Bacillus anthracis is a Gram-positive rod-shaped bacterium , 1 - 1.2 Â m in width and 3 - 5 Â m in length . Bacillus anthracis is a Gram-positive spore-forming , rod-shaped bacterium , with a width of 1-1 .2 Â m and a length of 3-5 Â m .