Describe Bacillus anthracis with its morphology, toxin, clinical forms and diagnosis. Add a note on other aerobic spore-bearers.

Bacillus Anthracis — Morphology

• Large, gram-positive, aerobic, spore-bearing bacillus with square-cut (truncated) ends, arranged in long chains giving a "bamboo stick" appearance
• Non-motile — which distinguishes it from almost all other Bacillus species, which are motile
• The only bacterium with a polypeptide capsule, made of poly-D-glutamic acid; being D-amino acid it resists proteolysis and is poorly immunogenic. The capsule is formed IN tissue (and in culture with bicarbonate and carbon dioxide), not on ordinary media
• Spores are central, oval and non-bulging, formed only in the presence of oxygen — so they are not formed in the living host, but appear once the carcass is opened. This is why anthrax carcasses must not be opened or skinned, and are burned or buried deeply

Anthrax Toxin and Pathogenesis

• Three components, each harmless alone — protective antigen (PA), which binds the cell and forms the channel; oedema factor (EF), a calmodulin-dependent adenylate cyclase that raises cAMP and causes massive oedema; and lethal factor (LF), a protease that cleaves map kinase kinases, killing macrophages and releasing cytokines
• PA + EF = oedema toxin; PA + LF = lethal toxin. Protective antigen is essential to both, which is why it is the component used in the vaccine — antibody to it blocks delivery of the other two
• Both toxin and capsule are plasmid-encoded (pXO1 and pXO2); loss of either plasmid attenuates the organism, which is the basis of the Sterne vaccine strain

Clinical Forms of Anthrax

• Treatment — ciprofloxacin or doxycycline (penicillin is effective against sensitive strains but resistance has been engineered), with combination therapy and antitoxin (raxibacumab) in systemic disease; the cutaneous lesion is not excised, since surgery may disseminate it
• Anthrax is a recognised bioterrorism agent, because spores are stable, easily disseminated and cause high mortality by inhalation; this is why inhalational anthrax in an unexplained setting is treated as a public health emergency

Other Aerobic Spore-bearers

Laboratory Diagnosis and Prevention of Anthrax

• Specimens — vesicle fluid or material from the eschar edge, blood, sputum, cerebrospinal fluid, and tissue or animal material
• Handle at BSL-3 where inhalational anthrax is suspected, and warn the laboratory, since anthrax is a recognised cause of laboratory-acquired infection
• Direct examination — Gram stain showing large square-ended bacilli in chains, and the McFADYEAN capsule reaction
• Culture on blood and nutrient agar, with Medusa head colonies, non-motility, non-haemolysis, gamma-phage susceptibility and the string of pearls test
• Serology and PCR for toxin and capsule genes; the ASCOLI test where material is decomposed
• Prevention — animal vaccination (the Sterne live attenuated non-capsulated strain) is the key control measure; safe carcass disposal by burning or deep burial with lime; protective equipment for those handling hides and wool; and human vaccination with protective antigen for high-risk occupational groups and military personnel. Post-exposure prophylaxis with ciprofloxacin for 60 days, because inhaled spores may germinate late

Applied Aspects

• Do not open or skin a carcass suspected of anthrax; spores form only on exposure to oxygen, and opening the animal contaminates the soil for decades. Carcasses are burned or buried deeply with lime
• Anthrax is a notifiable and occupational disease in India, occurring in agricultural and animal-handling communities in several states; control depends on animal vaccination and safe carcass disposal rather than on human treatment
• Suspect anthrax in a painless black eschar with disproportionate oedema in someone with animal contact — the painlessness distinguishes it from staphylococcal and streptococcal lesions
• Reheated rice is the classic vehicle for Bacillus cereus emetic disease; the spores survive boiling, germinate as the rice cools slowly, and the toxin is not destroyed by reheating — so cooked rice must be cooled rapidly and refrigerated
• Bacillus species in a blood culture are usually contaminants, but should not be dismissed automatically in a neutropenic patient or one with an intravascular device
• Spore-formers define sterilisation standards — the biological indicators for every sterilising process are Bacillus spores, because they are the most resistant form of life to that process
• Distinguish B. Anthracis from the anthracoid bacilli carefully; the latter are common contaminants, and the practical differences are motility, haemolysis, capsule and gamma-phage susceptibility
• Cutaneous anthrax is not excised, since surgery risks dissemination; it is treated with antibiotics and the eschar allowed to separate
• Post-exposure prophylaxis after inhalational exposure runs for 60 days, because spores may lie dormant in the mediastinal nodes and germinate long after exposure — a far longer course than intuition suggests
• Occupational health matters more than clinical medicine here; vaccinating livestock, disposing of carcasses safely and protecting hide and wool workers prevents almost all human anthrax
• Warn the laboratory when anthrax is suspected; several documented laboratory-acquired infections have followed routine handling of an unlabelled specimen on an open bench
• Inhalational anthrax presents as a widened mediastinum without pneumonia, which is an unusual radiological combination and should raise the possibility immediately
• An unexplained case of inhalational anthrax is a public health emergency and must be notified at once, since the natural form is now vanishingly rare
• Cook rice in the quantity needed and refrigerate promptly; the Bacillus cereus emetic toxin is not destroyed by reheating, so the prevention lies entirely in rapid cooling rather than in the final heating
• Anthrax spores survive in soil for decades, so land where an infected animal died remains hazardous long after any living memory of the event
• Bacillus cereus also causes severe eye infection after penetrating trauma, progressing to loss of the eye within a day, so a contaminated ocular injury must be treated urgently rather than observed