A man, after skinning a dead animal, developed a pustule on his hand. A smear prepared from the lesion showed Gram Positive Bacilli in long chains positive for McFadyean's reaction. The most likely aetiological agent is:

The patient in the question stem is showing features of Bacillus anthracis infection. Anthrax is a disease that occur in herbivores sheep, cattle, etc. Human cases in countries are commonly found in agricultural workers, and those involved in leather and hair processing. Bacillus anthracis is a gram positive, aerobic, non-motile, capsulated spore forming bacilli. Chain of bacilli present as bamboo stick or have a box car like appearance. It produces a heat stable spore under unourable conditions. The vegetative cell produces a polypeptide capsule and exotoxin, both responsible for its high virulence. When blood film containing anthrax bacilli is stained with polychrome methylene blue and then examined under microscope, amorphous purplish material representing capsular material is noticed. This is called Mc Fadyean's reaction (characteristic of anthrax bacilli). Ref: Clinical Bacteriology By J.Keith, Robe G.Westran, Page 171 ; Ananthnarayanan, 6th Edition, Chapter 27 ; Textbook of Microbiology and Immunology By Parija, Page 243-6, 415 ; Teaching Atlas of Chest Imaging By Mark Parker, Page 238