Bacillus anthracis is a Gram-positive, spore-forming rod bacterium that causes anthrax, and it kills through two proteins working in concert: a cell-binding component called protective antigen (PA) and two enzymatic toxins, lethal factor and edema factor. Untreated inhalation anthrax is fatal in roughly 85 to 90 percent of cases; treated cutaneous anthrax is fatal in under 1 percent. The bacterium itself is a weak invader — its deadliness comes from toxins and from spores that survive decades in soil. This explains why anthrax is simultaneously a veterinary disease, an occupational hazard, and a biothreat agent.
The spore: a dormant cell built for decades, not days
Bacillus anthracis grows as a vegetative rod about 1 to 1.5 micrometers wide and 3 to 5 micrometers long. When nutrients run out, it converts a fraction of its population into endospores — dehydrated, calcium-dipicolinate-rich cores wrapped in a multilayered protein coat. A spore contains almost no free water, which is why it resists heat, ultraviolet light, desiccation, and disinfectants that destroy ordinary bacteria.
The classic field marker is the "Medusa head" colony: aerobic growth on blood agar produces gray-white colonies with curled edges. The capsule is another distinguishing feature. B. anthracis is the only Bacillus species that carries a poly-gamma-D-glutamate capsule, and that capsule is what lets vegetative cells avoid phagocytosis in a host.
Spore survival is measured in years to decades. The most cited natural benchmark is Gruinard Island off Scotland, contaminated during a 1941–1942 biological weapons trial. Decontamination took 280 tonnes of formaldehyde and seawater applied over years, and the island was declared clear only in 1990 — roughly a half-century after the spores were released. In permafrost and burial sites, viable spores have been recovered after comparable spans. In plain terms: the spore is the bacterium's real survival strategy, and the vegetative cell is the brief, vulnerable phase.
Spores germinate when they sense favorable conditions — amino acids, purines, and a temperature near 37 degrees Celsius in a host, or specific nutrients in soil. Germination in the lung is the critical step for inhalation disease: inhaled spores are carried by alveolar macrophages toward lymph nodes, and they germinate inside those cells rather than in the air spaces themselves.
Two toxins, one shared delivery protein
Every anthrax toxin is an A-B structure. The B component is protective antigen (PA), which binds receptors on host cells, gets cleaved, and self-assembles into a heptameric or octameric pore. The A components are lethal factor (LF), a zinc-dependent metalloprotease, and edema factor (EF), a calmodulin-dependent adenylate cyclase.
Lethal factor cleaves mitogen-activated protein kinase kinases (MAPKKs), which breaks signaling cascades that control cell division and immune response. Edema factor converts ATP into cyclic AMP at a rate far above the host's own enzymes, flooding the cell with cAMP and driving the massive fluid accumulation behind anthrax edema. When LF and PA combine, they form lethal toxin; EF and PA form edema toxin. Each toxin has an LD50 in the low microgram-per-kilogram range in laboratory animals, which is why dose estimates for aerosol exposure are measured in inhaled spore counts, not milligrams.
The structural detail that matters for detection and therapy: PA is the shared gateway. Antibodies or drugs that block PA shut down both toxins at once. That is the mechanism behind raxibacumab and obiltoxaximab, two monoclonal antibodies licensed for inhalation anthrax, and it is why PA is the backbone of most anthrax vaccines.
A non-obvious correction: anthrax toxin does not simply poison cells uniformly. Lethal toxin is particularly effective at disabling macrophages and dendritic cells, which delays the immune response for days. The infection looks quiet early — fever, malaise, mild chest discomfort — while toxin accumulates. That silent phase is a core reason inhalation anthrax has historically been diagnosed after the point where treatment works best.
Three clinical forms and their case fatality rates
Anthrax presents in three main forms defined by route of entry. Cutaneous disease accounts for roughly 95 percent of naturally occurring human cases; inhalation and gastrointestinal forms are rarer but far more dangerous.
| Form | Route of entry | Typical incubation | Untreated fatality | Treated fatality |
|---|---|---|---|---|
| Cutaneous | Skin break, spore contact | 1–7 days (up to 12) | 20% (historical) | Under 1% |
| Inhalation | Aerosolized spores, 1–5 micrometers | 1–6 days (up to 43 reported) | 85–90%+ | 45–75% depending on stage at treatment |
| Gastrointestinal | Ingested contaminated meat | 2–5 days | 25–60% | Improved with early antibiotics, still high |
| Injection | Contaminated heroin, subcutaneous | Days to weeks | Not firmly established | Variable, often severe |
The inhalation numbers deserve a reconciliation sentence, because the table carries two different thresholds: the 85–90 percent figure applies to untreated or late-treated disease, while the 45–75 percent figure applies to the 2001 U.S. postal attacks, where patients treated after symptom onset still often died. The 2001 series produced 11 inhalation cases and 11 cutaneous cases across 22 total patients; 5 of the 11 inhalation patients died despite aggressive intensive care.
The 1979 Sverdlovsk outbreak — an accidental aerosol release from a military facility — produced at least 66 deaths over roughly six weeks, with most deaths in a narrow geographic band downwind of the plant. It remains the largest documented inhalation anthrax event and is the primary source for modern incubation-period estimates.
Gastrointestinal anthrax from undercooked contaminated meat is the form most often underdiagnosed in low-resource settings, because it mimics severe gastroenteritis. Injection anthrax, first described in heroin users in Europe from 2009 onward, is a newer category with atypical presentations and no long historical survival data.
Why the numbers differ by route and by hour
The fatality gap between cutaneous and inhalation disease is not about bacterial virulence — it is about toxin mass reaching tissue before antibiotics can clear the bacteria. Antibiotics kill vegetative cells; they do not neutralize toxin already circulating. In cutaneous disease the lesion stays local and toxin load is small. In inhalation disease, spores germinate in mediastinal lymph nodes, toxin enters the bloodstream, and hemorrhagic mediastinitis develops.
The time course of inhalation anthrax follows a recognizable arc. A prodromal phase of 1 to 3 days with fever, cough, and chest tightness is followed by a fulminant phase with sudden dyspnea, high fever, and mediastinal widening on chest imaging. The fulminant phase can progress to shock and meningitis within 24 to 48 hours.
Sepsis from anthrax carries a distinctive sign: massive edema and hemorrhage without the usual pus, because the bacteria do not provoke a typical neutrophil response. Anthrax meningitis, present in a substantial minority of systemic cases, raises mortality toward 90 percent even with treatment.
How much exposure is dangerous
Aerosol infectivity estimates for humans come largely from primate extrapolation. Published estimates put the human 50 percent infectious dose for inhaled spores at roughly 8,000 to 10,000 spores, though susceptible individuals may become ill at far lower doses, and the 2001 letters demonstrated that envelope contamination of a few grams could produce lethal exposures inside a building.
Environmental thresholds matter for response. The U.S. occupational standard for anthrax vaccination and monitoring applies to workplaces handling concentrations of B. anthracis that could generate aerosol. Surface sampling after a release is judged against spore counts per square centimeter rather than a single national limit, because no federal regulation sets one universal surface clearance number. That absence of a single threshold is a real limitation in remediation decisions.
The 2001 U.S. response used 60 days of post-exposure antibiotics as the standard course for exposed persons, later extended in some cohorts to 100 days after evidence of spore persistence in lung tissue. Per the CDC's 2001–2002 anthrax response guidance, post-exposure prophylaxis combines ciprofloxacin or doxycycline with a three-dose anthrax vaccine series.
The misconceptions worth correcting
Anthrax is not contagious person to person. No documented case of secondary transmission between humans exists, and public health guidance treats cases as non-transmissible. Transmission is environmental: soil, animal hides, wool, and intentional aerosol release. "In plain terms," this matters practically because contacts of a case do not need isolation, but they may need environmental risk assessment.
A second misconception: anthrax is not a disease of the past in livestock regions. Zoonotic anthrax remains endemic in parts of sub-Saharan Africa, Central Asia, and South Asia, where outbreaks follow floods alternating with dry spells that bring spores to the surface. The WHO reports recurring livestock and human case clusters in these regions; annual global human cases are estimated in the low thousands, but underreporting is substantial.
A third: the spore stain. Vegetative cells stain Gram-positive, but spores do not reliably Gram stain and are classically demonstrated with malachite green under steam, with the red vegetative cell and green spore described as the Schaeffer-Fulton method. Laboratory confirmation now relies on culture, polymerase chain reaction targeting the pXO1 and pXO2 plasmids, and matrix-assisted laser desorption/ionization mass spectrometry. Both plasmids are required for full virulence: pXO1 carries the toxin genes, pXO2 carries the capsule genes.
What to actually do
For clinicians, the practical sequence after suspected exposure is: start antibiotics before confirmation if exposure is credible, send blood and respiratory samples for culture and PCR, and obtain chest imaging for any inhalation-suspected case. First-line therapy is ciprofloxacin or doxycycline; severe systemic disease adds two antitoxin antibodies and aggressive supportive care. Antitoxin should be given as early as possible because it cannot reverse toxin already bound.
For people who handle animals, hides, or wool in endemic areas, the actions are concrete:
- Vaccinate livestock in endemic regions and report sudden livestock deaths rather than opening carcasses.
- Do not skin or butcher animals that died unexpectedly; anthrax carcasses should be buried or incinerated without opening, since spores form on exposure to air.
- Wear gloves and respiratory protection when processing high-risk animal products.
- Get the human anthrax vaccine if occupationally exposed in a covered workplace.
- For suspected cutaneous lesions, avoid incision and drainage; the eschar should be left intact, and antibiotics started promptly.
- After a known aerosol exposure, begin post-exposure prophylaxis immediately rather than waiting for symptoms.
Antibiotic choice during pregnancy shifts toward ciprofloxacin under current CDC guidance when the risk of inhalation anthrax is real, because the benefit outweighs the historically cited skeletal risk; that is a deliberate exception to older prescribing habits.
Where the science remains uncertain
Several core numbers rest on extrapolation rather than direct measurement. The 8,000 to 10,000 spore infectious dose for humans is derived from primate studies, not controlled human trials, and individual susceptibility varies with prior vaccination, smoking status, and immune function. The precise contribution of edema toxin versus lethal toxin to shock in humans is inferred from animal models; no human trial has separated them.
Duration of post-exposure prophylaxis is likewise unsettled in the extreme case. The 60-day course is grounded in spore germination timing under normal conditions, but spore persistence in non-human primate lungs after aerosol challenge has been documented beyond 60 days, which is why some guidance extended courses to 100 days after the 2001 attacks. Whether longer courses are needed after massive exposure remains unproven.
Treatment success in inhalation disease still depends mostly on speed. The 2001 data show that patients treated in the prodromal phase survived far more often than those treated after fulminant disease began, and no drug or antibody has changed that relationship. The strongest protective action remains early recognition, not a better late-stage therapy.