Overview: Bacillus anthracis is a spore-forming, rod-shaped, gram-positive bacterium best known as the cause of anthrax. Cells are typically a few micrometres long and occur singly or in short chains. In nature the organism cycles between a dormant spore state, capable of surviving harsh environments for long periods, and a vegetative state in a susceptible host.

Structure and genetics: B. anthracis possesses a polypeptide capsule and produces protein toxins that are central to its ability to cause disease. Key virulence determinants are encoded on two plasmids, commonly called pXO1 and pXO2, which encode the tripartite anthrax toxin components and the capsule synthesis genes, respectively. The toxin components interact to disrupt immune responses and cellular signalling.

Lifecycle and transmission: Spores in soil or contaminated animal products are the predominant infectious form. Animals typically acquire infection by ingesting or inhaling spores; humans can be infected through broken skin, inhalation, ingestion, or contaminated injection for people who inject drugs. Environmental persistence of spores is a major factor in epidemiology and control.

Clinical forms: Infection manifests mainly as cutaneous, inhalational, gastrointestinal, or injection-related disease. Cutaneous anthrax, the most common form historically, begins as a painless lesion that may develop a characteristic black eschar. Inhalational anthrax is less common but can be rapidly progressive and life-threatening. Severity depends on exposure route, dose, and timeliness of treatment.

Diagnosis, treatment and prevention: Diagnosis combines clinical assessment with microbiology, molecular tests, and serology. Effective antibiotic therapy includes agents such as penicillins, tetracyclines, and fluoroquinolones; severe cases may also be treated with antitoxin therapies and intensive supportive care. Vaccination is used for livestock in many regions and for selected high-risk human populations. Control strategies emphasize vaccination, rapid diagnosis, animal surveillance, and safe handling of suspect materials.

History and significance: B. anthracis played a central role in the development of modern bacteriology: it figured in Robert Koch's demonstration that bacteria can cause disease and in early vaccine development by Louis Pasteur. In the 20th and 21st centuries it has remained important in veterinary medicine, public health and biodefense following accidental and deliberate release events.

Notable facts

  • Spore resilience makes environmental decontamination challenging.
  • Virulence depends on both toxin production and capsule-mediated immune evasion.
  • Prevention combines animal vaccination, outbreak control, and targeted human vaccination for at-risk groups.