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

The history (animal contact), clinical picture (pustule in the area of contact) and lab findings (gram positive bacilli in chains with McFadyean reaction positive) are all characteristic of infection by Bacillus anthracis . Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition