A 28-year-old male farmer from rural Maharashtra presents with a painless, black, necrotic lesion on his left forearm that developed 5 days after a minor cut while handling animal hides. The lesion is surrounded by significant edema and is not suppurative. Blood culture and lesion swab are sent. Gram stain shows large, Gram-positive rods arranged in chains. The organism grows on blood agar as large, mucoid, non-hemolytic colonies. Which of the following is the most likely causative organism and the key virulence factor responsible for the characteristic black eschar?

Explanation

## Clinical Diagnosis: Cutaneous Anthrax **Key Point:** The clinical presentation—painless black necrotic eschar with surrounding edema following occupational exposure to animal hides—is pathognomonic for cutaneous anthrax caused by *Bacillus anthracis*. ### Identifying Features of *Bacillus anthracis* | Feature | *B. anthracis* | *B. cereus* | |---------|---|---| | **Gram stain** | Large, Gram-positive rods in chains | Similar morphology | | **Hemolysis** | Non-hemolytic | β-hemolytic | | **Motility** | Non-motile (penicillin-sensitive) | Motile | | **Spore location** | Central (swollen sporangium) | Eccentric | | **Capsule** | Poly-D-glutamic acid (antiphagocytic) | Absent | | **Virulence factors** | Edema factor (EF), Lethal factor (LF), Protective antigen (PA) | Cereulide, phospholipase C | ### Pathogenesis of the Black Eschar **High-Yield:** The characteristic black necrotic lesion (eschar) results from the synergistic action of: 1. **Edema Factor (EF)** — adenylate cyclase that increases cAMP → massive local edema 2. **Lethal Factor (LF)** — metalloprotease that cleaves MAPKK → cell death and tissue necrosis 3. **Protective Antigen (PA)** — binds to cell surface receptors and facilitates entry of EF and LF The black appearance is due to tissue necrosis and hemorrhage, not bacterial pigment. **Clinical Pearl:** The eschar is typically painless and non-suppurative—a key distinguishing feature from other skin infections. The surrounding edema can be massive and disproportionate to the lesion size. ### Occupational Context **Key Point:** Cutaneous anthrax is the most common form of anthrax (95% of cases) and typically occurs in: - Farmers and veterinarians - Wool sorters and hide processors - Individuals handling infected animal products The organism forms spores in soil and on animal products, which germinate when inoculated through skin breaks. ### Laboratory Confirmation - **Gram stain:** Large, Gram-positive bacilli in "boxcar" chains - **Culture:** Non-hemolytic colonies on blood agar (unlike *B. cereus*) - **PCR or immunofluorescence:** Rapid confirmation - **Anthrax toxin detection:** ELISA for PA, EF, and LF in serum or culture supernatant **Mnemonic: LEAP** — **L**ethal factor, **E**dema factor, **A**nthrax (disease), **P**rotective antigen (the three-component toxin system).