What is the Gram stain description of Bacillus anthracis?

• A. Gram-positive rods that are motile
• B. Gram-negative rods
• C. Gram-positive nonmotile rods
• D. Gram-positive cocci in clusters

Answer: (C)

Bacillus anthracis appears on Gram stain as Gram-positive, nonmotile rods. The Gram-positive description comes from its thick peptidoglycan cell wall that retains crystal violet, giving a purple appearance. The rod shape is characteristic of Bacillus species, but what sets B. anthracis apart is the absence of motility, so it does not move. This combination helps distinguish it from Gram-negative rods (which would stain pink) and from Gram-positive cocci in clusters (which describes organisms like Staphylococcus). Some Bacillus species are motile, so the nonmotile descriptor is key for identifying B. anthracis specifically.