, capsulated gram-positive bacilli in chains. Polychrome methylene blue (McFadyean stain) or India ink can be used to highlight the capsule. They grow on blood agar, forming grey-white, non-hemolytic colonies with a “bee’s eye” appearance.
Serological studies for antibodies are used only for retrospective diagnosis by comparing acute and convalescent sera tested by ELISA and immunoblotting techniques.
Rapid diagnosis can be done using PCR and direct fluorescent antibody tests.
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