Which organism is the causative agent of a malignant pustule?

• A. Bacillus anthracis
• B. Corynebacterium ulcerans
• C. Erysipelothrix rhusiopathiae
• D. Listeria monocytogenes

Answer: (A)

A malignant pustule is the skin manifestation of cutaneous anthrax. The organism responsible is Bacillus anthracis, a Gram-positive, spore-forming bacillus. Infection of the skin starts as a painless papule that rapidly becomes a vesicle and then a necrotic black eschar with surrounding edema. This lesion arises from the bacterium’s virulence factors, including a protective capsule and a tripartite toxin that disrupts immune responses and causes tissue necrosis. Spores in the environment, especially in soil or animal products, can contaminate skin through contact with an infected animal or product. Among the options, only Bacillus anthracis is associated with this malignant skin lesion; the others cause different diseases (for example, erysipeloid from Erysipelothrix, diphtheria-like illness from Corynebacterium ulcerans, and listeriosis from Listeria monocytogenes).