Bacillus anthracis

It is the causative organism of Anthrax. It derives its name from the Greek word for coal “anthrakis” due to 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 resulting in 5 deaths.

Morphology

It is a large, gram positive, spore forming , rod shaped bacillus frequently present in short chains in infected tissues. It has a capsule composed of D glutamate, the only bacteria to have this as capsules of all other bacteria are made of polysaccharides. Spore formation is not seen in host tissues as enough nutrients are available.

Classification

For epidemiological purposes it can be divided into 5 categories on the basis of variable number of tandem repeats in the VrrA gene region.

Human Pathology

Exotoxins are important for pathogenesis of anthrax. These are edema factor and lethal factor which are encoded on plasmids. Each of them exists as a binary A-B unit where B subunit called protective antigen forms pores in the cell allowing edema factor and lethal factor to enter the cell. A subunit has the enzyme activity. The capsule itself is antiphagocytic.

Infection occurs either on exposure to the spore through a break in the skin causing cutaneous anthrax, or spore ingestion causing gastrointestinal anthrax or spore inhalation causing pulmonary anthrax. It is an occupational hazard to factory workers processing wool and animal hides called “Woolsorter’s Disease” and 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: It starts with flu-like symptoms, 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 presence of blood in CSF.

Laboratory diagnosis of Anthrax

Presumptive diagnosis can be made by 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 only used 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.